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Climate change mitigation in cities: a systematic scoping of case studies

Mahendra Sethi 1,2 , William Lamb 2,3 , Jan Minx 2,4 and Felix Creutzig 1,2

Published 26 August 2020 • © 2020 The Author(s). Published by IOP Publishing Ltd Environmental Research Letters , Volume 15 , Number 9 Focus on Systematizing and Upscaling Urban Solutions for Climate Change Mitigation Citation Mahendra Sethi et al 2020 Environ. Res. Lett. 15 093008 DOI 10.1088/1748-9326/ab99ff

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1 Technical University of Berlin, Straße des 17. Juni 135, 10623, Berlin, Germany

2 Mercator Research Institute on Global Commons and Climate Change, Torgauer Straße 12–15, EUREF Campus #19, 10829, Berlin, Germany

3 School of Earth and Environment, University of Leeds, Leeds LS2 9JT, United Kingdom

4 Priestley International Centre for Climate, University of Leeds, Leeds LS2 9JT, United Kingdom

Mahendra Sethi https://orcid.org/0000-0003-1065-5484

William Lamb https://orcid.org/0000-0003-3273-7878

Jan Minx https://orcid.org/0000-0002-2862-0178

Felix Creutzig https://orcid.org/0000-0002-5710-3348

• Received 18 February 2019
• Accepted 5 June 2020
• Published 26 August 2020

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Method : Single-anonymous Revisions: 3 Screened for originality? Yes

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A growing number of researchers and stakeholders have started to address climate change from the bottom up: by devising scientific models, climate plans, low-carbon strategies and development policies with climate co-benefits. Little is known about the comparative characteristics of these interventions, including their relative efficacy, potentials and emissions reductions. A more systematic understanding is required to delineate the urban mitigation space and inform decision-making. Here, we utilize bibliometric methods and machine learning to meta-analyze 5635 urban case studies of climate change mitigation. We identify 867 studies that explicitly consider technological or policy instruments, and categorize these studies according to policy type, sector, abatement potential, and socio-technological composition to obtain a first heuristic of what is their pattern. Overall, we find 41 different urban solutions with an average GHG abatement potential ranging from 5.2% to 105%, most of them clustering in the building and transport sectors. More than three-fourth of the solutions are on demand side. Less than 10% of all studies were ex-post policy evaluations. Our results demonstrate that technology-oriented interventions in urban waste, transport and energy sectors have the highest marginal abatement potential, while system-wide interventions, e.g. urban form related measures have lower marginal abatement potential but wider scope. We also demonstrate that integrating measures across urban sectors realizes synergies in GHG emission reductions. Our results reveal a rich evidence of techno-policy choices that together enlarge the urban solutions space and augment actions currently considered in global assessments of climate mitigation.

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1. Introduction: summary of evidence gap and research question

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The role of urban areas in contributing to climate mitigation and adaptation, global sustainable development goals (SDG) and the New Urban Agenda (NUA) is undisputed (UN Habitat 2011 , IPCC 2014 , 2018 , UN-United Nations 2015 , 2016 ). In the last few decades, a growing number of cities and local governments have teamed-up to combat climate change from the bottom up: hundreds have developed and are implementing local climate action plans (UN Habitat 2015 , Deetjen et al 2018 , Reckien et al 2018 , C40 Cities 2019 , WRI 2019 ). Yet, the contribution of urban climate solutions to climate change mitigation remains poorly understood: we still have very little understanding of how well urban policy interventions work, under what conditions and why (Grubler et al 2012 , Revi et al 2014 , Seto et al 2014 , Rosenzweig et al 2015 , Minx et al 2017 , Bai et al 2018 ). The urban climate change literature is paved with individual case studies challenging our ability to maintain an oversight of new developments (Lamb et al 2019 ). Case-study evidence is used primarily in an anecdotal fashion leaving a large, untapped potential for systematic learning on urban climate solutions. There are at least two paths to upscale and systematize the study of urban-scale climate solutions (Creutzig et al 2019 ). One is data driven starting with city-scale datasets being combined with harmonized remote sensing or other land-use information to develop data-based typologies of cities and climate change (e.g. Creutzig et al 2015 , Baiocchi et al 2015 , Ahmed et al 2019 , Nangini et al 2019 , Solecki et al 2015 ). The other is evidence-driven synthesis starting with case studies to systematically compare and aggregate policy insights (Broto and Bulkeley 2013 , Kivimaa et al 2015 , Reckien et al 2018 ). Both can be eventually combined to match experience from case studies to urban drivers of energy use and climate change (Lamb et al 2019 , Creutzig et al 2019 ).

What useful information can one derive from urban case studies? A systematic scoping of these studies can reveal a spectrum of urban solutions available to policy makers—instruments, targeted sectors, expected (or documented) mitigation potentials, and social outcomes. This information could support fast learning among peer-cities particularly those responsible for large segments of global greenhouse gas (GHG) emissions (Creutzig et al 2015 , Baiocchi et al 2015 , Lamb et al 2018 ). In particular, there is a pressing need to identify solutions for smaller and medium sized cities, that too in developing countries. These cities will host the majority of future population growth, energy consumption and GHG emissions yet are most underequipped in financial and human resources to study and implement local climate action (GEA 2012 , Seto et al 2014 , Sethi and Puppim de Oliveira 2015 ).

A growing number of studies model climate mitigation potential in cities. Emission inventory exercises identify key priority areas for urban mitigation across multiple sectors—particularly when carried out in a comparative context (e.g. ICLEI 2009 , Kennedy et al 2009 , Chavez and Ramaswami 2013 ). For mid- and large- n samples of city inventories, parametric and non-parametric statistical approaches explain variations in urban CO 2 /GHG emissions due to socio-demographics, industrial structure, urban form, local geography and climatic conditions (Brown et al 2008 , Glaeser and Kahn 2010, Minx et al 2013 , Baiocchi et al 2015 ). Further refining such analysis, Creutzig et al ( 2015 ) use hierarchical regression-tree to endogenously cluster cities according to their GHG emission drivers and to estimate a global urban mitigation wedge. These studies are assimilative explorations into key drivers and thus potential areas to focus mitigation initiatives on, but they do not identify city-specific policy options that are directly available to urban policymakers.

Other studies have studied the ambition, focus, and regional distribution of urban climate actions (Broto and Bulkeley 2013 , Reckien et al 2014 , 2018 ). Yet, these studies abstract away from specific options and fall short of evaluating actual policy performance. As such, policy learning remains limited and insights are not actionable. This contrasts with a wealth of urban mitigation case studies available in the scientific literature (Lamb et al 2019 ) that offers the opportunity to systematically review this more granular evidence base and learn from experiences in pursuing technological solutions and urban policy instruments. We acknowledge the difficulties of such an undertaking, with inherent inconsistencies in methods, system boundaries, available data and desired outputs (Seto et al 2014 , Sethi 2017 ). Yet, in the absence of comprehensive and consistent evidence, working towards an initial heuristic for the urban climate solution space is a justifiable goal.

In this research, we apply a systematic scoping review methodology. A scoping review is guided by principles of transparency and reproducibility that follow a clear methodological protocol to analyse quantitative, qualitative or mixed evidence found in the scientific literature (Arksey and O'Malley 2005 ). As in other systematic evidence-synthesis approaches, it involves the following steps: (a) clearly defining the research question; (b) systematically searching defined literature databases for a defined time period; (c) justifying and making a transparent selection of the literature; (d) assessing the quality of the selected evidence; and (e) synthesizing the evidence based on a clear and transparent method (Berrang-Ford et al 2015 , Minx et al 2017 ). In this scoping review, we assess the urban case study literature pursuing four distinct, but inter-related research objectives: (1) to map global urban interventions, capturing the contributions across different mitigation sub-sectors, (2) to survey key urban mitigation solutions being practiced along with their GHG abatement potential, (3) to examine ex-post policy studies for specificity of opted policies and their governing mode, and (4) to capture trends and focus of the latest research and innovations in urban climate mitigation. In section 2 , we outline our review methodology, and in section 3 we describe analytical findings. In section 4 , we conclude with recommendations for future research.

2. Methodology

Climate change assessments, such as those by the Intergovernmental Panel on Climate Change (IPCC), gained status for evidence-based scientific policy advice. The progress in international climate governance would have not been possible without systematic learning in the scientific community. However, there has been little systematic learning on climate solutions from ex-post evidence. Systematic review methods as developed in health and educational sciences provide an adequate methodological toolkit for such learning, but have generally been neglected in climate and energy research. Only recently, a growing number of researchers have started applying systematic review methods in climate studies more widely (Berrang-Ford et al 2015 , Fuss et al 2018 , Minx et al 2018 , Nemet et al 2018 ). Such systematic reviews are challenging in that they deal with the vast and fast-growing evidence base. We call this new phenomenon 'big literature': resource-intensive systematic review methods are pushed to the brinks of feasibility (Minx et al 2017 ). Employing data science methods to assist during the systematic review process by lifting the burden of some of the most repetitive and resource-intensive tasks from human reviewers is a promising and crucial development in the field of evidence synthesis (Minx et al 2017 , Westgate et al 2018 , Nakagawa et al 2019 ).

In a recent experiment, Lamb et al ( 2019 ) apply data science and unsupervised machine learning (ML) methods to automatically map out the case study landscape on urban climate change mitigation. Rather than dozens or hundreds of case studies as analysed by urban climate change assessments (Seto et al 2014 ), it identifies more than 4000 cases, covering a broad range of topics from emission accounting to technology studies to scenario analysis to policy impact evaluations. We update this with an expanded database of 20 166 studies for our systematic review of technology and policy options in urban climate change mitigation. The detailed methodology for the review process is explained in annex 1 (available online at stacks.iop.org/ERL/15/094067/mmedia ) and summarized as a flowchart in figure 1 . As a first step, we search the Web of Science and Scopus with a broad query comprising synonyms for climate mitigation and urban policies (annex 1.1, table A1). We filter the resulting documents using a data bank of worldwide city names, resulting into 5635 case studies that mention cities in their title and abstracts. Next, we read a random sample of 250 papers to develop inclusion and exclusion criteria for our scoping review (annex 1.2, table A2). With the developed exclusion and inclusion criteria we then tested inclusion/exclusion for a further set of 200 papers (annex 1.2, table A3). We then used the coded papers as input for a supervised machine learning algorithm that calculates relevance rates for the remaining 5635 case studies. For the final review, we include all studies with a relevance rate of 0.6 or higher, resulting in 867 papers (annex 1.3). In the final stage, post-ML analysis and synthesis involves systematic coding and tagging of the content (annex 1.4) finding that 644 out of the 867 studies matched our inclusion criteria followed by an array of results. Section 3 reports our analytical findings sequentially for each research objective.

Figure 1.  The research methodology, scoping of case studies (in numbers) and results reporting for each objective.

3. Results discussion

The systematic review of case study literature leads to the following major outcomes: ( 3.1 ) Mapping of urban interventions globally, capturing the contribution of different mitigation sectors; ( 3.2 ) Exploring key urban mitigation solutions being practiced along with their GHG abatement potential; ( 3.3 ) Examining ex-post policy studies for specificity of policy mode opted in different urban mitigation solutions; ( 3.4 ) Identifying the focus of recent trends and innovations in urban climate mitigation.

3.1. Mapping of literature for GHG mitigation sectors

We map case study articles to the following sectors: buildings, energy, transport, waste; agriculture, forestry and other land uses (AFOLU) and industry. In the paper set, we find 548 studies focusing on a single sector, with buildings (249) and transport (148) most frequently investigated. There are 77 studies scrutinizing two sectors simulteneously, most often building and energy (21). A total of 19 studies cover 3 or more sectors (figure 1 , details in annex 2). Visualizing the intensity pattern of sectoral-interactions using a chord diagram (figure 2 - left) reveals a notable paucity of evidence observed between buildings–waste, industry- AFOLU/land, industry-waste and transport-industry sectors. Systematic reviews can be prone to inaccuracy in reporting if left unchecked for consistency of results. We hence hand-checked the relevance of post-ML results and validated these against the tested precision level (annex 3).

Figure 2.  Distribution of urban climate solutions- all (left) & ex-post policy (right), across different sectors. Source: Authors. The graphic demonstrates that volume and cross-connectivity between sectors for ex-post policy studies is significantly less in proportion to the entire urban climate solutions literature.

3.2. Key urban mitigation solutions and their mitigation potential

An in-depth analysis of 644 studies reveals several urban mitigation solutions evident across the six GHG mitigation sectors (figure 1 ). The most frequently identified interventions are demand-side management solutions that includes peak shaving or shifting (65 evidences), followed by energy efficiency (EE) measures (52), retrofitting a building completely (43), installing solar PV and PVT (43), integrated planning (41), fuel or technology shift (33), car free city (33), electric mobility (31), travel demand management (TDM) measures (26), biomass, bio-diesel, biomass gasification, ethanol production (26), waste to energy solutions (25), thermal insulation (23), public transport (22), urban form, design and planning (21), cool roofs (19), life-cycle assessments (19) and transit oriented development (18). Intrestingly, relatively ordinary, inexpensive and low-tech solutions e.g. water system efficiencies (12), mixed landuse (9), composting (7), walkability (5), non-motorised transport (4), greening/afforestation (4), parking management (3), and urban agriculture (2) find little evidence in case study literature.

Only 88 case studies across 46 solution typolgies provide quantitative data to estimate GHG abatement potential. Some cities like Vancouver, New York, Toronto, San Fransciso, London, Barcelona, Turin, Beijing, Tokyo, etc provide multiple studies in urban mitigation. These report results either as GHG mitigation, energy reduction or cost savings (essentially GHG abatement potential), all in percentage-points and ennumerated in annex 4. This enables us to evaluate relative opportunities offerred by these solutions. Out of 46 urban mitigation solutions, quantifiable data was available for only 41. We rank demand-side potential for climate change mitigation (figure 3 ), benchmarked against business-as-usual scenario (BAU) as defined in each individual study. Different studies can have a range of baselines and end-points for reporting percentage GHG reductions (Erickson and Broekhoff 2017 ). In this analysis, we consider studies with baseline ranging from 1979 to 2019 and report results on the basis of project vs non-project percentage variation, drawing inter-sectoral comparisons as has been the method in the recent AR5 (IPCC 2014 , p 92). Several variables contextualize the results:

• (a)   Geographical origin: Many studies originated in Europe and China (annex 5). The most notable heterogeneity is evident in cool-roof performance because of location. Cool roofs are less often deployed in higher latitudes (13%) and more frequently in lower latitudes (28%). Other climate policies are deployed similarly across geographic regions.
• (b)   Sector contributions: On an average, the highest mitigation values are observed in three chief sectors: buildings with net-zero emission buildings (NZEB) 105% (Wilkinson and Boehm 2005 ), transportation with E-mobility 94% (Van Duin, et al 2013, Prata et al 2015) & Waste with waste to energy 87% (Mustafa et al 2013 ). While a combination of certain sectors may give relatively moderate to low GHG abatement, in certain urban contexts these can potentially offer low-hanging fruits for decision makers and local bodies to deliberate on, especially if the scope and scale are controlled (explained below #5).
• (c)   Heterogeneity within an urban solution: The 41 urban solutions are categorized based on their techno-policy distinctiveness to be implemented as practical solutions within an urban setting. A particular solution could invariably include multiple sub-variants of technology. For example, 'electric mobility' includes studies examining full replacement of cars by e-bikes, replacement of individual conventional cars with light duty electric vehicles (EVs), and adoption of EVs in freight—all of which are different in terms of technology and their efficacy. To reflect policy relevance, we have segregated their use in public and private modes. For details see annex 4.
• (d)   Technological and social initiatives : Technology based interventions are more prevalent. Out of 41 urban climate solutions, 33 (80.5%) are technologically driven, 3 (7.31%) are society oriented and the remaining 5 interventions are technological and social. More than two thirds of solutions are on the demand-side, and less than one third on the supply side (figure 3 ).
• (e)   Scope (system boundaries) & Scalar effect : The scale of intervention controls mitigation potential at two levels. The first is at the systems level- with all things being equal, interventions with smaller absolute scope demonstrate greater marginal mitigation potential than more system-wide interventions. For instance, NZEB (105%) against retrofitting a precinct (50%) or sustainable urban form (23%) in the city. Similarly, waste to energy in a plant (87%) against integrated waste management (58%) in the city, or mitigation opportunities in public transportation system (51%) against complete TDM (27%) in the city. This information is crucial for local authorities and stakeholders to initiate policy action with small-scale manageable projects having high marginal impact, while simultaneously pursuing more system-wide approaches.

Figure 3.  Rank Diagram of average GHG abatement potential from BAU (in %) and categorization into technological and social-oriented solutions, and demand vs supply categories.

A further disaggregation of sectors for quantifiable solutions, in decreasing order of their average mitigation potential, indicates the following-

Waste (50%): The GHG abatement potential of 5 climate solutions in waste sector range from biomass, biomass gasification (21%) to waste to energy (87%) in project versus non-project scenario. The surge in climate mitigation potential with rising up the technology ladder are evident. On an average, the waste sector offers the maximum demand side GHG mitigation potential in cities with the most concentrated yet least number of measures, thus offering a low-hanging fruit to urban local bodies.

Transport (43%): The mitigation potential of 8 climate solutions in energy sector range from intelligent transportation system (ITS) (20%) to EV and hybrid EV (HEV) in public & private vehicles (94%), with the average being 43%. The results indicate that GHG savings from travel demand management, fuel shift and ITS plateau at 28%, beyond which deep mitigation can be attained only through pan-city expansion of public transportation system, particularly by introducing EV/HEVs. Most of these interventions are supply-driven and controlled by urban transport authorities and local governments.

Energy (38%): The mitigation potential of 14 climate solutions in energy sector range from expanding district heating/cooling (12%) to PV thermal and solar tri-generation (CPVT) solution (73%), with the average being 38%. The energy sector demonstrates a range of solutions, a lot of which are associated with the supply-side than in any other sector. These are district heating/cooling, PV thermal, solar tri-generation CPVT, etc though few demand side energy measure to reduce GHGs are also observed, like EE & conservation measures, consumer demand response models, optimization models in modulating energy consumption at local (community) level, in water systems, heat pumps, street-lighting optimizing energy demand with solar substitution and/or energy storage, demand adjustment for district heating, etc.

Buildings (35%): The relative mitigation potential of 13 climate solutions in building sector ranges from cool roof/facade, roof garden in higher latitudes (13%) to NZEB, carbon neutral building (105%), averaging 35% with all other variables being the same. Excluding NZEB, the mitigation potential in this subsector limits at 50% with building retrofit. The double savings in NZEB against retrofit signifies substantial untapped mitigation potential in creating new infrastructure or redeveloping old precincts to NZEB district than pursuing incremental retrofits.

Conventional insulation and thermal comfort solutions incorporated into the building during construction are twice more effective in reducing energy demand than operational/performance measures like automated building information system (BIS), intelligent controls, smart meters, etc or user driven EE measures. At the same time, urban bodies need to utilize these results with prudence. They should keep in view that the gross mitigation potential of NZEB versus retrofits would depend on multiple local factors, for instance (a) the relative prevalence of new buildings vs. old building stock; (b) how you locally define or interpret 'retrofitting'; and (c) relative cost-effectiveness of each solution, amongst others.

AFOLU (5.2%): There is o nly one urban solution- afforestation/greening with a mitigation potential of around 5%.

3.3. Review of ex-post policy studies

One of the key aims of this research is to examine ex-post policy studies for specificity of policy-governance instruments opted in different urban mitigation solutions. Firstly, only 73 (8.5%) out of 867 cases are ex-post policy studies, the most abundant in the buildings sector (26), followed by transport (16), energy (7), waste (5) and AFOLU/land (2). As the chord diagram of these evidences show (figure 2 - right), there are few cases observed in the nexus of buildings-energy (5), buildings and AFOLU/land (4), transport and AFOLU/land (2) while only seven urban solutions span through multiple sectors. The results suggest that the industry and waste sectors are most isolated and need integration with the rest of urban functions through innovations and policy convergence, to accrue greater GHG mitigation and climate co-benefits. Upon tagging these cases in accordance to four normative policy-governance modes, including overlapping (annex 6), we find that most of the urban solutions conform to enabling measures (46), regulatory instruments (45), voluntary, behavioural, awareness & education measures (37), followed by market/economic interventions (35). The following key observations emerge:

• (a)   There is a pre-occupancy of regulatory instruments that rely on legislations, standards/codes, certifications, etc across almost all GHG mitigation sectors, frequently observed in buildings and transport sector.
• (b)   Enabling and voluntary measures are not at all observed in waste sector, substantiating its isolation in urban GHG mitigation.
• (c)   There are only two evidences where all policy instruments are simultaneously employed in urban climate solutions. The case of Toronto highlights a mix-methods approach combining infrastructure provision, public acceptance, industry participation, regulating gasoline prices, tax incentives, subsidies for expanding EVs (Ing 2011 ). Also, local authorities like Leicester demonstrate different stakeholders can use multiple benefits approach with energy savings, job creation and community engagement to proactively meet national carbon reduction targets (Lemon et al 2015 ).

Surprisingly, initiatives such as car free cities, Fridays for future, odd–even car days, and congestion charges that capture active public interest, participation and media attention are missing in peer-reviewed scientific literature that we sampled. That contrasts with the high potential of transport-related lifestyle solutions to reduce individual carbon footprints (Ivanova et al 2020 ). A key reason might be that many urban-scale transport policies are primarily motivated by local concerns, such as congestion, air pollution, and quality of life, and thus may not occur in our literature data base. Secondly, policies need to be evaluated in terms of their relative effectiveness. For instance, in building projects, total renovation may not be optimal in all cases, while zero-cost measures like information campaigns could produce significant performance improvement (Pombo et al 2019, Calero et al 2018 ). Similarly, a better knowledge & propagation of energy codes, especially during early planning stage can have positive implications (Andrews et al 2016 ). Other effective intervention in the building sector include cap and trade for building energy emissions on lines of Tokyo (Nishida et al 2016 ), provision of incentives & subsidies, sustainable business models, focusing on thermal envelope system (than merely lighting); capacity building efforts and education campaigns, using of land-use planning and development approvals to expand green building market (Hou et al 2016 , Mellross and Fraser 2012 ).

For urban energy solutions, supply-side policies can be more effective if accompanied by participatory measures, as in Leicester, UK that simultaneously created jobs and engage communities in demand optimization (Lemon et al 2015 ). While technological solutions like solar PV accrue 30% cost savings (VanGeet et al 2008 ), yet complementary smart tariffication measures provide monetary incentives to households and motivate them to change their energy habits too (Kendel and Lazaric 2015 ). This certainly necessitates a greater role for enabling and voluntary modes of governance in supporting regulatory measures in urban mitigation.

Ex-post policies in the transportation sector further validate that demand-side interventions are indispensable for urban climate mitigation. For e.g. despite provisioning EV fleet in public transport in Jinhua, there is still an energy saving potential of 6.21% by optimizing departure time interval in line 1, with the same level of service (Wang et al 2017 ). Area traffic and parking restrictions with fuel taxation have substantial benefits in Athens (Goulas et al 2001 ), despite prevailing supply-side fuel efficiency norms. For significant outcomes, municipal plans should provide facilities that encourage increased use of transportation alternatives (walking, cycling, transit, etc.), that promote efficient vehicles and fuels (City of Vancouver 2005), enable enhanced involvement of people towards sustainable travel behaviour change as evident through holding the Big Green Commuter Challenge in Portsmouth City (Wall et al 2017 ), greater public transit use as in the case of Halifax, Moncton and Saint John in Canada (Gordon 2004 ), and adoption of smart parking system in London (Peng et al 2017 b).

Ex-post policy evaluation in AFOLU/land demonstrates that expanding of park area was the most appropriate initiative when considering both its effectiveness in reducing emissions, and its implementation cost in Bangkok (Kiewchaum et al 2017 ). In case of waste sector, different local conditions and waste composition were known to influence the choice of solution- landfill, incineration and composting (Assamoi and Lawryshyn 2012 , Hutton et al 2013 ). Urban mitigation across multiple sectors is scarce and spans across buildings, energy & transport. It involves technologies used for demand-side management that include natural-gas-based residential and commercial building heat pumps and chillers, cooking and water heating appliances developed for restaurant applications, and automobiles, buses, and trucks that use natural gas instead of gasoline (Wang et al 1995 ). These predominantly hinge on fuel-shift based rapid efforts or building-retrofit related sustained efforts, yet with significant health impacts (Tuomisto et al 2015 ).

In addition to the above cited evidence, there are certain plausible cross-cutting mitigation interventions viz. (a) Municipal waste-industry: demonstration of circular economy, biogas digestion, biomethanation & CO 2 e certificates, (b) AFOLU/land and waste: landfill site restoration to expand green cover and GHG mitigation, (c) Energy-transport: Instruments for bulk-purchase of green energy by transport companies, (d) Building-energy: Power purchase agreements between renewable energy plants, regional power grid, local electricity distribution companies on one end and townships, special zones, municipal councils, residential communities including prosumers on the other, and last but not the least (e) Ecocity/smart city developments: Integrated planned solutions encompassing solar PV, building EE measures, E-mobility, WTE and/or other combinations of the above interventions.

3.4. Recent trends, focus and innovations in urban mitigation

The past few years have witnessed advances in models, technologies and policies for climate urban mitigation, spanning all major GHG sectors except industry and AFOLU/land (table 1 ). Innovations advanced in the building sector (real time BIS, smart controls, roof-integrated solar technologies, efficient cooling & heating), albeit there is little evidence in their policy application. There is considerable use of technology in urban energy through heat pumps, solar PV, energy storage solutions, biomass gasification and energy-recovery demonstrator in district-heating. Meanwhile, policy innovations utilize community energy plans for utility-scale wind turbines, hybrid renewables and measures to re-evaluate national and local energy-efficiency design standards. There is fair mix of research, technological solutions and policy application evident in the transportation too. Complex computational models are built into apps, storage devices, breaking energy, and hybrid-fuel platforms supported by policy studies that optimize travel demand, improve street design, and strategize integrated and low-carbon transport planning. Other than EV/HEV, most of these measures enable incremental changes with no significant breakthrough from the status quo. The urban waste sector is well posited with research on life cycle assessment (LCA) of waste materials, analyzing optimal mix of different treatment and disposal technologies as well WTE applications. In addition, cross-sectoral interventions are experimenting with modelling & technologies in building-energy sectors (smart cities, green districts), land-transport related emissions as well as mechanisms to integrate climate goals with city master plans and setting up of demonstration projects. However, the cross-sectoral projects are few and need up-scaling to include non-contributing sectors. Also, policy innovations in unique sectors require expansion to apply models and technologies showing positive results for GHG mitigation.

Table 1.  Recent research in urban innovative models, technologies & policy solutions. For abbreviations, see notes at the bottom.

Notes: CO 2 : Carbon dioxide, DH: District heating, EV: Electric vehicles, GIS: Geographic information system, ITS: Intelligent transportation system, LU: landuse, RE: Renewable energy, SWM: Solid waste management, WTE: Waste to energy. For detailed tabulation see annex 7

4. Conclusion and recommendations

4.1. urban mitigation overwhelmingly presents demand-side solutions, yet it is still unsaturated.

Out of 41 quantifiable urban solutions, 33 (80.5%) exhibit demand-side interventions. Our findings support the prevailing literature (Lamb et al 2018 , Creutzig et al 2019 ) that topics like TDM, BEE, urban form, waste management dominate urban climate landscape, with irrefutably measureable evidence on the available mitigation choices and their relative efficacy. Our research pin-points technological and policy options and their GHG potential. For instance, most ex-post policy studies and current experiments are concentrated in the buildings and transport sectors, followed by energy and waste. The dearth of evidence in carbon sequestration initiatives indicates that (a) this topic is understudied in the urban literature, and/or that (b) cities designate insufficient importance to urban greening. However, most 'forward looking' studies (with futuristic scenarios) primarily deal with supply-side technologies in energy, CO 2 emission accounting, transportation and air-pollution (Lamb et al 2018 ). Thus advancing research should focus more on unexplored demand optimizing technologies and policies in urban industries, land and other cross-sectoral activities.

4.2. In urban climate literature, mitigation options that are frequency studied are not necessarily those with highest potential

The study of how literature is organized is important but the first step in any systematic review process. An earlier study, Lamb et al ( 2018 ) uses frequency mapping, topical modelling, clustering and bibliographic coupling networks to check representation of topics across sectors, time and geographical space. Yet, the meta-analysis of empirical and quantifiable data from global case studies is imperative for an informed decision-making. Comparing the results of both highlights some vital differences in representation and efficacy of urban climate solutions worldwide (figure 4 ). This bears vital inputs to agencies like IPCC, IEA/OECD that are now heavily relying on methodical reviews of case study literature for Assessment Report (AR6), Global Energy Outlooks, etc that (a) urban climate literature is still inadequately represented by technological and policy interventions that can effectively reduce or offset GHGs, and (b) climate solutions less frequently investigated in urban case studies (such as EVs that are typically addressed at national level transport policy) may nonetheless help cities to effectively address the 1.5 °C climate challenge.

Figure 4.  Comparison of key urban interventions for their marginal topic distribution results against their GHG abatement potential, in global case study literature. It is notable that solutions with high topical modelling (4.5%–9.7%) in literature like planning and governance, energy efficiency in buildings, travel demand management, urban form show a relatively lower demand-sensitivity (23%–27%) towards climate mitigation. Contrarily, solutions with lower topical representation (2.7%–4.4%) like integrated waste management, public transport, thermally insulated buildings, electric vehicles, transport provisioning show relatively higher climate mitigation potentials in the range of 47%–62%.

4.3. Technology coupling and synergistic interventions can upscale urban mitigation

Disruptive and synergetic technologies demonstrate that when it comes to GHG mitigation potential, the whole is greater than the sum of its parts (see figure 5 ). Out of 41 urban climate solutions, 33 (80.5%) are technologically driven, 3 (7.31%) are social oriented and the remaining 5 are both technology and social interventions. The results are in line with a previous study (Broto and Bulkeley 2013 ), wherein 76% of all experiments were technological, while only 50% were social innovation. The small proportion of social initiatives in our study partially reflect our exclusion of adaptation measures (Broto and Bulkeley 2013 observed that social innovations are more common in adaptation, carbon sequestration and urban form initiatives). Nevertheless, the significance of combining technological and social experiments to expand and upscale mitigation efforts in urban systems cannot be ruled out.

Figure 5.  The resultant mitigation potential (in %) of combining two urban interventions is greater than the net sum of individual interventions. District heating/cooling expansion provides 12% mitigation potential and PV Solar technology yields 14%, their coupling has steeper mitigation cut (37%). Similarly, buildings energy efficiency measures (23%) and PV Solar (14%) in tandem yield 65% mitigation potential. In transportation, expanding of existing technologies in public transport (62%) & electric mobility in private vehicles (39%) if combined, multiply benefits to 94%.

4.4. Expanding extra-regulatory and non-governmental actions is imperative in local climate governance

Technology is necessary but not a sufficient element to deepen urban mitigation. The overview of policy-governance measures corroborates an active role of local governments in leading urban climate solutions, through regulations (60.8%) or through partnerships between private or non-governmental actors (62%). A comparison with Broto and Bulkeley (Broto and Bulkeley 2013 ) shows that there is a slight shift observed in these last few years, when local governments' self-regulatory actions (66%) significantly outscored enabling measures (47%). This highlights an expanding form of horizontal partnerships by city governments with local agencies during 2013–19. The review of ex-post studies underscores that a mix of market mechanisms, user incentives, subsidies, voluntary measures, etc in cooperation with non-governmental actors is crucial for local climate mitigation.

Our scoping review is limited in certain ways. For example, it encounters a lot of heterogeneity and variabilities in cases. The unavailability of consistent data makes it hard to account for costs of mitigation options. We expect costs to vary with situations, depending on availability and price of the concerned resources & technologies, their in-direct costs, socio-economic costs, trade-offs, etc. The above nuances need further exploration through a full-systematic review and ought to be reasonably assessed while applying results to develop concerted urban policies and projects in different country & local contexts. Nevertheless, the research findings bear significant implication in analysing the efficacy of diverse demand-side climate solutions that will have a special reporting in the next IPCC report (AR6). In addition, the following insights or takeaways would help evolve a scientific and evidence based sustainable urbanism envisioned under the global SDGs & NUA, particularly for societies with rapidly developing infrastructures:

• (a)   Epistemological: Urban climate research requires a fusion of disciplinary knowledge in order to better utilize quantitative and qualitative data on how cities could respond to global warming. This investigation demonstrates how systematic learning from ongoing research and ex-post studies is still underutilized. With growing urban data collection across the globe (for more see Creutzig et al 2019 ), our study further affirms the significance of investing and learning through systematic review of case study and ex-post evidence by exploiting big literature and machine learning.
• (b)   Innovating technologies: Equipped with the above theoretical advances, urban innovation labs need to focus towards fostering ideas and technologies that not only enhance EE, fuel shift, etc (incremental solutions), in addition to innovating decarbonisation solutions exhibiting high effectiveness (like NZEB, EV, TDM) fully recognizing the obvious and the most infrequent linkages in urban systems. This aligns with the resilient urban systems discourse where all interventions (incremental, transitional and transformational) are considered equally applicable (Pearson et al 2014 , Chelleri et al 2015 , Meerow et al 2016 ). Our research underscores that energy-transport (e-mobility by prosumers and energy savings from breaking), transport-waste (route planning etc.), industry-waste (waste minimization, recycling), industry-AFOLU (mandatory sequestration, cap & trade, carbon capture) are inter-linkages that future innovations and research funding should explore.
• (c)   Policy design: Our review demonstrates significant GHG mitigation from cross-sectoral policy initiatives. For example, projects generating rooftop solar energy for consumption in green homes as well as both public and private EVs (Sugar and Kennedy 2012 , Litjens et al 2018 ). Similarly, integrated waste management demonstrates both GHG mitigation from households and industries, as well as lowered local air-pollution at the city scale (Liamsanguan and Gheewala 2008 ). Urban strategies need to develop NZEB and retrofits in mix-land use districts where non-motorised mobility can generate multiple benefits of climate mitigation, ordered urban-planning, improved local environment and jobs. The good news is that most of these portfolios are a mandate of urban local governments. The mainstreaming of climate change into national policies (UN Habitat and UNESCAP 2018 ), particularly national-urban frameworks (Sethi and de Oliveira 2018 ) present a good opportunity for systematic analysis of sustainability and local development issues in developing context. It can guide urban agencies for the necessary capacity building, financial support, performance targets, peer-competition and opportunities to be seen as acting decisively on environmental challenges.
• (d)   Societal applications: Soft infrastructures, such as habits and norms, shape behavior and consequently offer potential to reduce energy demand and GHGs (Hammer et al 2011 , Boyd and Juhola 2015 , Creutzig et al 2016 ). Global energy optimization models show that advanced technologies, behavioural changes and shared socioeconomic pathways, enable rapid and deep decarbonisation to limit global warming to below 2 °C whilst at the same time reducing reliance on negative emissions technologies (NETs) by up to ∼18% compared standard set of technologies (Riahi et al 2017 , Napp et al 2019 ). Grubler et al ( 2018 ) find that low-energy demand scenarios can further obliviate NETs within the 1.5 °C target. Our research demonstrates that multi-pronged policy solutions like incentives, subsidies, voluntary and behavioural measures, conscious choice of selecting low-energy consumption solutions and greener lifestyles like purchasing e-vehicles, travelling by public transport, garden composting, EE measures at home and office can play a pivotal role at urban scale supporting the realization of such scenarios. Cities can thus make socio-technical transitions required for climate stabilization.

Acknowledgments

The views presented by authors are independent without any influence or conflict of interest. The first author acknowledges the Alexander von Humboldt Foundation for the research fellowship. The internal review platform- APSIS scoping software is provisioned by Mercator Research Institute on Global Commons and Climate Change, Germany. Thanks is due to Max Callaghan for orientation to and troubleshooting in APSIS operations. Chord diagrams are prepared using open access tool on https://sites.google.com/site/e90e50fx/home/talent-traffic-chart-with-chord-diagram-in-excel .

Data availability statement

All data that support the findings of this study are included within the article (and any supplementary information files).

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Analysis of the Emergent Climate Change Mitigation Technologies

A climate change mitigation refers to efforts to reduce or prevent emission of greenhouse gases. Mitigation can mean using new technologies and renewable energies, making older equipment more energy efficient, or changing management practices or consumer behavior. The mitigation technologies are able to reduce or absorb the greenhouse gases (GHG) and, in particular, the CO 2 present in the atmosphere. The CO 2 is a persistent atmospheric gas. It seems increasingly likely that concentrations of CO 2 and other greenhouse gases in the atmosphere will overshoot the 450 ppm CO 2 target, widely seen as the upper limit of concentrations consistent with limiting the increase in global mean temperature from pre-industrial levels to around 2 °C. In order to stay well below to the 2 °C temperature thus compared to the pre-industrial level as required to the Paris Agreement it is necessary that in the future we will obtain a low (or better zero) emissions and it is also necessary that we will absorb a quantity of CO 2 from the atmosphere, by 2070, equal to 10 Gt/y. In order to obtain this last point, so in order to absorb an amount of CO 2 equal to about 10 Gt/y, it is necessary the implementation of the negative emission technologies. The negative emission technologies are technologies able to absorb the CO 2 from the atmosphere. The aim of this work is to perform a detailed overview of the main mitigation technologies possibilities currently developed and, in particular, an analysis of an emergent negative emission technology: the microalgae massive cultivation for CO 2 biofixation.

1. Introduction and State of the Art

Climatology with the terms climate change or climate change refers to changes in the Earth’s climate, i.e., variations at different spatial scales (regional, continental, hemispherical, and global) and historical-temporal (decade, secular, millennial, and over-millennial) of one or more environmental and climatic parameters in their average values: temperatures (average, maximum, and minimum), precipitation, cloudiness, ocean temperatures, distribution, and development of plants and animals. Climate change is caused for the most part by greenhouse gas emissions.

Some gases present in the atmosphere, called greenhouse gases, are responsible for the greenhouse effect, which plays a fundamental role in the growth and development of life forms. Without serra gases, the earth would be frozen and lifeless. These Serra gases, although present in small quantities, favor the reflection towards the ground of IR rays (which determine the heating of surfaces) coming from the sun. The heat, therefore, remains stored in the atmosphere, resulting in a warming of the air and climate in a ratio “directly proportional” to the presence of Greenhouse gases. GHG can be of both natural and anthropic origin. The main greenhouse gases can have both origins: water vapor, carbon dioxide, nitrogen dioxide, and methane. There is also a wide range of greenhouse gases exclusively produced by human activity, such as alocarbons, the best known of which are chlorofluorocarbons, the emissions of which are regulated by the Montreal Protocol. [ 1 , 2 ]. Yue and Gao statistically analyzed global greenhouse gas emissions from natural systems and anthropogenic activities and concluded that the Earth’s natural system can be considered as self-balancing and that anthropogenic emissions add extra pressure to the Earth system [ 1 ].

In recent years, there has been a continuous energy consumption increase due to various activities. Reducing and decarbonizing energy consumption, especially with actions aimed at the most energy sectors (industry, buildings, and mobility) is, therefore, a way to go in order to reach “eco-sustainable” community [ 3 ]. Since 1870, more than 70% of anthropogenic greenhouse gas (GHG) emissions have resulted from the combustion of fossil fuels [ 4 ]. Constructing projections of future fossil fuel emissions for studies of future climate change is a challenging task. Workers in 19th-century mines could have scarcely imagined the technologies used by today’s coal industry. The same context is faced today when pondering an outlook for coal in the global energy system of the 21st-century [ 5 ]. Chakrabarty and Wang [ 6 ] highlighted that multinational enterprises must adapt their strategies to changes in the external business environment to perform environmentally, economically, and socially. This is because, on the one hand, their globalized activities across the world will have an important impact on the climate or on society [ 7 ]. Multinational enterprises can accelerate or slow sustainable development process both locally and globally by acting as one of the main actors in the international globalized economy [ 8 , 9 , 10 ]. Considering that cities are responsible for a large part of energy consumption (from which about 80% of carbon emissions derive) greening urban areas can also make a difference. In this case, it is necessary to intervene both in the reduction in emissions for energy consumption (due, for example, to domestic heating) and on emissions due to transport [ 11 ].

The decarbonization of development in the context of the Anthropocene was critically evaluated by Lugo–Morin D.R. [ 12 ] in a global scale. The study assess that the possibility of transitioning to a decarbonized global economy, or zero carbon emissions, is not encouraging indeed global energy production and carbon dioxide emissions are concentrated in a dozen countries and these nations are part of societies with an advanced social metabolism that negatively impacts the emission of CO 2 .

A new model for disaggregating the total observed changes into a number of source of change, thus opening to an estimated quantification of the effects originating from local policies alone was provided by Avezedo et al. [ 13 ]. Indeed, Avezedo et al. [ 13 ] remark that the evaluation of the effects of local actions for climate change mitigation is fundamental for the assessment of successful policies, but the existing methodologies do not bring to an single quantification of the effects of local policy actions. This happens because many causes of change in the local energy use and accountable emissions are not planned or controlled by the local authorities and national policies.

Considering the health risk point of view, Tong and Ebi [ 14 ] highlight that the global environmental changes are altering our planet in ways that be a model to current threats to human health, with the magnitude of these threats projected to grow over coming years if additional, proactive actions are not taken. The health risks of climate modification will become increasingly acute as climate change affects the quantity and quality of food and water, improves air pollution, changes the distribution of pathogens and disease transmission dynamics, and make less eco-physical buffering against extraordinary weather and climate events. Health systems urgently need to be upgraded to effectively address these emerging issues. In their study [ 14 ], the authors provide a global view of the health consequences of climate change, and discusses how health risks can be resized and avoided using mitigation and adaptation pathways.

Amelung et al. [ 15 ] describe effects of this type of health information on stated readiness to choose mitigation actions, as well as on simulation-based carbon emission reductions in a pre-recorded experimental setup among 308 households in 4 mid-size case-study cities in 4 European high-income countries: France, Germany, Norway, and Sweden. For every mitigation action from the food, housing, and mobility sectors, half of the sample received the amount of CO 2 equivalents (CO 2 -eq) saved and the financial costs or savings the corresponding action generated. The remaining half additionally obtained information on direct health co-benefits, if applicable. For households, obtaining information on direct health co-benefits, a higher mean willingness to adopt food and housing actions was found, and a higher proportion very willing to choose one or more mitigation actions; and an increased simulated reduction in overall carbon footprint: difference in percent reduction equals to −2.70%, overall and −4.45%, for food.

A heated debate is open about the relationship between primary pollutants (air pollution) and greenhouse gases (GHG): there seems to be a link between the two, but the scientific evidence is not yet sufficient to state this with certainty. It is certain that the problem of primary pollution (PM, NOx, SOx, etc.) must always rediscover a central role given the consequences associated with it. From the point of view of GHG modeling, which is used for the production of future scenarios, it should be highlighted that existing models are not always able to adequately represent them. One of the most consolidated data are that direct emissions of atmospheric pollutants, such as black carbon, or those of secondary origin, such as sulphates and ozone, influence the radiative balance and, therefore, the climate change. However, if reducing the emissions of black carbon and the concentration of ozone (through a decrease in its precursors both anthropogenic and natural) could lead to a decrease in global temperature, a reduction in others pollutants, such as sulfates, would not have the same effect: in fact, these have on the atmosphere a cooling effect due to their ability to reflect solar radiation. From the point of view of the influence on climate change, it should be stressed that the increase in the greenhouse gas concentrations modifies the radiative balance between the atmosphere and the Earth’s surface, leading to a change in environmental conditions, including the temperatures and the meteorological regime increase. These evidences can determine changes in the atmosphere chemical transformations and, therefore, in the chemical composition of the same atmosphere. In particular, an increase in the temperatures and irradiation conditions could increase the ozone and secondary pollutants concentrations. The most important GHG is carbon dioxide, which persists in the atmosphere for thousands of years. Other important GHGs are methane, nitrous oxide and fluorinated gases. The first legally binding global climate agreement adopted by 195 countries in Paris (COP 21) in December 2015 includes the goal of limiting global warming to a maximum of two degrees in the long term (Paris Agreement). This will not be easy to achieve unless there are major improvements, particularly in the field of energy efficiency, which we know to be one of the main causes of global warming (Bel and Joseph, in the press). The IPPC has also pointed out in its fifth assessment report that it is necessary to reduce the global GHG emissions by 40–70% from the 2010 level before 2050, and to reduce the global GHG emissions to the level of near zero by the end of the 21st century [ 2 ]. To achieve the objectives set by IPCC, the development and use of adequate climate change mitigation technologies will play a pivotal and indispensable role [ 15 , 16 , 17 ]. In order to obtain this result for the GHG (and, in particular, for the carbon dioxide) it is certainly necessary to study and analyze the so-called “negative emission technologies” (technologies that allow a carbon dioxide concentrations reduction). It is necessary to emphasize that, considering the strategies in order to limit the climate change and, at the same time, to improve the air quality, it is necessary to assess the impacts of both these phenomena and, therefore, seek to identify synergies (win–win) and avoiding solutions that improve one of the two phenomena and worsen the other one (win–lose) [ 18 ]. The aim of this work is the analysis of the climate change mitigation phenomena coupled with the “negative technologies” particularly focusing on microalgae biofixation.

Therefore, the methodological approach used in the drafting of this paper was as follows: after a careful analysis of the state of the art of the phenomenon and of the technology currently available (also highlighting the state of maturity of the same), the topic of micro algae biofixation as an emerging and promising technology in the field of climate change mitigation was examined.

2. Climate Change Mitigation

The main GHG are carbon dioxide, methane, nitrous oxide, and the fluorinated gases, such as hydrofluorocarbons, perfluorocarbons, and sulphur hexafluoride [ 19 ].

These are the gas more analyzed in the scientific literature and defined and treated in the Kyoto Protocol and in the Paris Agreement [ 19 ].

As reported in the scientific literature the GHG are emitted by many economic activities: in particular the power generation is responsible for about 26%; the industries sector is responsible for about 19%; the transports sector is responsible for about the 13%; and finally the deforestation and forest degradation sector are responsible for about the 17% [ 20 , 21 ].

In the year 2018, the total GHG emissions were equal to 55.3 GtCO 2 e. These data come from the report prepared in the year 2019 by the United Nations Environment Programme (UNEP) [ 19 ].

Overall, 37.5 GtCO 2 of the total 55.3 GtCO 2 e are attributed to fossil CO 2 emissions deriving in particular from energy and power generation and from the industrial sector [ 19 ].

It is possible to note an increase of 2% in the year 2018 in comparison to an annual increase of 1.5% relative to the last decade for both the global GHG emissions and for the fossil Carbon dioxide emissions. This last increase (the increase in the emissions of the fossil CO 2 ) is due, in particular, to the higher energy demand. The land-use change emissions amounted to 3.5 GtCO 2 in the year 2018. In this year (2018), the emissions due to the land-use change and to the fossil CO 2 accounted for approximately 74% of the total global greenhouse gas emissions.

A recent Intergovernmental Panel on Climate Change (IPCC) report reported that the anthropogenic activities have caused until now a global warming until the 1.0 °C. In the same report are reported that global warming is likely to reach 1.5 °C between 2030 and 2052 if the current emissions will be not cut in the next years [ 19 ].

The terms “adaptation” and “mitigation” are two important terms that are fundamental in the climate change debate.

Climate mitigation is any action taken to permanently eliminate or reduce the long–term risk and hazard of climate change to human life. The Intergovernmental Panel on Climate Change (IPCC) [ 22 ] defines mitigation as: “An anthropogenic intervention to reduce the sources or enhance the sinks of greenhouse gases” [ 22 ].

Climate adaptation refers to the ability of a system to adjust to climate change (including climate variability and extremes) to moderate potential damage [ 23 ]. The IPCC defines adaptation as the “adjustment in natural or human systems to a new or changing environment”.

Mitigation refers to actions or policies that both reduce emissions of greenhouse gases that can cause climate change, or that increase the climate system’s capacity to treat such gases directly from the atmosphere (e.g., reforestation). The main gases that actively contribute to climate change can be indicated as carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O). Some human activities, for example energy generation from burning fossil fuels and deforestation and agriculture, emit these gases and contribute to increase their concentrations in the atmosphere. The actual concentrations of these gases in the atmosphere have reached values never seen for some 800,000 years or more [ 24 ].

As energy has a fundamental importance in modern economies, CO 2 emissions have continued to increase rapidly in accordance with economic activity and population, despite international efforts under the UN Framework Convention on Climate Change (UNFCCC) to put under control the amount of CO 2 in the atmosphere. The presence of methane and N 2 O in the atmosphere are also increasing. It must be considered that there is also a wide range of other substances that are relevant greenhouse gases but are presently in the atmosphere at much lower concentrations [ 24 ].

3. Applied Technologies for Mitigation

In order to ensure proper protection of the environment, it is necessary to operate in two ways: by means of pollution prevention techniques and by means of control techniques. These two techniques can be operated separately but to obtain good results it is better if they are applied jointly [ 25 , 26 , 27 ].

Prevention technologies act upstream, that is, before the pollutant is formed and by means of appropriate measures aim at its non-formation [ 27 ]. Pollution control technologies (also called end of pipe technologies) act downstream of the process, that is, when the pollutant is now formed: these technologies provide specific techniques or processes for the removal of the pollutant generated [ 25 ].

Wang (2017) [ 18 ] propose to classify the environmental technologies into 5 types:

• Eco-efficiency technology, the aim of this technology is reducing pollution by reducing the amount of required power and material in input to the process maintaining the same level of production; The purpose can be achieved by installing more energy-efficient equipment, by modifying the process, etc. The main advantages are related to the environmental and economic benefits: in fact, thanks to the use of eco-efficient technologies, significant economic savings are possible;
• Higher costs of procuring or generating low carbon energy than conventional energy;
• Integrating low-carbon energy into existing energy supply systems can disrupt the current operational process.
• Green design technology, the aim of these kind of technologies is to reduce the pollutant contents of the products modifying, in particular, the design of the products (generally using more environmentally sustainable materials). The polluting contents of the product are usually measured on the basis of the life cycle analysis. Looking at technology from the demand side, “green” products are likely to become more attractive to environmentally conscious consumers. By analyzing the technology from the side of the company, we have that the design for regeneration or circulation can improve the recovery value of the products. Therefore, these technologies can have advantages both from an environmental point of view and from an economic point of view;
• Pollution control technology, these kind of technologies (call also end of pipe) aims to eliminate the pollution at the end of the process with appropriate instruments. Pollution control technology usually involves burning, recycling, filtering, and catalyzing pollutants. Typical pollution control technologies include electro-filters, bag filters, scrubbers (dry and wet), use of activated carbon, SNCR, or SCR systems [ 25 , 26 ];
• Management system technology, these kind of technologies try to reduce pollutants upstream, before their formation [ 27 ] adapting the way operations are handled. This is generally achieved through monitoring, reporting of pollution events and through employee training programs to raise awareness of climate change issues.
• Fawzy et al. [ 19 ] state that there are three main ways to mitigate climate change:
• Use of de-carbonization technologies and techniques in order to reduce CO 2 emissions. These include the use of renewable energy instead of fossil fuels, the use of nuclear energy, the storage and use of carbon capture. These are, in the first case, well-established technologies;
• Use of so-called negative emissions technologies. These are recent technologies, which have not yet been studied in detail. These technologies are able to absorb CO 2 present in the atmosphere. These include bioenergy carbon capture and storage, biochar, enhanced weathering, direct air carbon capture and storage, ocean fertilization [ 28 ];
• Technologies based on the principle of altering the balance of terrestrial radiation through the management of solar and terrestrial radiation. Such techniques are deducted forced radiative geoengineering technologies, and the main objective is stabilization or temperature reduction. At present, radiative techniques of forced geoengineering are studied only from the point of view of scientific research and are not included in policy frameworks.

3.1. The Negative Emissions Technologies

As reported from the European Parliamentary Research Service (EPRS) [ 29 ] there are a lot of technologies able to remove the CO 2 from the atmosphere. In order to compare them with the traditional carbon abatement technologies an important parameter to consider is the amount of energy needed. In this sense most of the negative emissions technologies (NET) required only a minor increase in the fraction of energy that must be dediacted to these kind of technologies. This is a very important point that must be taken into consideration.

The negative emissions technologies (NET) are able to absorb the CO 2 at low concentration present into the atmosphere. In order to perform this process the NET using an “industrial” process, which, depending on the NET used, can have different characteristics. This kind of process happens naturally during the photosynthesis and during the growth of the biomasses [ 29 ].

The Intergovernmental Panel on Climate Change (IPPC) in most of its scenarios of stabilization involves the use of NETs.

It is important to note that the use of NETs must not replace the indications relating to the cutting of GHG emissions. Using NETs and cutting emissions must implemented together in order to achieve good results [ 29 ].

The main negative emissions technologies are reported in the following. The information reported was elaborated starting from the reports present in the scientific literature [ 24 , 29 , 30 ]:

• Forestation, afforestation and reforestation would involve planting forests on unused land;
• Biochar, biochar involves the production of enriched carbon material by the slow pyrolysis process;
• Soil Carbon Management, agricultural land management practices such as reduced tilling, cover crops and certain grazing practices increase organic carbon levels in soils;
• Ocean Fertilization, involves adding nutrients to the ocean in order to stimulate the growth of planktonic algae and other microscopic plants that take up CO 2 ;
• Augmented Ocean Disposal (“ocean liming”), uses lime in oceans to trap CO 2 in a stable, dissolved inorganic form;
• Enhanced Weathering and Mineral Carbonation, implies the application of finely ground silicate or carbonate minerals to seawater or soils;
• Bio-energy with Carbon Capture and Storage (BECCS), is the combination of two mitigation options: biomass combustion to generate energy and Carbon Capture and Storage (CCS). The BECCS process achieve negative emissions by storing the carbon dioxide resulting from the combustion of plants, which have previously removed CO 2 from the air through photosynthesis;
• Direct Air Capture, refers to industrial methods for removing carbon dioxide from the air by putting the air in contact with a chemical sorbent that are able to absorb the carbon dioxide. An example of this are the “Artificial Trees” technology: this is a technology that mimics the processes by plant life to withdraw CO 2 from the atmosphere;
• Lime–Soda Process, this process is similar to artificial trees, but uses a chemical scrubbing method to enhance CO 2 capture;
• Carbon Storage and CO 2 utilization, both BECCS and Direct Air Capture need carbon storage to achieve permanent removal of the carbon from the atmosphere. The most common method is geological storage in depleted oil and gas fields, coal beds, and saline aquifers; however total storage capacity is uncertain and requires further geological studies.

Table 1 reports the net estimate costs (expressed as €/tCO 2 removed) for the negative emissions technologies above reported [ 20 ].

Cost indication of the different negative emissions technologies (data from [ 20 ]).

Analyzing Table 1 shows that all the negative technologies reported have quite high CO 2 removal costs. Enhanced weathering and mineral carbonation technology is the most expensive (about $1000 per ton of CO 2 removed) followed by Ocean Fertilization (about $500 per ton of CO 2 removed). The remaining technologies have quite similar costs (of the order of 100–150 $ per ton of CO 2 removed).

These technologies are a concept rather, however, given the role that these technologies can play in mitigating climate change (absorbing a share of CO 2 now present in the atmosphere) it is necessary to be able to give the right importance to these technologies and the right study or development.

Negative emission technologies to become industrially mature will also have to be studied and tested more individually. To date, in fact, most of the studies proposed by the IPCC include the implementation of negative emission technologies together with conventional decarbonization technologies [ 28 ], this is to achieve the objectives set by the Paris Agreement. Actually, only two negative technologies have been included in the IPCC’s assessments: bioenergy carbon capture and storage and afforestation and afforestation [ 30 ].

3.2. Microalgae for BIO-Fixation

The use of microalgae to actively bio fix CO 2 is an activity strictly related with the growing factor of these microorganisms. In the past, a lot of research works were published where the focus was to produce enough biomass to economically sustain the production of biodiesel. This scope in the last years was slowly changed to production of food, cosmetic, and high value compounds. However, it is a fact that microalgae to successfully perform photosynthesis needs CO 2 , in fact it was reported that microalgae cells contain about 50% carbon, in which 1.8 kg of carbon dioxide are fixed by producing 1 kg of microalgae biomass [ 31 , 32 ]. Using photosynthesis process the CO 2 is fixed by microalgae cells to support their growth by using the carbon to produce carbohydrate and consequently, the carbohydrates are used to build proteins, nucleic acids, and lipids [ 33 ]. Because of their simple cell structure and fast growth rate, microalgae are expected to have a 10 to 50 times higher CO 2 bio fixation efficiency than terrestrial plants [ 34 , 35 ]. This aspect related to the microalgae metabolism could be usefully considered for carbon capture mitigation when the microalgae were cultivated on a high efficiency system that is strongly integrated with an industrial plant. In this scenario, the CO 2 is provided by an existing industrial stream, as well as for the nutrient elements. There has been increasing interests on the use of microalgae growing technology for both bio fixation of carbon dioxide from flue gases [ 36 , 37 , 38 , 39 , 40 , 41 ] and removal of nutrients from wastewater [ 40 , 41 , 42 , 43 , 44 ]. Another consideration that needs to be done is related with the kind of light energy provided to the microalgae growing system. If the light is coming from natural source, it has been proven that the theoretical maxima of solar energy conversion efficiencies in photosynthesis is around 10%–8% solar-to-biomass [ 32 ]. As confirmed by other experimental works the best-case scenario (lab and small scale green microalgal productivities) has achieved only about 40% (or less) of the theoretical maximum productivity. At the same time, the best case solar to biomass energy conversion efficiency obtained with green microalgae did not exceed the 3% value [ 45 , 46 ]. Several studies reported by Melis (2009) [ 32 ], highlight solar to biomass energy conversion efficiencies for Spirulina and switchgrass that are lower than 1%. Obviously, biomass productivity is much lower for traditional C3 crop [ 47 ] and wild land plants, where the solar to biomass conversion efficiency values are below 0.1%.

If the light is provided by an artificial source that can provide the right quantity and quality of photon flux the global efficiency of the system can increase. First of all an optimized light spectrum that is specifically designed for the selected microalgae strain can double the energy conversion efficiency bringing the value from 8–10% to 20–22%. Secondary, the energy source used to power the growing system must come from a national energy mix that does not have coal, or from a renewable one with a high-power density value, for instance last generation of PV panels, wind, or tidal turbine. Only in this way the CO 2 balance of the microalgae growing system can be compared with one that use natural light source. Finally, the growing system must allow a CO 2 fixation rate, or removal percentage, as near as possible to the growth rate of the strain or to the 100% of removal. To achieve this result the system must be closed, not in direct contact with the atmosphere, with a well-known gas mass coefficient and with a CO 2 inlet and O 2 outlet mechanism carefully designed. In this way, the dissolved carbon dioxide inside the growing medium can be efficiently bio fixed by the microalgae, and the release of oxygen will not become a limiting factor. Using a closed growing system can allow to achieve another interesting result that is related with the protection of environment. Indeed, a closed system can allow to avoid microalgae contamination on the external environment and using a managed system for the water treatment. In this way, it can be possible to completely reuse the amount of water needed to the process, limiting its consumption and drastically reducing make-up cost with a positive environmental footprint. There is any industrial standard related with this specific kind of technology, even the scientific literature presents several difficulties as authors release research that cannot be directly compared to each other and the few commercial-industrial plant do not release any technical specifications. Despite the low efficiency obtainable from system placed under natural light and the scarcity of information related to technology that uses artificial light, the bio fixation of CO 2 with microalgae is still under study by different research groups as its potential for climate change mitigation is only just beginning. In the work of Lim et al. [ 48 ], this aspect was deeply investigated as the authors selected a higher variety of published results. Additionally, they proposed a direct comparison of several experiments where the considered parameters were grouped under the carbon content quantification method used, then the evaluate carbon dioxide fixation rate and, consequently, the CO 2 fixation efficiency. The first parameter is directly related with the biomass productivity and the second is more related with the growing technology used. At the end, after a comparison of more than 150 experimental tests the authors concluded assessing those microalgae have great potential not only to reduce the CO 2 levels in the atmosphere, but the obtained biomasses are also useful in different type of applications. At the same time, they confirmed our initial thesis in relation with the fact that CO 2 fixation quantification methods have not been critically analyzed and explicitly discussed in the scientific literature. For example, Almomani et al. [ 49 ] tested a pilot scale plant for a period of 20 months, verifying the bio fixation capacity and the growth rate of two different type of strains. The experiment was conducted with the use of a specific designed photobioreactor, where it was possible to use flue gas and wastewater as a viable source of nutrients. The conclusion was positive, but to be sure about the technology application the authors themselves suggested to improve the study with a scale up of the system. Only in this way will it be possible to recover data that can be used for assessing a valid economic feasibility and to perform a Life Cycle Analysis.

4. Conclusions

In this work, the analysis of the climate change mitigation phenomena was performed. In particular, a review of the main “negative emissions technologies” or the technologies able to absorb the CO 2 present in the atmosphere was conducted. For these technologies, the advantages and disadvantages and the cost are analyzed and reported (based on the literature data). The results of this first part show that the costs of these technologies are still very high and much still needs to be done in the field of research to make them industrially competitive. However, it is clear that these technologies can play a key role in achieving the objectives of the Paris Agreement. New information are added concerning an emergent negative emissions technologies: the microalgae.

Regardless, concerning the use of microalgae for CO 2 bio-fixing more work is required, as the bio fixation efficiency is strictly related with the microalgae growth rate and to obtain a valid technology many difficult aspects still need to be solved. In this way, it will be possible to reduce the energy demand and reduce investment and operating costs.

Author Contributions

Conceptualization, D.P., V.A.R., and M.Z.; methodology D.P., V.A.R., and M.Z.; validation, D.P., V.A.R., and M.Z.; formal analysis, D.P., V.A.R., and M.Z.; investigation, D.P., V.A.R., and M.Z.; resources, D.P., V.A.R., and M.Z.; data curation, D.P., V.A.R., and M.Z.; writing—original draft preparation, D.P., V.A.R., and M.Z.; writing—review and editing, D.P., V.A.R., and M.Z.; visualization, D.P., V.A.R., and M.Z.; supervision, D.P., V.A.R., and M.Z.; project administration, D.P., V.A.R., and M.Z. All authors have read and agreed to the published version of the manuscript.

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Conflicts of interest.

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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New study finds continued loss of coastal ecosystems may jeopardize millions of lives in the face of tropical storms

Research suggests urgent need for conservation and nature-based solutions as human reliance on coastal ecosystems for protection increases

November 15, 2023 | Arlington, VA

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Claire Griffin Media Relations Coordinator Email: [email protected]

68 million people in low-lying coastal areas around the world are at risk from tropical cyclones every year, and ecosystems like mangroves, coral reefs, salt marsh and wetlands serve as some of their best defenses. As climate change increases the frequency and intensity of natural disasters around the world, the value of natural protection against impacts like storm surge, erosion, and wave energy is greater than ever. But a new study in Environmental Research Letters shows that many people in coastal communities are at risk of losing that protection in the face of habitat degradation.

The new paper, titled “Global protection from tropical cyclones by coastal ecosystems – past, present, and under climate change,” showed that over the last 30 years, the destruction of coastal ecosystems was the primary driver of 1.4 million people annually losing the natural protection they previously enjoyed. The research model then estimated that, with climate change, 40% more people in coastal areas (annually around 27 million more annually than are already at risk) could be impacted by tropical cyclones each year. That increase would not even include effects of future population growth or sea level rise, which can be expected to further exacerbate the dangers.

“Coastal communities are vulnerable and losing nature’s protection,” says study co-author Rob McDonald, Lead Scientist for Nature-Based Solutions at The Nature Conservancy. “We hope this paper opens eyes to the fact that we need to prioritize the conservation of coastal ecosystems now, to ensure the defenses people rely on in times of disaster not only remain intact but grow and thrive.”

The challenge coastal communities face to maintain existing protection, much less add to it, is twofold: for one, many ecosystems are struggling to adapt and survive to the harsher conditions caused by climate change, from increasing temperatures to harmful fluctuations in salinity and nutrients. Combine that with the historic levels of habitat degradation caused by increased human development in recent decades, and the natural ecosystems people rely upon for protection are put even more at risk. While daunting, this reality demonstrates the promise of nature-based solutions to climate change. By investing in the health of natural ecosystems like reefs and mangroves, which are notorious carbon sinks known to help counteract climate change, it is possible to protect people from the negative impacts of climate change while simultaneously lessening the severity of climate change worldwide.

“Governments, international organizations and NGOs, communities, and the private sector must act now,” says the study’s lead author Sarah Hülsen, PhD at the Institute for Environmental Decisions, ETH Zürich. “If we don’t, more ecosystems and the protection they provide could be lost, putting millions more people at risk.”

The first and most cost-effective step to ensure coastal ecosystems and the protection services they provide continue into the future is by conserving existing ecosystems. But since climate change will only increase humanity’s need for natural protections, then rebuilding and restoring ecosystems that were previously destroyed (in addition to protecting what already exists) is also worth pursuing in certain geographies. This is particularly true for island states where coastal ecosystems have the capacity to protect large percentages of the population; in Bermuda, for example, mangrove restoration could increase the share of people protected from tropical cyclones by coastal ecosystems from 43% to 81%.

The study was done in collaboration with scientists from The Nature Conservancy, ETH Zurich, World Wildlife Fund, the University of Minnesota St. Paul, and the University of Cambridge. The full paper is available here:  https://iopscience.iop.org/article/10.1088/1748-9326/ad00cd

Sarah Hülsen  et al  2023  Environ. Res. Lett.  18 124023

DOI: 10.1088/1748-9326/ad00cd

The Nature Conservancy is a global conservation organization dedicated to conserving the lands and waters on which all life depends. Guided by science, we create innovative, on-the-ground solutions to our world’s toughest challenges so that nature and people can thrive together. We are tackling climate change, conserving lands, waters and oceans at an unprecedented scale, providing food and water sustainably and helping make cities more sustainable. Working in more than 70 countries and territories, we use a collaborative approach that engages local communities, governments, the private sector, and other partners. To learn more, visit nature.org or follow @nature_press on Twitter.

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Original research article, climate change adaptation and mitigation strategies for small holder farmers: a case of nyanga district in zimbabwe.

• 1 Department of Space Science and Applied Physics, University of Zimbabwe, Harare, Zimbabwe
• 2 Discipline of Geography, School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, Pietermaritzburg, South Africa
• 3 Meteorological Services Department of Zimbabwe, Corner Bishop Gaul/Hudson, Harare, Zimbabwe
• 4 Geography Department, Bindura University of Science Education, Bindura, Zimbabwe

Climate change encompassing mostly hydro-meteorological hazards is a reality affecting the world in diverse ways. It is manifesting in various ways such as increases in frequency and intensity of floods, droughts, and extreme temperatures. In recent years, climate change has induced droughts, other extreme weather events and meteorological disasters in many countries including Zimbabwe. Effective management of climate change induced challenges require localized strategies which may vary from one part of the world to another and even within a country. In view of the need to understand localized impacts and responses to climate change, the main objectives of the study were to (i) assess the impact of climate change on livelihoods and food security, (ii) identify and evaluate adaptation and mitigation strategies that small holder farmers in Ward 17, Nyanga, Zimbabwe have developed. The research used both qualitative and quantitative approaches with data collection methods comprising of questionnaires (56), observations and interviews (8). The tools were used to gather information which included encounters with extreme weather events, climatic trends as well as adaptive responses. The findings showed that climate change had a significant negative impact on the livelihoods and food security status of small holder farmers in ward 17 of Nyanga district. The identified climate change adaptation strategies implemented in the study area included food aid, use of traditional grains and other drought resistant crops, early planting, multiple planting, barter trade and livelihood diversification. The mitigation strategies used included afforestation and reforestation programs, avoiding veld fires and preservation of wetlands. The research identified challenges to climate change adaptation which include lack of markets to sell farming produce, inefficient institutions, poverty and high climate variability and increased uncertainty in the behavior of seasons. The findings of this study indicated the need for similar assessment in other parts of the country as impacts of climate change and responses thereof should vary from place to place.

Introduction

Global warming during the 20th Century in Africa has been estimated at between 0.26 and 0.5°C per decade ( IPCC, 2007 ). This trend is expected to continue and even to increase significantly, with a consequent impact on livelihoods. According to the Intergovernmental Panel on Climate Change ( IPCC, 2007 ), a medium-high emission scenario would see an increase in annual mean surface air temperatures of between 3 and 4°C by 2080. This implies difficult times ahead for local people that depend directly/indirectly on agriculture especially rain fed agriculture for their livelihoods and have few assets or strategies to cope with the changes to come. Other observed effects of climate change include reduced reliability of rainfall, increased frequency of extreme events such as prolonged dry spells, droughts and floods as well as poor intra-season spatial and temporal distribution of seasonal rainfall. In Zimbabwe, recorded temperatures have risen by about 1°C over the last 40 years of the twentieth century, while rainfall and runoff decreased by ~20 and 30 percent, respectively ( Watson et al., 1998 ). The frequency of droughts has also increased from once per decade to about once every 3 years in Zimbabwe ( FAO, 2004 ). Unganai (2009) points out that rainfall exhibits considerable spatial and temporal variability characterized by shifts in the onset of rains and increases in the frequency and intensity of heavy rainfall activity. This significantly compromise crop production especially for small holder farmers who depend heavily on agriculture and lack in irrigation and water harvesting technologies. Small holder farmers, whose livelihood depends on the use of natural resources and rain fed agriculture, are likely to suffer most the brunt and adverse effects of climate change ( IPCC, 2001b ). With rain fed agriculture failing, the situation is worsened by lack of other sources of any income needed to buy food supplements in the area of study ( Deressa and Rashid, 2010 ). Hence drought and climate change often turn into disasters since the copying mechanisms in the region are limited in capacity ( Gwimbi, 2009 ).

According to the Zimbabwe Draft Response to Climate Change (2013), the government of Zimbabwe regards climate change as a potential threat which undermines positive development made so far in the country in meeting the developmental goals like the millennium development goal number one which was aimed at eradicating extreme hunger and poverty. Zimbabwe has demonstrated its willingness to contribute to the preservation of the global climate for sustainable development through the formulation of the Zimbabwe National Environmental Policy and Strategies which broadly covers issues to do with climate change. However, Chagutah (2010) noted that capacity in African counties is limited by high levels of poverty and underdevelopment and Zimbabwe is not an exception. The study on the impact of climate change on the local populations' livelihoods is increasingly forwarded as an urgent research need. Bunce et al. (2010) noted that the African continent is increasingly becoming a major global food crisis spot if there are no efforts to address climate change at the local level. As Nath and and Behera (2011 ) argue that local assessment of vulnerability enables a better understanding of how and why communities respond to the same type of environmental stress in ways that are different. As such the impact of climate change across the globe also differs. With this in mind, it becomes imperative as Nath and and Behera (2011 ) notes, to understand the actual dynamics of climate change impact and responses at the lowest levels of the society, such as households, communities and districts so as to influence responsive relief interventions.

Climate change adaptation and mitigation strategies vary from place to place even within the same country. Understanding of strategies employed in an area is important for modification and adoption of strategies in other areas. Studies which look at area specific responses are important for creating a national and global database for climate change adaptation and mitigation strategies. In light of this, while other studies have looked at strategies such as in Bikita ( Mushore et al., 2013 ) and recently in Chipinge ( Mavhura et al., 2017 ) it is also important to understand how climate change is being tackled in other areas such as Nyanga District in the eastern parts of Zimbabwe. To the best of our knowledge, no emphasis has been placed on these climatic transformations which are threatening the sustainability of subsistence or smallholder agriculture in Nyanga. Nyanga is located in the semi-arid climatic regions hence understanding adaptation and mitigation strategies in such a dry area would benefit wetter areas as they adjust to projected decline in rainfall. The objectives of this study are thus to (i) assess the impact of climate change on livelihoods and food security, (ii) identify climate change adaptation and mitigation strategies employed in Ward 17 of Nyanga District in Zimbabwe, and (iii) highlight the ward level challenges to climate change adaptation and mitigation strategies.

Methodology

Description of the study area.

Nyanga district is located in Manicaland province in the Eastern Highlands of Zimbabwe ( Figure 1 ). The district consists of 31 wards. Ward 17 of the district consists of 12 villages. The target population comprises of communal farmers in Ward 17 of Nyanga District. ZIMSTAT (2012) , indicated that Nyanga Rural District has a population of 126 577. Ward 17, which is under study has a total population of 10,605 of which 4,040 are males and 6,555 females. The Ward has a total of 1,370 households. The larger part of the district is located in Natural Farming Region 1, but Ward 17 is located in natural Natural Farming Region 4, which is characterized by low and erratic rainfall. The type of vegetation is characterized by thorn bushes, baobab trees and acacia, implying that the area generally receives low rainfall. There are prominent agricultural practices which are primarily subsistence farming.

Figure 1 . Location of Zimbabwe in Africa (A) , study area in Zimbabwe (B) , and map of Ward 17 of Nyanga District (C) .

Target Population

One thousand three hundred and seventy (1,370) households were too large to work with due to shortage of time and financial resources. Saunders et al. (2003) suggested that a sample size can be defined by using 10–20% of the targeted population. A total of 56 subsistence farmers was selected using simple random sample and purposive sampling technique. This sample was large enough to make some generalizations about the ward since the respondents were selected from different spaced households.

Research Instruments

For a comprehensive evaluation of the effectiveness of strategies in mitigation and adaptation to climate change programs in food insecurity reduction, multiple data collection techniques were needed. The data collection instruments used include questionnaires, interviews and observations. The usage of multiple data collection tools was meant to ensure that the weakness of one tool would be covered by another. During field surveys, observations were made to identify information on poverty levels, evidence of adaptation and mitigation as well as livelihood strategies.

The questionnaires were designed to solicit information from the general villagers. The questionnaire focused on issues such as: adaption and mitigation, types of crops grown, food intakes per day, type of livestock and other assets or resources owned by the farmers and involved in helping the farmers. The challenges being faced in implementing new farming methods were also of interest.

Structured interview guides were designed to solicit the same information from key informant members of the community. The purpose of key informant interviews was to collect information from a wide range of people including community leaders, professionals, or residents especially the elderly who had historical glimpse of weather and climate change and have first-hand knowledge about the community. Observation guides were also designed to help the researcher to obtain information based on related indicators that would be seen while moving around the area. This was used by the researcher to identify some of the issues that may not be aired out clearly during interviews and questionnaires.

The usability of the instruments was tested before use for actual data collection, known as pre-testing. Questionnaire was tested at a random office at Environment Management Agency while interview guides were administered in the ward before the actual data collection. This helped to check the reliability and validity of these instruments as well as to ensure that the tools were clear enough to gather data covering all the objectives. Correction and rephrasing of issues which were confusing to the respondents were done.

Data Collection Procedures

The researcher acquired approval from the Ward 17 Councilor and village heads before embarking on the study after vividly explaining the objectives and aim of the study to community leaders. After clearing issues which have something to do with permission and approval the researcher went on to the participants of the farmer's questionnaire. The researcher also made appointments with key informants from, Environment Management Agency (EMA), Agricultural and Technical Extension (AGRITEX) officers, non-governmental organizations which offer climate related technical assistance to farmers and ward 17 village heads. Key informants were composed of one individual from EMA, two from AGRITEX, three village heads and two from Non-Governmental Organizations (NGOs). The selection of these key informants is based on their proximity to Ward 17 and also the in depth knowledge on the problem under study. After the appointments had been done the researcher conducted face to face interviews with above mentioned key informants ( Table 1 ). Before the interview meeting with the key informants, the researcher practiced and familiarized with the script and questions to ensure there were no biases and confusion during the interview. The researcher also observed the types of crops which were under cultivation to evaluate if the varieties are resilient to climate change and variability.

Table 1 . Distribution of respondents.

Three key informants consisting of one from the Ministry of Environment and Climate Change specifically from EMA and two from Agriculture Extension (AGRITEX) Office were targeted and reached and the other two were targeted from NGOs which are doing a DRR/disaster risk reduction program in the ward. Both were reached through interviews to provide information on climate change and its effects on food security and livelihoods in the area of study ( Table 1 ). They also elaborated the work being done by government through the role they are playing in reducing the effects of climate change. The AGRITEX officers provided information on the implementation of adaptation and coping mechanisms as well as accounts of vulnerabilities of people on the ground to the effects of climate change.

Reliability and Validity

Reliability and validity were ensured through the use of multiple instruments referred to as triangulation and in this study questionnaire, interviews and observations were utilized. Validity was catered for by cross checking for consistency. Triangulation is a powerful technique that facilitates validation of data through cross verification from more than two instruments ( Kimchi et al., 1991 ). In this study information from questionnaires, interviews and observations were validated by inter-comparison. Similar data from these different sources were compared to check for consistence and collect as much information from all the methods used as possible. Errors and suspicious data from any of the methods were identified and corrected using information from the other complementary sources.

Data Analysis Procedure

The collected data were analyzed using Microsoft Excel as well as Statistical Package for Social Scientists (SPSS) where simple descriptive statistics were obtained and results were summarized as graphs and pie charts for discussions. Quantitative data from the questionnaires were analyzed using Statistical Package for Social Science (SPSS), Microsoft excel and were analyzed thematically. Transcripts from interviews were analyzed using the participant's own words and without preconceived classification. The participant's language and phrases were examined; categorized and recurrent themes were identified. Recurrent themes are the similar and consistent ways people think about, and give accounts on concerning particular issues. For open ended questionnaires, the researcher looked into the themes as they emerged from the data as they were coded and then put into conceptual categories and the results were described.

Responses collected using the different instruments mentioned above were sorted into classes namely climate change adaptation strategies, mitigation strategies, impact of climate change to livelihoods, challenges to climate change mitigation and adaptation and impact of drought mitigation strategies.

The first analysis involved identification and assessment of the effectiveness of climate change adaptation strategies. The percentages of the participants who view each of the technique as effective or otherwise were recorded. The second analysis involved identification of climate change mitigation strategies in use and counting of the number of people who pointed out each of the strategies in Ward 17 of the Nyanga District. Another analysis was done which identified challenges faced in adaptation and mitigation of climate change in Ward 17 of the Nyanga District and counting the number of people who pointed out each as a challenge.

Presentation and Discussion of Research Findings

Climate change is affecting a variety of socio-economic activities in Zimbabwe including Ward 17 of Nyanga District. The evidence include rising temperature, increased frequency of floods, dry spells, droughts and other extreme events ( Releifweb, 2011 ; Brazier, 2015 ). While the economy of Zimbabwe strongly relies on agriculture which is largely rain-fed, hydro-meterological extremes are compromising productivity. For instance, the start and end of season have increasingly become uncertain making it difficult to decide on setting planting dates and selecting crop varieties to grow ( Mushore et al., 2017 ). Even in a season where rainfall totals are within or exceed long term average, quality of the season is causing decline in productivity such as through prolonged and frequent mid season dry spells. The combination of declining trend in rainfall and rising temperatures has meant increase of evapo-transpiration speeding up crop growth cycles and affecting proper maturity of crops ( Manatsa et al., 2017 ). Livestock production has also not been spared by adverse impacts of diminishing water resources and severely high temperatures not suitable for animals during some periods ( Mutekwa, 2009 ). In view of these challenges, communities embark on a variety of efforts to survive within the changing climate (adaptation) and to reduce further changes in climate (mitigation). In the context of background changes in climate, based on information gathered using qualitative and quantitative techniques, adaptation and mitigation efforts applied in Ward 17 of Nyanga District are discussed in this section.

Livelihoods Strategies Employed to Curb Climate Change Impacts in Ward 17 of Nyanga District in Zimbabwe

As noted from Figure 2 , 40% of the respondents are into food and cash crop production, 38% are into barter and petty trade and 22% are into livelihoods diversification ( Figure 2 ). This did not mean that some respondents are not involved in all the three livelihood strategies. The people in the area mainly rely on food and cash crop production, petty trade and barter trade and livelihoods diversification which include migration to Mozambique, selling of livestock for them to get money to supplement food requirements. This reduces their ability to productively carry out their farming activities in the coming seasons thus making them more vulnerable to food insecurity and other negative effects of climate change as they spent their time doing other off farm livelihoods activities. Sometimes they sell their livestock which reduce their draft power to meet their immediate food needs at the expense of their long time coping mechanisms. This makes some of the adaptations less effective and less sustainable. This observation confirms De Waal (1990) 's assumption that people's need to consume food drives their actions. Consequently it results in depleted draft power, which in turn limits the farmers' capacity to farm productively in the coming seasons thus making them more vulnerable to food insecurity, a phenomenon echoed by the AGRITEX officer. This therefore reverses the development made by the people confirming Davidson et al. (2003) who noted that climate change will affect the achievement of the MDGs in particular the one to do with achieving alleviating hunger and poverty.

Figure 2 . Livelihood strategies of respondents (multiple response) n = 58.

As a result food security continues to deteriorate in the area. This was confirmed by Bunce et al. (2010) who noted that Africa has become a place for food crises. As such events mainly affect the poor and the situation is exacerbated by poor governance of the available resources ( Feyissa, 2007 ). Climate change undermines the government's capacity to cope with the rising demands for food in Africa as food production is very low particularly in Zimbabwe. Food security is really an issue in ward 17 as it is a semi-arid region and the circumstances are being worsened by the adverse effects of climate change. Most of the families in the study have at least one or two meals a day. Thus they rely mostly on handouts from NGOs. Crops like groundnuts ( Arachis hypogaea ) and round nuts ( Vigna subterranea ) are prominent there and in a way they improve on nutrition circles since they are rich in proteins.

Climate Change Adaptation Strategies Used in Ward 17 of Nyanga District in Zimbabwe

Most of the small holder farmers rely on food aid, using small grains like sorghum which are drought resistant, use of short seasoned varieties, barter trade, multiple cropping, livelihood diversification, dry planting, and early planting as climate change copying and adaptation strategies. Most of the adaptation strategies are sustainable as the smallholder farmers seem to favor them since they are less costly and are Indigenous Knowledge System/IKS based. This is usually the second response to climate change as it is hinged on finding alternative ways for instance; initiatives and policies to reduce the susceptibility of people and the environment to the harsh effects of climate change. Adaptation is usually divided into two broad categories namely ethno-science and techno-science ( Matanga and Jere, 2011 ). Ethno-science comprise of techniques based on local people's knowledge of their physical environment while techno-science involves modern technologies. Farmers in ward 17 villages use both methods for its adaptation to climate change. However, the former are mostly used as the method is not expensive like the latter. Figure 3 shows that 25% of the respondents opted for early planting because it is less costly. This shows that the respondents indicated that they are willing to take up any kind of adaptation strategy if it proves to be less costly, thus they encouraged AGRITEX and EMA to carry out adaptation and mitigation strategies which can be at zero budget.

Figure 3 . Adaptation strategies (multiple responses) n = 58.

Ethno-Science Adaptive Measures

Ethno-science is most commonly expressed as Indigenous Knowledge Systems ( Matanga and Jere, 2011 ) and it usually consist of adaptation methods such as growing of drought tolerant crops, multiple planting, early planting, barter trade, planting, hiring labor, selling, and begging.

Growing Drought Tolerant Crops or Small Grains

Growing of drought tolerant small grain crops such as millet, sorghum, and rapoko is usually done to curb issues of low and unreliable rainfall ( Chazovachii et al., 2010 ), which are caused by climate change. Current weather conditions are making it impossible to grow maize which is the staple cereal for Zimbabwe and for villagers of ward 17, since the area does not have enabling conditions for the crop to have a good yield as it is an arid area. As a result, small grain crops are suitable because they can survive in dry conditions. This is helpful because it will mostly ensure the availability of food even during drought seasons.

This is in contrast with the findings of this research as the dominant food crop being grown is maize. The majority of the subsistence farmers prefer to grow maize crops even in marginal areas like Ward 17, resulting in persistent food shortages. Although, some households now shift from maize to sorghum, there is need for awareness campaigns and education from AGRITEX on the importance of small grains in marginal areas. Also lack of access to seeds such as millet and rapoko has contributed to the ineffectiveness of this climate change adaptation strategy.

Multiple Cropping to Increase Chances of Getting Yield in Harsh Climatic Conditions

This involves planting a variety of crops such that if other crop types fail due to the given weather conditions the surviving crops would act as safety nets. Thus, they mix crops like pumpkins, maize and beans together. This helps in promoting soil fertility as the legumes are nitrogen fixing crops. Again they create soil cover which helps in soil moisture retention and preservation. Multiple cropping ensured families would get some yields to harvest even when other crops fail. In Maereka, Kuwenyi, Chimonyo, and Dzimbiti villages, they usually multi and mix crops especially groundnuts ( Arachis hypogaea ), round nuts ( Vigna subterranea ), cow peas, sugar beans, maize, sorghum, rapoko, and millet, most of which are drought tolerant.

Early Planting to Take Advantage of Early Rains and Full Length of Season

Based on the deep knowledge of their agro-ecological conditions by local people and expectation of a good rainfall season (based on indigenous indicators) crops are planted as soon as the first rains fall. CARE (2009) however, noted that some people in ward 17 village avoid this method as it can be risky since there can be some instances when the rains would go away after they had planted and that would be wastage of seeds. Some would want to practice this method but might face hindrances like lack of inputs such as seeds during the time of the first rains.

Dry Planting to Counter Uncertainty in Start of Season

Farmers prepare the land and saw their crops in September and October before the rainfall come. It is also known in the district as kupandira in the local Manyika dialect. This is done so that when the rains come the already sowed seeds will sprout with the first rains. Itis done to counter the unpredictability of rain. The types of crops which are normally dry planted include maize, ground nuts ( Arachis hypogaea ) and round nuts ( Vigna subterranea ) and rapoko. This method gives smallholder farmers a chance to focus on other off farm activities like petty trade if it is properly done. This method also helps in moisture preservation, thus making crops thrive even in dry conditions.

Barter Trade in Exchange for Food Items in Times of Deficit or Poor Crop Yields

Barter trade is also another way which people in ward 17 are using to adapt to the effects of climate change. Since they mostly grow small grain crops they usually do not have crops like maize and their households' gardens do not grow much to sustain them with vegetables, tomatoes or onions. Therefore, they have to practice barter trade within Nyanga District. One bucket of approximately 10 kg of millet and sorghum can be barter traded with 2 kg of sugar or a bar of washing soap, or vegetables, or tomatoes, or onions, or beans, or fish with people from Chimonyo village. Poultry or livestock is usually traded with maize from areas like Mutare or Mutasa. The major setback with this adaptation method is that, sometimes the villagers may be treated unfairly in terms of the standard valuation of livestock and other asserts. In some situations livestock may be exchanged for very small quantities of grains for example a heifer can be exchanged with 500 kg of maize.

Livelihood Diversification to Sustain Lives Even When Agricultural Production Is Limited

Casual labor, selling and begging are some of the livelihood diversification strategies which they employ. People in ward 17 especially in Chimonyo village usually go to nearby areas like Muozi and Nyanga Forests to work as casual or full time laborers so as to get money or other basic commodities they would be in need of. However, in some cases whereby drought would have hit hard, selling labor can be less practical as the areas people usually go to work would also be affected. This then means that the sustainability of the livelihoods is threatened. Some will be involved in activities of buying and selling clothes or household equipment and utensils that they get from urban areas or across the border, most notably from Mozambique.

Others take another unreliable source of livelihood strategy which is begging for food or money. Begging better known as kutsunza or kupemh a in the manyika dialect, is one of the least practiced adaptation method to climate change. This is so because rarely people will have excess supply of agricultural produce to spare. However, begging reduce one's self esteem as one has to reduce his or her pride so that the potential donors can sympathize with them and give them something. Also most vulnerable households can benefit from chieftain granary reserve as it is still being practiced by the local chief Saunyama. The findings are in tandem with the notion that there is need for the agricultural sector to diversify and start producing sorghum, millet, rapoko, sweet potatoes, cassava and yams at a large scale to meet the country's food requirements to ensure food security in Zimbabwe in the wake of climate change ( Mudimu, 2003 ).

Techno-Science Adaptive Methods

Techno-science adaptation methods include small supplementary feeding and reliance on food aid. Africa as a continent given that it is still developing, lacks the capacity and resources to adapt to climate change for this requires a lot of money. Zimbabwe's government therefore as one of the Third world countries has weak inter- and intra-sectoral co-ordination in as far as climate change is concerned Gukurume (2013) . Therefore, the country has narrow capacity for climate change policy analysis, implementation and has limited resources to fund climate change adaptation and mitigation programmes. Adaptation to climate change more often than not heavily depends on donor funds.

Food Aid Programmes

Various NGOs in Nyanga district particularly ward 17; most notably World Food Program (WFP) are involved in food aid programmes whereby the community will be given food free of charge or on food for work basis. Out of all the adaptation measures most of the community members seem to favor this intervention method. However, it has dangers of making the community develop donor dependency syndrome. It is progressive in situations whereby the community has to do food for work. For instance in 2020 villagers were promised to go for food for work programs, in which case the community would benefit at the same time developing their community through a development initiative such as building of a bridge or repairing eroded roads.

Food aid can also be in form of supplementary feeding programs which are also significant in adapting to climate change in Chimonyo and Kuwenyi villages this is whereby schools especially primary schools receive food aid from the government or NGOs. The community would be responsible for preparing and feeding pupils in school. This initiative has a double impact of alleviating the education and health sector at the same time as it acts as a pull factor of sending children to school. This was also supported by Munro and Scoular (2012) who stated that unsustainable relief on vulnerable households lasted for only a few days before the next distribution date due to inadequate quantities during the 1991/2 drought in Zimbabwe. Besides aid create a donor syndrome which does not create a sense of innovativeness in trying to cope with the ills of climate change.

Climate Change Mitigation Implemented Strategies in Ward 17 of Nyanga District

Figure 4 shows that about 56.8% of the respondents prefer reforestation as a mitigation strategy. And 43.2% of the respondents are mainly into forestry preservation by avoiding veld fires. Mitigation is a process that involves humans reducing their anthropogenic causes of climate change and this is usually through limiting pollutants such as carbon dioxide. The African continent is so unfortunate that it only contributes about 3.8% of the total GHGs ( Bjurström and Polk, 2011 ) yet its inhabitants and resources are the most vulnerable to the impacts of climate change. People in ward 17 in Nyanga contribute to the mitigation of climate change by planting and maintaining the already existing indigenous and exotic trees in their homesteads. And they also mitigate by avoiding veld fires.

Figure 4 . Prominent mitigation strategies (multiple response) n = 58, Source: primary data.

This will assist in providing and enhancing adequate carbon sinks for GHGs. Besides that, trees are very important for life. They protect the soil from erosion, provide food and shelter for some animals as well as medicines to mention but a few. The main reforestation strategy which can affect the sustainability of the mitigation strategies is the use of gum trees in forests. They have an advantage in that they grow fast, but they make use of the underground water source and deplete it. The other issue is that if you clear a gum forest you can never use that land for crop production since they produce chemicals which affect soil structure in the long run.

Challenges Faced in Climate Change Adaptation and Mitigation in Ward 17

The respondents are facing quite a number of drawbacks in trying to reduce the impacts of climate change and this has contributed to the ineffectiveness of some of the adaptation and mitigation strategies. The respondents pointed out that poverty, inefficient institutions for example EMA and AGRITEX, increased frequency of extreme weather events and remoteness of the area were the major challenges faced in trying to mitigate the impacts of droughts as shown in Figure below.

From Figure 5 households clearly face a major challenge of increased weather and climate variability or extreme weather events. Thus, extreme weather events are now rampant as a result of climate change and that droughts are becoming more frequent in the area as a result of global warming. The increase in extreme weather events proved to be one of the major challenges as it is one of the more unpredictable challenges. This is evidenced by the fact that the farmers testified that every other season is unique to itself, such that the farmer should be prepared to change at each and every season to meet the ever changing seasonal trends. The human activities are also a critical issue in exacerbating vulnerability to climate change, ranging from anthropogenic climate change at one extreme to local deforestation ( Munro and Scoular, 2012 ).

Figure 5 . Challenges faced by Farmers (multiple response) n = 58.

Climate variability acts as a dynamic pressure which worsens the vulnerability of rural populations to natural slow onset disasters like drought. Climate changes are a threat to rural agricultural livelihoods through increased drought frequency. In particular, climate change may configure drought, which may lead to decrease in agricultural yield since it is associated with an increasing drought frequency.

For the majority of the population, absolute lack of assets and means of livelihood and precarious economies with low coping or adaptive capacity present one key factor that configures risk to drought. Poverty is the major problem which is exacerbated by drought effects as indicated by Maphosa (1994) . The households are very poor in such a way that they rely on food handouts because of food insecurity. The respondents mentioned that due to lack of capital and collateral security they do not access loans and they do not have access to inputs such as fertilizer, seeds and farming equipment.

During the research, the households revealed that there are limited markets for their produce especially vegetables and drought resistant crops such as millet and rapoko so the majority cultivated maize for food production. A similar finding was reported by Chazovachii et al. (2010) , who stated that there is no market for drought resistant crops and people are only relying on the local market. This implies that the livelihoods in the area are still threatened and a lot still needs to be done to alleviate and cope with the adverse impacts of climate change. They said the ears of millet, rapoko and sorghum plants might not ripe at the same time thus they may have to be more than one harvest hence, the majority of villagers opted for maize which is the staple crop.

From the findings, physical geographic location and remoteness of the ward makes it more vulnerable to inaccessibility of useful climate change information since the area is in Region 4 and 5 and is isolated far from other areas like Growth Points. The poor road networks and communication networks makes the area inaccessible. The road is very poor in such a way that the donors and investors shun away from this ward. The households are isolated and marginalized therefore development and other opportunities will not be attained. Isolation due to a lack of infrastructure may limit choices and coping strategies during times of stress and other climate change related disasters like drought.

There is lack of integration and coordination among Government departments, NGOs and other institutions in disaster management. Institutions face a number of challenges which include political interference, lack of resources and lack of coordination in climate change and drought management. During the interviews respondents stated that only a few benefit from programmes since the institutions are always bickering with each other and battling for supremacy, therefore this made them ineffective. The households were benefitting from institutions at a lesser extent since most of the assistance is helpful in the short run but in the long run there are persistent food shortages and the adverse impacts of climate change persist.

After a consideration of the research findings we deduced that climate change negatively affects livelihoods and food security in rural communities which rely on rain fed agriculture as shown by the situation in ward 17 of Nyanga district. The people in the area of study rely on rain fed agriculture as their source of livelihood and the continuous poor yields obtained mean that the people face food challenges. As a result, the majority of the people in the area of study are vulnerable to food insecurity and their livelihoods are threatened as they have limited or no lasting coping strategies with the food challenges they face. The respondents noted that in the late 1980s and 1990s serve for the 1992 drought; people could harvest large quantities of maize as well as crops like sunflowers ground nuts (Vigna subterranea) and round nuts (Vigna subterranea) which attracted a lot of people to the area. As noted by the respondents, back then, the people could afford to choose nutritional food unlike in the present day that the people now consider looking for nutritional food as a luxury not as a basic need. The small holder farmers' livelihoods are being negatively affected by the changes in climate and weather. This is evidenced by the fact that most indicated that they no longer take three meals per day as they used to do, some only take one but the majority are now having two meals per day citing that this is caused by the change in climate which is affecting food output during harvesting.

The challenges in the mitigation and adaptation to climate change are rampant and the coping methods are very limited because of the state of development and resource scarcity especially in Sub-Saharan Africa and ward 17 is not an exception. The coping mechanisms are limited due to institutional poverty in state institutions like the EMA, which makes the efforts to cope difficult to achieve. As shown, the effects of climate change have been increasing and are getting worse over time. About mitigation strategies the farmers were mainly interested in talking of avoiding veld fires and reforestation in which the forests act as carbon sinks. Most farmers indicated that these mitigation strategies are less costly to them thus they opted for them.

Recommendations

As a contribution toward alleviating climate change effects to small holder farmers in the study area, the following recommendations are suggested:

➢The government should take a bottom up approach toward alleviation of climate change effects by working with community based leadership structures.

➢Government should give adequate resource prioritization to climate line institutions like AGRITEX and EMA so as to educate and inform the rural people with adequate copying information.

➢The government should work hand in hand with other development agencies so as to share or pool resources together in order to mitigate and adapt to climate change.

➢In this regard the government is recommended to put up effective measures in ensuring access of small holder farmers to credit and small loan facilities to improve output and livelihoods.

The research findings of this study show that climate change negatively affects livelihoods and food security in rural communities which rely on rain fed agriculture as shown by the situation in ward 17 of Nyanga district. The climate change adaptation strategies in Ward 17 in Nyanga District are food aid, use of small grains and other drought resistant crops, early planting, multiple planting, barter trades and livelihood diversification. The mitigation strategies used include afforestation and reforestation programs, avoiding veld fires and preservation of wetlands. The research identified challenges to climate change adaptation which include lack of markets to sell farming produce, inefficient institutions, poverty and sudden change of weather, seasons and climate trends. The research recommends that the government should increase resource availability to the AGRITEX and EMA which are line ministries toward agriculture and climate change as this is how some of the challenges can be resolved.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Ethics Statement

The studies involving human participants were reviewed and approved by Bindura University of Science Education. The patients/participants provided their written informed consent to participate in this study.

Author Contributions

TDM conceptualized the paper and was involved in literature review, tool design and data collection, analysis, paper writing and response to reviewers. TM was involved in literature review, tool design and data collection, analysis, paper writing and response to reviewers. MM, LM, EMat, EMas, CM, JG, and GM were involved in review of tools, data collection, analysis and preparation of manuscript which was an iterative process. All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher's Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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Keywords: climate change, Nyanga district, rainfall, agriculture, mitigation, Zimbabwe

Citation: Mushore TD, Mhizha T, Manjowe M, Mashawi L, Matandirotya E, Mashonjowa E, Mutasa C, Gwenzi J and Mushambi GT (2021) Climate Change Adaptation and Mitigation Strategies for Small Holder Farmers: A Case of Nyanga District in Zimbabwe. Front. Clim. 3:676495. doi: 10.3389/fclim.2021.676495

Received: 05 March 2021; Accepted: 06 July 2021; Published: 06 August 2021.

Reviewed by:

Copyright © 2021 Mushore, Mhizha, Manjowe, Mashawi, Matandirotya, Mashonjowa, Mutasa, Gwenzi and Mushambi. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Terence Darlington Mushore, tdmushore@science.uz.ac.zw ; mushoret@ukzn.ac.za

This article is part of the Research Topic

Climate Risk Management in Smallholder Agriculture

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The Fifth National Climate Assessment

The Fifth National Climate Assessment is the US Government’s preeminent report on climate change impacts, risks, and responses. It is a congressionally mandated interagency effort that provides the scientific foundation to support informed decision-making across the United States.

Fifth National Climate Assessment 1. Overview Understanding Risks, Impacts, and Responses

• Addressing Climate Change
• Experiencing Climate Change
• Current and Future Risks
• Determining the Future
• A Resilient Nation

How the United States Is Addressing Climate Change

The effects of human-caused climate change are already far-reaching and worsening across every region of the United States. Rapidly reducing greenhouse gas emissions can limit future warming and associated increases in many risks. Across the country, efforts to adapt to climate change and reduce emissions have expanded since 2018, and US emissions have fallen since peaking in 2007. However, without deeper cuts in global net greenhouse gas emissions and accelerated adaptation efforts, severe climate risks to the United States will continue to grow.

Future climate change impacts depend on choices made today

The more the planet warms, the greater the impacts. Without rapid and deep reductions in global greenhouse gas emissions from human activities, the risks of accelerating sea level rise, intensifying extreme weather, and other harmful climate impacts will continue to grow. Each additional increment of warming is expected to lead to more damage and greater economic losses compared to previous increments of warming, while the risk of catastrophic or unforeseen consequences also increases. { 2.3 , 19.1 }

However, this also means that each increment of warming that the world avoids—through actions that cut emissions or remove carbon dioxide (CO 2 ) from the atmosphere—reduces the risks and harmful impacts of climate change. While there are still uncertainties about how the planet will react to rapid warming, the degree to which climate change will continue to worsen is largely in human hands. { 2.3 , 3.4 }

In addition to reducing risks to future generations, rapid emissions cuts are expected to have immediate health and economic benefits (Figure 1.1 ). At the national scale, the benefits of deep emissions cuts for current and future generations are expected to far outweigh the costs. { 2.1 , 2.3 , 13.3 , 14.5 , 15.3 , 32.4 ; Ch. 2, Introduction }

US emissions have decreased, while the economy and population have grown

Annual US greenhouse gas emissions fell 12% between 2005 and 2019. This trend was largely driven by changes in electricity generation: coal use has declined, while the use of natural gas and renewable technologies has increased, leading to a 40% drop in emissions from the electricity sector. Since 2017, the transportation sector has overtaken electricity generation as the largest emitter. { 11.1 , 13.1 , 32.1 ; Figures 32.1 , 32.3 }

As US emissions have declined from their peak in 2007, the country has also seen sustained reductions in the amount of energy required for a given quantity of economic activity and the emissions produced per unit of energy consumed. Meanwhile, both population and per capita GDP have continued to grow. { 32.1 ; Figures 32.1 , 32.2 }

Recent growth in the capacities of wind, solar, and battery storage technologies is supported by rapidly falling costs of zero- and low-carbon energy technologies, which can support even deeper emissions reductions. For example, wind and solar energy costs dropped 70% and 90%, respectively, over the last decade, while 80% of new generation capacity in 2020 came from renewable sources (Figures 1.2 , 1.3 ). { 5.3 , 12.3 , 32.1 , 32.2 ; Figure A4.17 }

Across all sectors, innovation is expanding options for reducing energy demand and increasing energy efficiency, moving to zero- and low-carbon electricity and fuels, electrifying energy use in buildings and transportation, and adopting practices that protect and improve natural carbon sinks that remove and store CO 2 from the atmosphere, such as sustainable agricultural and land-management practices. { 11.1 , 32.2 , 32.3 ; Boxes 32.1 , 32.2 ; Focus on Blue Carbon }

Accelerating advances in adaptation can help reduce rising climate risks

As more people face more severe climate impacts, individuals, organizations, companies, communities, and governments are taking advantage of adaptation opportunities that reduce risks. State climate assessments and online climate services portals are providing communities with location- and sector-specific information on climate hazards to support adaptation planning and implementation across the country. New tools, more data, advancements in social and behavioral sciences, and better consideration of practical experiences are facilitating a range of actions (Figure 1.3 ). { 7.3 , 12.3 , 21.4 , 25.4 , 31.1 , 31.5 , 32.5 ; Table 31.1 }

Actions include:

Implementing nature-based solutions—such as restoring coastal wetlands or oyster reefs—to reduce shoreline erosion { 8.3 , 9.3 , 21.2 , 23.5 }

Upgrading stormwater infrastructure to account for heavier rainfall { 4.2 }

Applying innovative agricultural practices to manage increasing drought risk { 11.1 , 22.4 , 25.5 }

Assessing climate risks to roads and public transit { 13.1 }

Managing vegetation to reduce wildfire risk { 5.3 }

Developing urban heat plans to reduce health risks from extreme heat { 12.3 , 21.1 , 28.4 }

Planning relocation from high-risk coastal areas { 9.3 }

Despite an increase in adaptation actions across the country, current adaptation efforts and investments are insufficient to reduce today’s climate-related risks and keep pace with future changes in the climate. Accelerating current efforts and implementing new ones that involve more fundamental shifts in systems and practices can help address current risks and prepare for future impacts (see “Mitigation and adaptation actions can result in systemic, cascading benefits” below). { 31.1 , 31.3 }

Climate action has increased in every region of the US

Efforts to adapt to climate change and reduce net greenhouse gas emissions are underway in every US region and have expanded since 2018 (Figure 1.3 ; Table 1.1 ). Many actions can achieve both adaptation and mitigation goals. For example, improved forest- or land-management strategies can both increase carbon storage and protect ecosystems, and expanding renewable energy options can reduce emissions while also improving resilience. { 31.1 , 32.5 }

Climate adaptation and mitigation efforts involve trade-offs, as climate actions that benefit some or even most people can result in burdens to others. To date, some communities have prioritized equitable and inclusive planning processes that consider the social impacts of these trade-offs and help ensure that affected communities can participate in decision-making. As additional measures are implemented, more widespread consideration of their social impact can help inform decisions around how to distribute the outcomes of investments. { 12.4 , 13.4 , 20.2 , 21.3 , 21.4 , 26.4 , 27.1 , 31.2 , 32.4 , 32.5 ; Box 20.1 }

Meeting US mitigation targets means reaching net-zero emissions

The global warming observed over the industrial era is unequivocally caused by greenhouse gas emissions from human activities—primarily burning fossil fuels. Atmospheric concentrations of carbon dioxide (CO 2 )—the primary greenhouse gas produced by human activities—and other greenhouse gases continue to rise due to ongoing global emissions. Stopping global warming would require both reducing emissions of CO 2 to net zero and rapid and deep reductions in other greenhouse gases. Net-zero CO 2 emissions means that CO 2 emissions decline to zero or that any residual emissions are balanced by removal from the atmosphere. { 2.3 , 3.1 ; Ch. 32 }

Once CO 2 emissions reach net zero, the global warming driven by CO 2 is expected to stop: additional warming over the next few centuries is not necessarily “locked in” after net CO 2 emissions fall to zero. However, global average temperatures are not expected to fall for centuries unless CO 2 emissions become net negative, which is when CO 2 removal from the atmosphere exceeds CO 2 emissions from human activities. Regardless of when or if further warming is avoided, some long-term responses to the temperature changes that have already occurred will continue. These responses include sea level rise, ice sheet losses, and associated disruptions to human health, social systems, and ecosystems. In addition, the ocean will continue to acidify after the world reaches net-zero CO 2 emissions, as it continues to gradually absorb CO 2 in the atmosphere from past emissions. { 2.1 , 2.3 , 3.1 ; Ch. 2, Introduction }

National and international commitments seek to limit global warming to well below 2°C (3.6°F), and preferably to 1.5°C (2.7°F), compared to preindustrial temperature conditions (defined as the 1850–1900 average). To achieve this, global CO 2 emissions would have to reach net zero by around 2050 (Figure 1.4 ); global emissions of all greenhouse gases would then have to reach net zero within the following few decades. { 2.3 , 32.1 }

While US greenhouse gas emissions are falling, the current rate of decline is not sufficient to meet national and international climate commitments and goals. US net greenhouse gas emissions remain substantial and would have to decline by more than 6% per year on average, reaching net-zero emissions around midcentury, to meet current national mitigation targets and international temperature goals; by comparison, US greenhouse gas emissions decreased by less than 1% per year on average between 2005 and 2019. { 32.1 }

Many cost-effective options that are feasible now have the potential to substantially reduce emissions over the next decade. Faster and more widespread deployment of renewable energy and other zero- and low-carbon energy options can accelerate the transition to a decarbonized economy and increase the chances of meeting a 2050 national net-zero greenhouse gas emissions target for the US. However, to reach the US net-zero emissions target, additional mitigation options need to be explored and advanced (see “Available mitigation strategies can deliver substantial emissions reductions, but additional options are needed to reach net zero” below). { 5.3 , 6.3 , 32.2 , 32.3 }

Jay, A.K., A.R. Crimmins, C.W. Avery, T.A. Dahl, R.S. Dodder, B.D. Hamlington, A. Lustig, K. Marvel, P.A. Méndez-Lazaro, M.S. Osler, A. Terando, E.S. Weeks, and A. Zycherman, 2023: Ch. 1. Overview: Understanding risks, impacts, and responses. In: Fifth National Climate Assessment. Crimmins, A.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, B.C. Stewart, and T.K. Maycock, Eds. U.S. Global Change Research Program, Washington, DC, USA. https://doi.org/10.7930/NCA5.2023.CH1

How the United States Is Experiencing Climate Change

As extreme events and other climate hazards intensify, harmful impacts on people across the United States are increasing. Climate impacts—combined with other stressors—are leading to ripple effects across sectors and regions that multiply harms, with disproportionate effects on underserved and overburdened communities.

Current climate changes are unprecedented over thousands of years

Global greenhouse gas emissions from human activities continue to increase, resulting in rapid warming (Figure 1.5 ) and other large-scale changes, including rising sea levels, melting ice, ocean warming and acidification, changing rainfall patterns, and shifts in timing of seasonal events. Many of the climate conditions and impacts people are experiencing today are unprecedented for thousands of years (Figure 1.6 ). { 2.1 , 3.1 ; Figures A4.6 , A4.7 , A4.10 , A4.13 }

As the world’s climate has shifted toward warmer conditions, the frequency and intensity of extreme cold events have declined over much of the US, while the frequency, intensity, and duration of extreme heat have increased. Across all regions of the US, people are experiencing warming temperatures and longer-lasting heatwaves. Over much of the country, nighttime temperatures and winter temperatures have warmed more rapidly than daytime and summer temperatures. Many other extremes, including heavy precipitation, drought, flooding, wildfire, and hurricanes, are becoming more frequent and/or severe, with a cascade of effects in every part of the country. { 2.1 , 2.2 , 3.4 , 4.1 , 4.2 , 7.1 , 9.1 ; Ch. 2, Introduction ; App. 4 ; Focus on Compound Events }

Risks from extreme events are increasing

One of the most direct ways that people experience climate change is through changes in extreme events. Harmful impacts from more frequent and severe extremes are increasing across the country—including increases in heat-related illnesses and death, costlier storm damages, longer droughts that reduce agricultural productivity and strain water systems, and larger, more severe wildfires that threaten homes and degrade air quality. { 2.2 , 4.2 , 12.2 , 14.2 , 15.1 , 19.2 ; Focus on Western Wildfires }

Extreme weather events cause direct economic losses through infrastructure damage, disruptions in labor and public services, and losses in property values. The number and cost of weather-related disasters have increased dramatically over the past four decades, in part due to the increasing frequency and intensity of extreme events and in part due to increases in assets at risk (through population growth, rising property values, and continued development in hazard-prone areas). Low-income communities, communities of color, and Tribes and Indigenous Peoples experience high exposure and vulnerability to extreme events due to both their proximity to hazard-prone areas and lack of adequate infrastructure or disaster management resources. { 2.2 , 4.2 , 17.3 , 19.1 ; Focus on Compound Events }

In the 1980s, the country experienced, on average, one (inflation-adjusted) billion-dollar disaster every four months. Now, there is one every three weeks, on average. Between 2018 and 2022, the US experienced 89 billion-dollar events (Figure 1.7 ). Extreme events cost the US close to $150 billion each year—a conservative estimate that does not account for loss of life, healthcare-related costs, or damages to ecosystem services. { 2.2 , 19.1 ; Ch. 2, Introduction ; Figures 4.1 , A4.5 }

Cascading and compounding impacts increase risks

The impacts and risks of climate change unfold across interacting sectors and regions. For example, wildfire in one region can affect air quality and human health in other regions, depending on where winds transport smoke. Further, climate change impacts interact with other stressors, such as the COVID-19 pandemic, environmental degradation, or socioeconomic stressors like poverty and lack of adequate housing that disproportionately impact overburdened communities. These interactions and interdependencies can lead to cascading impacts and sudden failures. For example, climate-related shocks to the food supply chain have led to local to global impacts on food security and human migration patterns that affect US economic and national security interests. { 11.3 , 17.1 , 17.2 , 17.3 , 18.1 , 22.3 , 23.4 , 31.3 ; Introductions in Chs. 2 , 17 , 18 ; Focus on Compound Events ; Focus on Risks to Supply Chains ; Focus on COVID-19 and Climate Change }

The risk of two or more extreme events occurring simultaneously or in quick succession in the same region—known as compound events—is increasing. Climate change is also increasing the risk of multiple extremes occurring simultaneously in different locations that are connected by complex human and natural systems. For instance, simultaneous megafires across multiple western states and record back-to-back Atlantic hurricanes in 2020 caused unprecedented demand on federal emergency response resources. { 2.2 , 3.2 , 15.1 , 22.2 , 26.4 ; Focus on Compound Events ; Ch. 4, Introduction }

Compound events often have cascading impacts that cause greater harm than individual events. For example, in 2020, record-breaking heat and widespread drought contributed to concurrent destructive wildfires across California, Oregon, and Washington, exposing millions to health hazards and straining firefighting resources. Ongoing drought amplified the record-breaking Pacific Northwest heatwave of June 2021, which was made 2° to 4°F hotter by climate change. The heatwave led to more than 1,400 heat-related deaths, another severe wildfire season, mass die-offs of fishery species important to the region’s economy and Indigenous communities, and total damages exceeding $38.5 billion (in 2022 dollars). { 27.3 ; Ch. 2, Introduction ; Focus on Compound Events , Focus on Western Wildfires }

Climate change exacerbates inequities

Some communities are at higher risk of negative impacts from climate change due to social and economic inequities caused by ongoing systemic discrimination, exclusion, and under- or disinvestment. Many such communities are also already overburdened by the cumulative effects of adverse environmental, health, economic, or social conditions. Climate change worsens these long-standing inequities, contributing to persistent disparities in the resources needed to prepare for, respond to, and recover from climate impacts. { 4.2 , 9.2 , 12.2 , 14.3 , 15.2 , 16.1 , 16.2 , 18.2 , 19.1 , 20.1 , 20.3 , 21.3 , 22.1 , 23.1 , 26.4 , 27.1 , 31.2 }

For example, low-income communities and communities of color often lack access to adequate flood infrastructure, green spaces, safe housing, and other resources that help protect people from climate impacts. In some areas, patterns of urban growth have led to the displacement of under-resourced communities to suburban and rural areas with less access to climate-ready housing and infrastructure. Extreme heat can lead to higher rates of illness and death in low-income neighborhoods, which are hotter on average (Figure 1.8 ). Neighborhoods that are home to racial minorities and low-income people have the highest inland (riverine) flood exposures in the South, and Black communities nationwide are expected to bear a disproportionate share of future flood damages—both coastal and inland (Figure 1.9 ). { 4.2 , 11.3 , 12.2 , 15.1 , 22.1 , 22.2 , 26.4 , 27.1 ; Ch. 2, Introduction }

These disproportionate impacts are partly due to exclusionary housing practices—both past and ongoing—that leave underserved communities with less access to heat and flood risk-reduction strategies and other economic, health, and social resources. For example, areas that were historically redlined—a practice in which lenders avoided providing services to communities, often based on their racial or ethnic makeup—continue to be deprived of equitable access to environmental amenities like urban green spaces that reduce exposure to climate impacts. These neighborhoods can be as much as 12°F hotter during a heatwave than nearby wealthier neighborhoods. { 8.3 , 9.2 , 12.2 , 15.2 , 20.3 , 21.3 , 22.1 , 26.4 , 27.1 , 32.4 ; Ch. 2, Introduction }

Harmful impacts will increase in the near term

Even if greenhouse gas emissions fall substantially, the impacts of climate change will continue to intensify over the next decade (see “Meeting US mitigation targets means reaching net-zero emissions” above; Box 1.4 ), and all US regions are already experiencing increasingly harmful impacts. Although a few US regions or sectors may experience limited or short-term benefits from climate change, adverse impacts already far outweigh any positive effects and will increasingly eclipse benefits with additional warming. { 2.3 , 19.1 ; Ch. 2, Introduction ; Chs. 21–30}

Table 1.2 shows examples of critical impacts expected to affect people in each region between now and 2030, with disproportionate effects on overburdened communities. While these examples affect particular regions in the near term, impacts often cascade through social and ecological systems and across borders and may lead to longer-term losses. { 15.2 , 18.2 , 20.1 ; Figure 15.5 ; Ch. 20, Introduction }

Current and Future Climate Risks to the United States

Climate changes are making it harder to maintain safe homes and healthy families; reliable public services; a sustainable economy; thriving ecosystems, cultures, and traditions; and strong communities. Many of the extreme events and harmful impacts that people are already experiencing will worsen as warming increases and new risks emerge.

Safe, reliable water supplies are threatened by flooding, drought, and sea level rise

More frequent and intense heavy precipitation events are already evident, particularly in the Northeast and Midwest. Urban and agricultural environments are especially vulnerable to runoff and flooding. Between 1981 and 2016, US corn yield losses from flooding were comparable to those from extreme drought. Runoff and flooding also transport debris and contaminants that cause harmful algal blooms and pollute drinking water supplies. Communities of color and low-income communities face disproportionate flood risks. { 2.2 , 4.2 , 6.1 , 9.2 , 21.3 , 24.1 , 24.5 , 26.4 ; Figure A4.8 }

Between 1980 and 2022, drought and related heatwaves caused approximately $328 billion in damages (in 2022 dollars). Recent droughts have strained surface water and groundwater supplies, reduced agricultural productivity, and lowered water levels in major reservoirs, threatening hydropower generation. As higher temperatures increase irrigation demand, increased pumping could endanger groundwater supplies, which are already declining in many major aquifers. { 4.1, 4.2 ; Figure A4.9 }

Droughts are projected to increase in intensity, duration, and frequency, especially in the Southwest, with implications for surface water and groundwater supplies. Human and natural systems are threatened by rapid shifts between wet and dry periods that make water resources difficult to predict and manage. { 2.2 , 2.3 , 4.1 , 4.2 , 5.1 , 28.1 }

In coastal environments, dry conditions, sea level rise, and saltwater intrusion endanger groundwater aquifers and stress aquatic ecosystems. Inland, decreasing snowpack alters the volume and timing of streamflow and increases wildfire risk. Small rural water providers that often depend on a single water source or have limited capacity are especially vulnerable. { 4.2 , 7.2 , 9.2 , 21.2 , 22.1 , 23.1 , 23.3 , 25.1 , 27.4 , 28.1 , 28.2 , 28.5 , 30.1 ; Figure A4.7 }

Many options are available to protect water supplies, including reservoir optimization, nature-based solutions, and municipal management systems to conserve and reuse water. Collaboration on flood hazard management at regional scales is particularly important in areas where flood risk is increasing, as cooperation can provide solutions unavailable at local scales. { 4.3 , 9.3 , 26.5 ; Focus on Blue Carbon }

Disruptions to food systems are expected to increase

As the climate changes, increased instabilities in US and global food production and distribution systems are projected to make food less available and more expensive. These price increases and disruptions are expected to disproportionately affect the nutrition and health of women, children, older adults, and low-wealth communities. { 11.2 , 15.2 }

Climate change also disproportionately harms the livelihoods and health of communities that depend on agriculture, fishing, and subsistence lifestyles, including Indigenous Peoples reliant on traditional food sources. Heat-related stress and death are significantly greater for farmworkers than for all US civilian workers. { 11.2 , 11.3 , 15.1 , 15.2 , 16.1 ; Focus on Risks to Supply Chains }

While farmers, ranchers, and fishers have always faced unpredictable weather, climate change heightens risks in many ways:

Increasing temperatures, along with changes in precipitation, reduce productivity, yield, and nutritional content of many crops. These changes can introduce disease, disrupt pollination, and result in crop failure, outweighing potential benefits of longer growing seasons and increased CO 2 fertilization. { 11.1 , 19.1 , 21.1 , 22.4 , 23.3 , 24.1 , 26.2 }

Heavy rain and more frequent storms damage crops and property and contaminate water supplies. Longer-lasting droughts and larger wildfires reduce forage production and nutritional quality, diminish water supplies, and increase heat stress on livestock. { 23.2, 25.3 , 28.3 }

Increasing water temperatures, invasive aquatic species, harmful algal blooms, and ocean acidification and deoxygenation put fisheries at risk. Fishery collapses can result in large economic losses, as well as loss of cultural identity and ways of life. { 11.3 , 29.3 }

In response, some farmers and ranchers are adopting innovations—such as agroecological practices, data-driven precision agriculture, and carbon monitoring—to improve resilience, enhance soil carbon storage, and reduce emissions. Across the Nation, Indigenous food security efforts are helping improve community resilience to climate change while also improving cultural resilience. Some types of aquaculture have the potential to increase climate-smart protein production, human nutrition, and food security, although some communities have raised concerns over issues such as conflict with traditional livelihoods and the introduction of disease or pollution. { 10.2 , 11.1 , 29.6 , 25.5 ; Boxes 22.3 , 27.2 }

Homes and property are at risk from sea level rise and more intense extreme events

Homes, property, and critical infrastructure are increasingly exposed to more frequent and intense extreme events, increasing the cost of maintaining a safe and healthy place to live. Development in fire-prone areas and increases in area burned by wildfires have heightened risks of loss of life and property damage in many areas across the US. Coastal communities across the country—home to 123 million people (40% of the total US population)—are exposed to sea level rise (Figure 1.10 ), with millions of people at risk of being displaced from their homes by the end of the century. { 2.3 , 9.1 , 12.2 , 22.1 , 27.4 , 30.3 ; Figures A4.10 , A4.14 ; Focus on Western Wildfires }

People who regularly struggle to afford energy bills—such as rural, low-income, and older fixed-income households and communities of color—are especially vulnerable to more intense extreme heat events and associated health risks, particularly if they live in homes with poor insulation and inefficient cooling systems. For example, Black Americans are more likely to live in older, less energy efficient homes and face disproportionate heat-related health risks. { 5.2 , 15.2 , 15.3 , 22.2 , 26.4 , 32.4 ; Figure A4.4 }

Accessible public cooling centers can help protect people who lack adequate air-conditioning on hot days. Strategic land-use planning in cities, urban greenery, climate-smart building codes, and early warning communication can also help neighborhoods adapt. However, other options at the household scale, such as hardening homes against weather extremes or relocation, may be out of reach for renters and low-income households without assistance. { 12.3 , 15.3 , 19.3 , 22.2 }

Infrastructure and services are increasingly damaged and disrupted by extreme weather and sea level rise

Climate change threatens vital infrastructure that moves people and goods, powers homes and businesses, and delivers public services. Many infrastructure systems across the country are at the end of their intended useful life and are not designed to cope with additional stress from climate change. For example, extreme heat causes railways to buckle, severe storms overload drainage systems, and wildfires result in roadway obstruction and debris flows. Risks to energy, water, healthcare, transportation, telecommunications, and waste management systems will continue to rise with further climate change, with many infrastructure systems at risk of failing. { 12.2 , 13.1 , 15.2 , 23.4 , 26.5 ; Focus on Risks to Supply Chains }

In coastal areas, sea level rise threatens permanent inundation of infrastructure, including roadways, railways, ports, tunnels, and bridges; water treatment facilities and power plants; and hospitals, schools, and military bases. More intense storms also disrupt critical services like access to medical care, as seen after Hurricanes Irma and Maria in the US Virgin Islands and Puerto Rico. { 9.2 , 23.1 , 28.2 , 30.3 }

At the same time, climate change is expected to place multiple demands on infrastructure and public services. For example, higher temperatures and other effects of climate change, such as greater exposure to stormwater or wastewater, will increase demand for healthcare. Continued increases in average temperatures and more intense heatwaves will heighten electricity and water demand, while wetter storms and intensified hurricanes will strain wastewater and stormwater management systems. In the Midwest and other regions, aging energy grids are expected to be strained by disruptions and transmission efficiency losses from climate change. { 23.4 , 24.4 , 30.2 }

Forward-looking designs of infrastructure and services can help build resilience to climate change, offset costs from future damage to transportation and electrical systems, and provide other benefits, including meeting evolving standards to protect public health, safety, and welfare. Mitigation and adaptation activities are advancing from planning stages to deployment in many areas, including improved grid design and workforce training for electrification, building upgrades, and land-use choices. Grid managers are gaining experience planning and operating electricity systems with growing shares of renewable generation and working toward understanding the best approaches for dealing with the natural variability of wind and solar sources alongside increases in electrification. { 5.3 , 12.3 , 13.1 , 13.2 , 22.3 , 24.4 , 32.3 ; Figure 22.17 }

Climate change exacerbates existing health challenges and creates new ones

Climate change is already harming human health across the US, and impacts are expected to worsen with continued warming. Climate change harms individuals and communities by exposing them to a range of compounding health hazards, including the following:

More severe and frequent extreme events { 2.2 , 2.3 , 15.1 }

Wider distribution of infectious and vector-borne pathogens { 15.1 , 26.1 ; Figure A4.16 }

Air quality worsened by smog, wildfire smoke, dust, and increased pollen { 14.1 , 14.2 , 14.4 , 23.1 , 26.1 }

Threats to food and water security { 11.2 , 15.1 }

Mental and spiritual health stressors { 15.1 }

While climate change can harm everyone’s health, its impacts exacerbate long-standing disparities that result in inequitable health outcomes for historically marginalized people, including people of color, Indigenous Peoples, low-income communities, and sexual and gender minorities, as well as older adults, people with disabilities or chronic diseases, outdoor workers, and children. { 14.3 , 15.2 }

The disproportionate health impacts of climate change compound with similar disparities in other health contexts. For example, climate-related disasters during the COVID-19 pandemic, such as drought along the Colorado River basin, western wildfires, and Hurricane Laura, disproportionately magnified COVID-19 exposure, transmission, and disease severity and contributed to worsened health conditions for essential workers, older adults, farmworkers, low-wealth communities, and communities of color. { 15.2 ; Focus on COVID-19 and Climate Change }

Large reductions in greenhouse gas emissions are expected to result in widespread health benefits and avoided death or illness that far outweigh the costs of mitigation actions. Improving early warning, surveillance, and communication of health threats; strengthening the resilience of healthcare systems; and supporting community-driven adaptation strategies can reduce inequities in the resources and capabilities needed to adapt as health threats from climate change continue to grow. { 14.5 , 15.3 , 26.1 , 30.2 , 32.4 }

Ecosystems are undergoing transformational changes

Together with other stressors, climate change is harming the health and resilience of ecosystems, leading to reductions in biodiversity and ecosystem services. Increasing temperatures continue to shift habitat ranges as species expand into new regions or disappear from unfavorable areas, altering where people can hunt, catch, or gather economically important and traditional food sources. Degradation and extinction of local flora and fauna in vulnerable ecosystems like coral reefs and montane rainforests are expected in the near term, especially where climate changes favor invasive species or increase susceptibility to pests and pathogens. Without significant emissions reductions, rapid shifts in environmental conditions are expected to lead to irreversible ecological transformations by mid- to late century. { 2.3 , 6.2 , 7.1 , 7.2 , 8.1 , 8.2 , 10.1 , 10.2 , 21.1 , 24.2 , 27.2 , 28.5 , 29.3 , 29.5 , 30.4 ; Figure A4.12 }

Changes in ocean conditions and extreme events are already transforming coastal, aquatic, and marine ecosystems. Coral reefs are being lost due to warming and ocean acidification, harming important fisheries; coastal forests are converting to ghost forests, shrublands, and marsh due to sea level rise, reducing coastal protection; lake and stream habitats are being degraded by warming, heavy rainfall, and invasive species, leading to declines in economically important species. { 8.1 , 10.1 , 21.2 , 23.2 , 24.2 , 27.2 ; Figures 8.7 , A4.11 }

Increased risks to ecosystems are expected with further climate change and other environmental changes, such as habitat fragmentation, pollution, and overfishing. For example, mass fish die-offs from extreme summertime heat are projected to double by midcentury in northern temperate lakes under a very high scenario (RCP8.5). Continued climate changes are projected to exacerbate runoff and erosion, promote harmful algal blooms, and expand the range of invasive species. { 4.2 , 7.1 , 8.2 , 10.1 , 21.2 , 23.2 , 24.2 , 27.2 , 28.2 , 30.4 }

While adaptation options to protect fragile ecosystems may be limited, particularly under higher levels of warming, management and restoration measures can reduce stress on ecological systems and build resilience. These measures include migration assistance for vulnerable species and protection of essential habitats, such as establishing wildlife corridors or places where species can avoid heat. Opportunities for nature-based solutions that assist in mitigation exist across the US, particularly those focused on protecting existing carbon sinks and increasing carbon storage by natural ecosystems. { 8.3 , 10.3 , 23.2 , 27.2 ; Focus on Blue Carbon }

Climate change slows economic growth, while climate action presents opportunities

With every additional increment of global warming, costly damages are expected to accelerate. For example, 2°F of warming is projected to cause more than twice the economic harm induced by 1°F of warming. Damages from additional warming pose significant risks to the US economy at multiple scales and can compound to dampen economic growth. { 19.1 }

International impacts can disrupt trade, amplify costs along global supply chains, and affect domestic markets. { 17.3 , 19.2 ; Focus on Risks to Supply Chains }

While some economic impacts of climate change are already being felt across the country, the impacts of future changes are projected to be more significant and apparent across the US economy. { 19.1 }

States, cities, and municipalities confront climate-driven pressures on public budgets and borrowing costs amid spending increases on healthcare and disaster relief. { 19.2 }

Household consumers face higher costs for goods and services, like groceries and health insurance premiums, as prices change to reflect both current and projected climate-related damages. { 19.2 }

Mitigation and adaptation actions present economic opportunities. Public and private measures—such as climate financial risk disclosures, carbon offset credit markets, and investments in green bonds—can avoid economic losses and improve property values, resilience, and equity. However, climate responses are not without risk. As innovation and trade open further investment opportunities in renewable energy and the country continues to transition away from fossil fuels, loss and disposal costs of stranded capital assets such as coal mines, oil and gas wells, and outdated power plants are expected. Climate solutions designed without input from affected communities can also result in increased vulnerability and cost burden. { 17.3 , 19.2 , 19.3 , 20.2 , 20.3 , 27.1 , 31.6 }

Many regional economies and livelihoods are threatened by damages to natural resources and intensifying extremes

Climate change is projected to reduce US economic output and labor productivity across many sectors, with effects differing based on local climate and the industries unique to each region. Climate-driven damages to local economies especially disrupt heritage industries (e.g., fishing traditions, trades passed down over generations, and cultural heritage–based tourism) and communities whose livelihoods depend on natural resources. { 11.3 , 19.1 , 19.3 }

As fish stocks in the Northeast move northward and to deeper waters in response to rapidly rising ocean temperatures, important fisheries like scallops, shrimp, and cod are at risk. In Alaska, climate change has already played a role in 18 major fishery disasters that were especially damaging for coastal Indigenous Peoples, subsistence fishers, and rural communities. { 10.2 , 21.2 , 29.3 }

While the Southeast and US Caribbean face high costs from projected labor losses and heat health risks to outdoor workers, small businesses are already confronting higher costs of goods and services and potential closures as they struggle to recover from the effects of compounding extreme weather events. { 22.3 , 23.1 }

Agricultural losses in the Midwest, including lower corn yields and damages to specialty crops like apples, are linked to rapid shifts between wet and dry conditions and stresses from climate-induced increases in pests and pathogens. Extreme heat and more intense wildfire and drought in the Southwest are already threatening agricultural worker health, reducing cattle production, and damaging wineries. { 24.1 , 28.5 }

In the Northern Great Plains, agriculture and recreation are expected to see primarily negative effects related to changing temperature and rainfall patterns. By 2070, the Southern Great Plains is expected to lose cropland acreage as lands transition to pasture or grassland. { 25.3 , 26.2 }

Outdoor-dependent industries, such as tourism in Hawai‘i and the US-Affiliated Pacific Islands and skiing in the Northwest, face significant economic loss from projected rises in park closures and reductions in workforce as continued warming leads to deterioration of coastal ecosystems and shorter winter seasons with less snowfall. { 7.2 , 8.3 , 10.1 , 10.3 , 19.1 , 27.3 , 30.4 }

Mitigation and adaptation actions taken by businesses and industries promote resilience and offer long-term benefits to employers, employees, and surrounding communities. For example, as commercial fisheries adapt, diversifying harvest and livelihoods can help stabilize income or buffer risk. In addition, regulators and investors are increasingly requiring businesses to disclose climate risks and management strategies. { 10.2 , 19.3 , 26.2 }

Job opportunities are shifting due to climate change and climate action

Many US households are already feeling the economic impacts of climate change. Climate change is projected to impose a variety of new or higher costs on most households as healthcare, food, insurance, building, and repair costs become more expensive. Compounding climate stressors can increase segregation, income inequality, and reliance on social safety net programs. Quality of life is also threatened by climate change in ways that can be more difficult to quantify, such as increased crime and domestic violence, harm to mental health, reduced happiness, and fewer opportunities for outdoor recreation and play. { 11.3 , 19.1, 19.3 }

Climate change, and how the country responds, is expected to alter demand for workers and shift where jobs are available. For example, energy-related livelihoods in the Northern and Southern Great Plains are expected to shift as the energy sector transforms toward more renewables, low-carbon technologies, and electrification of more sectors of the economy. Losses in fossil fuel–related jobs are projected to be completely offset by greater increases in mitigation-related jobs, as increased demand for renewable energy and low-carbon technologies is expected to lead to long-term expansion in most states’ energy and decarbonization workforce (Figure 1.12 ). Grid expansion and energy efficiency efforts are already creating new jobs in places like Nevada, Vermont, and Alaska, and advancements in biofuels and agrivoltaics (combined renewable energy and agriculture) provide economic opportunities in rural communities. { 10.2 , 11.3 , 19.3 , 25.3 , 26.2 , 29.3 , 32.4 }

Additional opportunities include jobs in ecosystem restoration and construction of energy-efficient and climate-resilient housing and infrastructure. Workforce training and equitable access to clean energy jobs, which have tended to exclude women and people of color, are essential elements of a just transition to a decarbonized economy. { 5.3 , 19.3 , 20.3 , 22.3 , 25.3 , 26.2 , 27.3 , 32.4 }

Climate change is disrupting cultures, heritages, and traditions

As climate change transforms US landscapes and ecosystems, many deeply rooted community ties, pastimes, Traditional Knowledges, and cultural or spiritual connections to place are at risk. Cultural heritage—including buildings, monuments, livelihoods, and practices—is threatened by impacts on natural ecosystems and the built environment. Damages to archaeological, cultural, and historical sites further reduce opportunities to transfer important knowledge and identity to future generations. { 6.1 , 7.2 , 8.3 , 9.2 , 10.1 , 12.2 , 16.1 , 22.1 , 23.1 , 26.1 , 27.6 , 28.2 ; Introductions in Chs. 10 , 30 }

Many outdoor activities and traditions are already being affected by climate change, with overall impacts projected to further hinder recreation, cultural practices, and the ability of communities to maintain local heritage and a sense of place. { 19.1 }

For example:

The prevalence of invasive species and harmful algal blooms is increasing as waters warm, threatening activities like swimming along Southeast beaches, boating and fishing for walleye in the Great Lakes, and viewing whooping cranes along the Gulf Coast. In the Northwest, water-based recreation demand is expected to increase in spring and summer months, but reduced water quality and harmful algal blooms are expected to restrict these opportunities. { 24.2 , 24.5 , 26.3 , 27.6 }

Ranges of culturally important species are shifting as temperatures warm, making them harder to find in areas where Indigenous Peoples have access (see Box 1.3 ). { 11.2 , 24.2 , 26.1 }

Hikers, campers, athletes, and spectators face increasing threats from more severe heatwaves, wildfires, and floods and greater exposure to infectious disease. { 22.2 , 15.1 , 26.3 , 27.6 }

Nature-based solutions and ecosystem restoration can preserve cultural heritage while also providing valuable local benefits, such as flood protection and new recreational opportunities. Cultural heritage can also play a key role in climate solutions, as incorporating local values, Indigenous Knowledge, and equity into design and planning can help reaffirm a community’s connection to place, strengthen social networks, and build new traditions. { 7.3 , 26.1 , 26.3 , 30.5 }

The Choices That Will Determine the Future

With each additional increment of warming, the consequences of climate change increase. The faster and further the world cuts greenhouse gas emissions, the more future warming will be avoided, increasing the chances of limiting or avoiding harmful impacts to current and future generations.

Societal choices drive greenhouse gas emissions

The choices people make on a day-to-day basis—how to power homes and businesses, get around, and produce and use food and other goods—collectively determine the amount of greenhouse gases emitted. Human use of fossil fuels for transportation and energy generation, along with activities like manufacturing and agriculture, has increased atmospheric levels of carbon dioxide (CO 2 ) and other heat-trapping greenhouse gases. Since 1850, CO 2 concentrations have increased by almost 50%, methane by more than 156%, and nitrous oxide by 23%, resulting in long-term global warming. { 2.1 , 3.1 ; Ch. 2, Introduction }

The CO 2 not removed from the atmosphere by natural sinks lingers for thousands of years. This means that CO 2 emitted long ago continues to contribute to climate change today. Because of historical trends, cumulative CO 2 emissions from fossil fuels and industry in the US are higher than from any other country. To understand the total contributions of past actions to observed climate change, additional warming from CO 2 emissions from land use, land-use change, and forestry, as well as emissions of nitrous oxide and the shorter-lived greenhouse gas methane, should also be taken into account. Accounting for all of these factors and emissions from 1850–2021, emissions from the US are estimated to comprise approximately 17% of current global warming. { 2.1 }

Carbon dioxide, along with other greenhouse gases like methane and nitrous oxide, is well-mixed in the atmosphere. This means these gases warm the planet regardless of where they were emitted. For the first half of the 20th century, the vast majority of greenhouse gas emissions came from the US and Europe. But as US and European emissions have been falling (US emissions in 2021 were 17% lower than 2005 levels), emissions from the rest of the world, particularly Asia, have been rising rapidly. The choices the US and other countries make now will determine the trajectory of climate change and associated impacts for many generations to come (Figure 1.13 ). { 2.1 , 2.3 ; Ch. 32 }

Rising global emissions are driving global warming, with faster warming in the US

The observed global warming of about 2°F (1.1°C) over the industrial era is unequivocally caused by greenhouse gas emissions from human activities, with only very small effects from natural sources. About three-quarters of total emissions and warming (1.7°F [0.95°C]) have occurred since 1970. Warming would have been even greater without the land and ocean carbon sinks, which have absorbed more than half of the CO 2 emitted by humans. { 2.1 , 3.1 , 7.2 ; Ch. 2, Introduction ; Figures 3.1 , 3.8 }

The US is warming faster than the global average, reflecting a broader global pattern: land areas are warming faster than the ocean, and higher latitudes are warming faster than lower latitudes. Additional global warming is expected to lead to even greater warming in some US regions, particularly Alaska (Figure 1.14 ). { 2.1 , 3.4 ; Ch. 2, Introduction ; App. 4 }

Warming increases risks to the US

Rising temperatures lead to many large-scale changes in Earth’s climate system, and the consequences increase with warming (Figure 1.15 ). Some of these changes can be further amplified through feedback processes at higher levels of warming, increasing the risk of potentially catastrophic outcomes. For example, uncertainty in the stability of ice sheets at high warming levels means that increases in sea level along the continental US of 3–7 feet by 2100 and 5–12 feet by 2150 are distinct possibilities that cannot be ruled out. The chance of reaching the upper end of these ranges increases as more warming occurs. In addition to warming more, the Earth warms faster in high and very high scenarios (SSP3-7.0 and SSP5-8.5, respectively), making adaptation more challenging. { 2.3 , 3.1 , 3.4 , 9.1 }

How Climate Action Can Create a More Resilient and Just Nation

Large near-term cuts in greenhouse gas emissions are achievable through many currently available and cost-effective mitigation options. However, reaching net-zero emissions by midcentury cannot be achieved without exploring additional mitigation options. Even if the world decarbonizes rapidly, the Nation will continue to face climate impacts and risks. Adequately and equitably addressing these risks involves longer-term inclusive planning, investments in transformative adaptation, and mitigation approaches that consider equity and justice.

Available mitigation strategies can deliver substantial emissions reductions, but additional options are needed to reach net zero

Limiting global temperature change to well below 2°C (3.6°F) requires reaching net-zero CO 2 emissions globally by 2050 and net-zero emissions of all greenhouse gases from human activities within the following few decades (see “Meeting US mitigation targets means reaching net-zero emissions” above). Net-zero emissions pathways involve widespread implementation of currently available and cost-effective options for reducing emissions alongside rapid expansion of technologies and methods to remove carbon from the atmosphere to balance remaining emissions. However, to reach net-zero emissions, additional mitigation options need to be explored (Figure 1.16 ). Pathways to net zero involve large-scale technological, infrastructure, land-use, and behavioral changes and shifts in governance structures. { 5.3 , 6.3 , 9.2 , 9.3 , 10.4 , 13.2 , 16.2 , 18.4 , 20.1 , 24.1 , 25.5 , 30.5 , 32.2 , 32.3 ; Focus on Blue Carbon }

Scenarios that reach net-zero emissions include some of the following key options:

Decarbonizing the electricity sector, primarily through expansion of wind and solar energy, supported by energy storage { 32.2 }

Transitioning to transportation and heating systems that use zero-carbon electricity or low-carbon fuels, such as hydrogen { 5.3 , 13.1 , 32.2 , 32.3 }

Improving energy efficiency in buildings, appliances, and light- and heavy-duty vehicles and other transportation modes { 5.3 , 13.3 , 32.2 }

Implementing urban planning and building design that reduces energy demands through more public transportation and active transportation and lower cooling demands for buildings { 12.3 , 13.1 , 32.2 }

Increasing the efficiency and sustainability of food production, distribution, and consumption { 11.1 , 32.2 }

Improving land management to decrease greenhouse gas emissions and increase carbon removal and storage, with options ranging from afforestation, reforestation, and restoring coastal ecosystems to industrial processes that directly capture and store carbon from the air { 5.3 , 6.3 , 8.3 , 32.2 , 32.3 ; Focus on Blue Carbon }

Due to large declines in technology and deployment costs over the last decade (Figure 1.2 ), decarbonizing the electricity sector is expected to be largely driven by rapid growth in renewable energy. Recent legislation is also expected to increase deployment rates of low- and zero-carbon technology. To reach net-zero targets, the US will need to add new electricity-generating capacity, mostly wind and solar, faster than ever before. This infrastructure expansion may drastically increase demand for products (batteries, solar photovoltaics) and resources, such as metals and critical minerals. Near-term shortages in minerals and metals due to increased demand can be addressed by increased recycling, for example, which can also reduce dependence on imported materials. { 5.2, 5.3 , 17.2 , 25.3 , 32.2 , 32.4 ; Focus on Risks to Supply Chains }

Most US net-zero scenarios require CO 2 removal from the atmosphere to balance residual emissions, particularly from sectors where decarbonization is difficult. In these scenarios, nuclear and hydropower capacity are maintained but not greatly expanded; natural gas–fired generation declines, but more slowly if coupled with carbon capture and storage. { 32.2 }

Nature-based solutions that restore degraded ecosystems and preserve or enhance carbon storage in natural systems like forests, oceans, and wetlands, as well as agricultural lands, are cost-effective mitigation strategies. For example, with conservation and restoration, marine and coastal ecosystems could capture and store enough atmospheric carbon each year to offset about 3% of global emissions (based on 2019 and 2020 emissions). Many nature-based solutions can provide additional benefits, like improved ecosystem resilience, food production, improved water quality, and recreational opportunities. { 8.3 ; Boxes 7.2 , 32.2 ; Focus on Blue Carbon }

Adequately addressing climate risks involves transformative adaptation

While adaptation planning and implementation has advanced in the US, most adaptation actions to date have been incremental and small in scale (see Table 1.3 ). In many cases, more transformative adaptation will be necessary to adequately address the risks of current and future climate change. { 31.1 , 31.3 }.

Transformative adaptation involves fundamental shifts in systems, values, and practices, including assessing potential trade-offs, intentionally integrating equity into adaptation processes, and making systemic changes to institutions and norms. While barriers to adaptation remain, many of these can be overcome with financial, cultural, technological, legislative, or institutional changes. { 31.1 , 31.2 , 31.3 }.

Adaptation planning can more effectively reduce climate risk when it identifies not only disparities in how people are affected by climate change but also the underlying causes of climate vulnerability. Transformative adaptation would involve consideration of both the physical and social drivers of vulnerability and how they interact to shape local experiences of vulnerability and disparities in risk. Examples include understanding how differing levels of access to disaster assistance constrain recovery outcomes or how disaster damage exacerbates long-term wealth inequality. Effective adaptation, both incremental and transformative, involves developing and investing in new monitoring and evaluation methods to understand the different values of, and impacts on, diverse individuals and communities. { 9.3 , 19.3 , 31.2 , 31.3 , 31.5 }

Transformative adaptation would require new and better-coordinated governance mechanisms and cooperation across all levels of government, the private sector, and society. A coordinated, systems-based approach can support consideration of risks that cut across multiple sectors and scales, as well as the development of context-specific adaptations. For example, California, Florida, and other states have used informal regional collaborations to develop adaptation strategies tailored to their area. Adaptation measures that are designed and implemented using inclusive, participatory planning approaches and leverage coordinated governance and financing have the greatest potential for long-term benefits, such as improved quality of life and increased economic productivity. { 10.3 , 18.4 , 20.2 , 31.4 }

Mitigation and adaptation actions can result in systemic, cascading benefits

Actions taken now to accelerate net emissions reductions and adapt to ongoing changes can reduce risks to current and future generations. Mitigation and adaptation actions, from international to individual scales, can also result in a range of benefits beyond limiting harmful climate impacts, including some immediate benefits (Figure 1.1 ). The benefits of mitigation and proactive adaptation investments are expected to outweigh the costs. { 2.3 , 13.3 , 14.5 , 15.3 , 17.4 , 22.1 , 31.6 , 32.4 ; Introductions in Chs. 17 , 31 }

Accelerating the deployment of low-carbon technologies, expanding renewable energy, and improving building efficiency can have significant near-term social and economic benefits like reducing energy costs and creating jobs. { 32.4 }

Transitioning to a carbon-free, sustainable, and resilient transportation system can lead to improvements in air quality, fewer traffic fatalities, lower costs to travelers, improved mental and physical health, and healthier ecosystems. { 13.3 }

Reducing emissions of short-lived climate pollutants like methane, black carbon, and ozone provides immediate air quality benefits that save lives and decrease the burden on healthcare systems while also slowing near-term warming. { 11.1 , 14.5 , 15.3 }

Green infrastructure and nature-based solutions that accelerate pathways to net-zero emissions through restoration and protection of ecological resources can improve water quality, strengthen biodiversity, provide protection from climate hazards like heat extremes or flooding, preserve cultural heritage and traditions, and support more equitable access to environmental amenities. { 8.3 , 15.3 , 20.3 , 24.4 , 30.4 ; Focus on Blue Carbon }

Strategic planning and investment in resilience can reduce the economic impacts of climate change, including costs to households and businesses, risks to markets and supply chains, and potential negative impacts on employment and income, while also providing opportunities for economic gain. { 9.2 , 19.3 , 26.2 , 31.6 ; Focus on Risks to Supply Chains }

Improving cropland management and climate-smart agricultural practices can strengthen the resilience and profitability of farms while also increasing soil carbon uptake and storage, reducing emissions of nitrous oxide and methane, and enhancing agricultural efficiency and yields. { 11.1 , 24.1 , 32.2 }

Climate actions that incorporate inclusive and sustained engagement with overburdened and underserved communities in the design, planning, and implementation of evidence-based strategies can also reduce existing disparities and address social injustices. { 24.3 , 31.2 , 32.4 }

Transformative climate actions can strengthen resilience and advance equity

Fossil fuel–based energy systems have resulted in disproportionate public health burdens on communities of color and/or low-income communities. These same communities are also disproportionately harmed by climate change impacts. { 13.4 , 15.2 , 32.4 }

A “just transition” is the process of responding to climate change with transformative actions that address the root causes of climate vulnerability while ensuring equitable access to jobs; affordable, low-carbon energy; environmental benefits such as reduced air pollution; and quality of life for all. This involves reducing impacts to overburdened communities, increasing resources to underserved communities, and integrating diverse worldviews, cultures, experiences, and capacities into mitigation and adaptation actions. As the country shifts to low-carbon energy industries, a just transition would include job creation and training for displaced fossil fuel workers and addressing existing racial and gender disparities in energy workforces. For example, Colorado agencies are creating plans to guide the state’s transition away from coal, with a focus on economic diversification, job creation, and workforce training for former coal workers. The state’s plan also acknowledges a commitment to communities disproportionately impacted by coal power pollution. { 5.3 , 13.4 , 14.3 , 15.2 , 16.2 , 20.3 , 31.2 , 32.4 ; Figure 20.1 }

A just transition would take into account key aspects of environmental justice:

Recognizing that certain people have borne disparate burdens related to current and historical social injustices and, thus, may have different needs

Ensuring that people interested in and affected by outcomes of decision-making processes are included in those procedures through fair and meaningful engagement

Distributing resources and opportunities over time, including access to data and information, so that no single group or set of individuals receives disproportionate benefits or burdens

{ 20.3 ; Figure 20.1 }

An equitable and sustainable US response to climate change has the potential to reduce climate impacts while improving well-being, strengthening resilience, benefiting the economy, and, in part, redressing legacies of racism and injustice. Transformative adaptation and the transition to a net-zero energy system come with challenges and trade-offs that would need to be considered to avoid exacerbating or creating new social injustices. For example, transforming car-centric transportation systems to emphasize public transit and walkability could increase accessibility for underserved communities and people with limited mobility—if user input and equity are intentionally considered. { 13.4 , 20.3 , 31.3 , 32.4 ; Ch. 31, Introduction }

Equitable responses that assess trade-offs strengthen community resilience and self-determination, often fostering innovative solutions. Engaging communities in identifying challenges and bringing together diverse voices to participate in decision-making allows for more inclusive, effective, and transparent planning processes that account for the structural factors contributing to inequitable climate vulnerability. { 9.3 , 12.4 , 13.4 , 20.2 , 31.4 }

Cover image

Two volunteers help demonstrate and install solar panels in Highland Park, Michigan, in May 2021. The event was hosted by the local nonprofit Soulardarity, which teaches local residents about solar power, installs solar-powered streetlights that also provide wireless internet access, and helps local communities build a just and equitable energy system. Adopting energy storage with decentralized solutions, such as microgrids or off-grid systems, can promote energy equity in overburdened communities. Photo credit: Nick Hagen.

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• 14 November 2023

The Amazon’s record-setting drought: how bad will it be?

• Meghie Rodrigues

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Rivers that cut through the Amazon rainforest are falling to their lowest levels on record. Credit: Bruno Kelly/Reuters

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Last month, a portion of the Negro River in the Amazon rainforest near Manaus, Brazil, shrank to a depth of just 12.7 metres — its lowest level in 120 years, when measurements began. In Lake Tefé, about 500 kilometres west, more than 150 river dolphins were found dead, not because of low water levels, but probably because the lake had reached temperatures close to 40 °C.

‘We are killing this ecosystem’: the scientists tracking the Amazon’s fading health

These are symptoms of the unprecedented drought gripping the Amazon rainforest this year. Climate change is involved. But researchers who study the rainforest say other factors have come together to exacerbate this crisis, which has cut river communities off from supplies including food, and has forced Indigenous residents to use dirty, contaminated water, resulting in gastrointestinal and other illnesses.

The drought is the sum of three things, says Luciana Gatti, a climate-change researcher at Brazil's National Institute for Space Research (Inpe) in São José dos Campos. The first is deforestation, “which is killing the rainforest’s resilience and turning it into a drier, hotter place”, she says.

Fire season

Deforestation in the Brazilian Amazon dropped between January and July this year — by 42.5% compared with the same period in 2022, according to data from Inpe — but this follows a number of years of record destruction. The main culprit, say researchers who spoke to Nature , is agribusiness. Ranchers and farmers have cleared trees to expand Brazil’s agricultural area by about 50% over the past four decades, mostly in the Amazon, according to a report from MapBiomas , a consortium of academic, business and non-governmental organizations that monitor land use in the country.

About 20% of the Amazon rainforest is deforested, and 40% is degraded — which means trees are still standing, but their health has faded and they are prone to fire and drought, Gatti says. “That was all done by humans.”

Researchers study a freshwater dolphin ( Inia geoffrensis ) found dead in Lake Tefé amid a record drought in the Amazon rainforest. Credit: Gustavo Basso/NurPhoto via Getty

Making matters worse is the second factor contributing to the drought: an El Niño climate pattern has begun this year.

El Niño is a phase of a phenomenon called the El Niño–Southern Oscillation, and occurs every two to seven years. During El Niño, winds that usually blow east to west along the Equator weaken or reverse, and warm water pushes into the eastern tropical Pacific Ocean. Precipitation patterns change in South America, causing dry air in the north, where the rainforest lies, and damp air in the south. As a result, Uruguay is currently being slammed by heavy rains. In the past few months, Paraguay, Argentina and southern Brazil have experienced floods that have killed dozens of people and left thousands of others without shelter.

But in northern and northeast Brazil, 8 states have had the lowest July to September precipitation levels in 40 years, according to the Brazilian National Center for Early Warning and Monitoring for Natural Disasters (Cemaden) in São José dos Campos. These months are the peak of the ‘fire season’ in most of the Amazon.

Oil from the Amazon? Proposal to drill at river’s mouth worries researchers

Dry spells in the Amazon have consequences in addition to low water levels. Ranchers and others clearing the rainforest don’t burn trees when it’s rainy or when the air is humid, says Erika Berenguer, an ecosystems researcher at the University of Oxford, UK. But because El Niño has made the rainforest’s air dry, those who are clearing trees have been burning them, Berenguer says. This has added to the harsh conditions and has sparked some uncontrolled fires — something she experienced at first hand when she visited the town of Belterra in the northern state Pará in September.

“We would sleep and wake up surrounded by smoke,” Berenguer says. Ironically, she was there with a team to study how vulnerable the rainforest is to fire. Things got so bad that she had to evacuate for ten days. “I was shorter of breath than when I got COVID — and I am among those who can leave and get medicine. What about those who can’t?” she asks. “This is collective poisoning.”

A visible pattern

The third factor responsible for the Amazon’s severe drought is an unusual warming of the water in the northern Atlantic Ocean. Climate change is contributing to this anomaly, says Maria Assunção Dias, a climatologist at the University of São Paulo in Brazil. The warming of these waters has affected the intertropical convergence zone. This region, which circles Earth close to the Equator, “is one of the main meteorological systems acting in the tropics and is a region of intense cloud and rain formation”, says Karina Lima, a geographer at the Federal University of Rio Grande do Sul in Porto Alegre. The zone has shifted north, taking the storms with it, away from northern Brazil.

When will the Amazon hit a tipping point?

All of this adds up to a record-setting year for the Amazon. The rainforest has experienced dry spells in the past, but severe droughts “are becoming more frequent”, Dias says. There is a visible pattern, she adds, citing extreme droughts in 1912, 1925, 1983, 1987, 1998, 2010, 2016 and now 2023.

One big problem is that the current El Niño is just getting started. So “things are not going to get any better”, Gatti says.

It might even turn into a ‘super’ El Niño, Dias says. This could occur if the sea surface temperature in the tropical Pacific reaches 2.5 °C higher than average — a possibility, given that 2023 looks set to be the hottest year ever recorded on Earth . Last week, the World Meteorological Organization issued a statement that there is a 90% likelihood that El Niño will persist at least until the end of April.

Although it is hard to predict when the next drought might grip the Amazon, studies have shown that climate change is messing with the timing of El Niño. “The tendency is that we have stronger and more frequent episodes,” Lima says. This could be catastrophic for the Amazon rainforest, already battered by deforestation and a warming and drying climate. “The forest’s tipping point is coming closer — and it’s coming quick.”

Nature 623 , 675-676 (2023)

doi: https://doi.org/10.1038/d41586-023-03469-6

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India: A Case Study in Climate Mitigation and Adaptation

This article explores the difficult trade-offs that need to be made between the competing claims of climate mitigation, adaptation, and economic development..

Thursday, September 14, 2023

By Maxine Nelson

This article has been extensively updated, incorporating new COP 27 commitments, Reserve Bank of India (RBI) statements and current green bond issuance. It was originally published Oct. 18, 2021.

After decades of population growth and economic development, India is now the third largest emitter of greenhouse gases in the world. In addition, India is among the countries most vulnerable to climate change due to its geography and dependence on agriculture.

It has been estimated that if emissions are not significantly reduced, India could suffer economic losses of USD 35 trillion . Indeed, much of India has been experiencing annual heatwaves followed by intense flooding, and in 2021 alone it experienced even more  extreme weather events — including cyclones and a glacier collapse. Thus, India makes a thought-provoking case study for policymakers and risk professionals given the difficult trade-offs that need to be made between the competing claims of climate mitigation, adaptation and economic development.

Climate Change’s Effect on India

The banking regulator, Reserve Bank of India (RBI) , explains that “India has witnessed changes in climatic patterns in line with the rest of the world… the rainfall pattern, particularly with respect to the [south west monsoon] SWM, which provides around 75 percent of the annual rainfall, has undergone significant changes. Moreover, the occurrence of extreme weather events like floods/unseasonal rainfall, heat waves and cyclones has increased during the past two decades, and data reveal that some of the key agricultural states in India have been the most affected by such events.”

A more recent, detailed RBI study points out that “it is the increased frequency of extreme weather occurrences that is breaking the back of our capability to cope with natural disasters.” As shown by India’s nationally determined contributions (NDCs) — the actions it has committed to take to reduce its emissions and adapt to the impacts of climate change — it is among the most vulnerable countries in the world to the impact of accelerated sea level rise from global warming. This is due to its long coastline, large number of islands and population of 170 million living in coastal regions.

The RBI also notes that precipitation and temperature — the two key climate indicators — “play a crucial role in the overall health of the Indian economy.” As well as affecting food production, the extreme weather in agricultural states impacts employment and GDP, with approximately 44% of the working population employed in agriculture and allied sectors which contribute about 20% of GDP, according to M.K. Jain, the deputy governor of the Reserve Bank . Several challenges confronting Indian agriculture, including diminishing and degrading natural resources and unprecedented climate change, need to be tackled for the long-term sustainability and viability of Indian agriculture.

However, there is uncertainty over how large the impacts might be. The Swiss Re Institute , for example, estimates a 35% reduction in the level of India’s GDP by 2050 if greenhouse gas emissions are not reduced globally, and approximately a 6% GDP reduction even if the Paris Agreement goals are met. An Oxford Economics report “Estimating the Economic Impact of Global Warming” has framed the impact differently, estimating that India’s GDP could be 90% lower in 2100 than it would be if there was no climate change, suggesting that climate change has the potential to absorb all of India’s future prospective growth in income per capita. And  Deloitte  has estimated USD 35 trillion of economic losses by 2070. While these different approaches produce diverse estimates, they all show that the impact will be big and require additional investments in both mitigation and adaptation.

India’s Effect on Climate Change

Not only will the changing climate have a significant impact on India, but India is also expected to have a significant impact on the climate. Although historically it has not had high emissions, India rose to the number three spot in the national emissions rankings 15 years ago, behind China and the U.S. The RBI noted that “With the increase in population, the cumulative level of greenhouse gas (GHG) emissions has increased, resulting in a rise of average temperature. According to a study by the International Energy Agency (IEA), India emitted 2,299 million tonnes of carbon dioxide (CO 2 ) in 2018, a rise of 4.8% over the previous year.”

Unfortunately, India’s future potential emissions are not yet aligned with the Paris Agreement goals. India’s NDCs currently correspond to temperature increases above 3°C, according to Climate Action Tracker . (You can find out more about NDCs and their place in the Paris Agreement in this  short article . ) India increased its commitment to reduce greenhouse gas emissions at COP 26, the 2021 annual meeting of the signatories of the Paris Agreement, where it pledged to cut its emissions to net zero by 2070. While this was a large increase in commitment, it isn’t yet aligned with the worldwide goal of cutting emissions to net zero by 2050 needed to limit global warming to 1.5°C.

Maxine Nelson

In advance of COP 27, India has again increased its commitment and pledged to a 45% reduction in GDP emissions intensity by 2030 — marking an 10% increase from the previous pledge. Any emissions reduction is helpful to mitigate climate change. However, as the pledge is based on emissions intensity and not absolute emissions, emissions can continue increasing as the economy expands. This pledge, therefore, doesn’t meet the net-zero goal of reducing emissions by 45% by 2030. Still, the effort required to overcome the challenge of rapidly expanding an economy while decreasing emissions intensity needs to be appreciated.

To further mitigate climate change, India may need to agree to reduce its emissions even more — a big task for a developing economy with average annual energy consumption of a third the global average, and per capita emissions already 10 times lower than that of the U.S., four times lower than China, and three times lower than Europe. With IPCC reports highlighting the urgency of tackling climate change quickly to reduce the loss and damage for humans and ecosystems, it is even more important that emissions reductions are ambitious.

Financing Mitigation and Adaptation

A 2021 RBI Financial Stability Report noted that climate change and the associated mitigating policy commitments are “set to reshape the macroeconomic and financial landscape”. Extensive funding is needed both to reduce future emissions and to finance the adaptation needed to manage the impacts of climate change. In their 2016 NDC, India estimated that at least USD 2.5 trillion (at 2014-15 prices) would be required for meeting its climate change actions between 2016 and 2030. And the International Energy Agency estimates that nearly 60% of India’s CO 2 emissions in the late 2030s will be coming from infrastructure and machines that do not exist today. If this investment is to be sustainable, USD 1.4 trillion extra funding (above that required for current policies) is needed over the next 20 years.

Like most of the world, green bond issuance in India — which could provide some of this funding — is currently a small proportion of all bond issuance.  The rate of issuance is increasing, however, with USD 21.6 billion of green, sustainable or social bonds issued in 2022. And in 2023, the Government of India entered the green finance market issuing USD 2 billion of green bonds to finance their spending on a range of projects including solar power, green hydrogen and afforestation. As they obtained a greenium (lower financing costs than other equivalent bonds), we should expect to see more of these issued in the future.

There are also substantial opportunities in other financial markets, such as the  development of a derivatives market   to aid adaptation via products such as:

• agricultural commodity derivatives, which can help reduce risks by enabling continuous price discovery and providing hedging
• weather derivatives, which can hedge the risks of high-probability, low-risk events

Of course, meeting the needs of climate change financing carries the usual financial risk implications of any lending. An RBI analysis shows that banks’ direct exposure to fossil fuels (through electricity, chemicals and cars) is 10% of total outstanding non-retail bank credit, so it should have a limited impact on the banking system. However, it notes that many other industries indirectly use fossil fuels and their impacts also need to be closely monitored.

Regulatory Response

The RBI has noted that policy measures such as a deepening of the corporate bond market, standardization of green investment terminology, consistent corporate reporting and removing information asymmetry between investors and recipients can make a significant contribution in addressing some of the shortcomings of the green finance market.

Like in most of the rest of the world, there is an increasing regulatory focus on climate risk. The RBI Governor has stated that guidelines will be issued about disclosure of climate-related risks, and also scenario analysis and stress testing. This followed last year’s RBI consultation which asked for inputs on a comprehensive range of topics from climate risk governance to strategy, and risk monitoring, management and mitigation at regulated entities. This consultation, in turn, built on the results of an RBI survey of banks that was also published last year. The survey found that “although banks have begun taking steps in the area of climate risk and sustainable finance, there remains a need for concerted effort and further action in this regard.” It also found that board-level engagement is inadequate, and few banks had a strategy for incorporating climate risk into their risk management framework. To see what leading climate risk firms are doing globally look at GARP’s whitepaper: “ Climate Risk Leadership: Lessons From 4 Annual Surveys .”

Given the widespread impact of climate change, it isn’t just the banking regulator that is looking at how climate risk will affect firms in its jurisdiction. In 2021, the Securities and Exchange Board of India (SEBI) mandated that the largest 1,000 listed firms complete a Business Responsibility and Sustainability Report . The report asks for information like material ESG risks and opportunities and their financial implications; sustainability related targets and performance; and their greenhouse gas emissions. Companies’ value chains also need to be assessed. This requirement is being progressively rolled out from 2023 to 2027, with the largest companies also required to get assurance of their disclosures.

In addition, SEBI has altered the rules for mutual funds , allowing them to have multiple ESG schemes with different strategies; in the past, a mutual fund could only have one ESG fund. This increase in scope follows one for green debt securities , which was expanded to include bonds such as blue bonds (sustainable water management and marine sector), yellow bonds (solar energy generation and transmission), transition bonds and adaptation bonds. Both of these expansions in scope should increase financing for sustainability related initiatives.

Reflecting the fact that addressing climate change is a global problem, needing both local and global solutions, the RBI joined the Network for Greening the Financial System (NGFS) in April 2021. The NGFS’s purpose is to strengthen the global response required to meet the goals of the Paris Agreement and to enhance the role of the financial system to manage risks and to mobilize capital for green and low-carbon investments. These goals align very well with the work India needs to undertake to make not just its financial system resilient to the risks from climate change, but to balance mitigation, adaptation, and economic development across the country.

Maxine Nelson , Ph.D, Senior Vice President, GARP Risk Institute, currently focusses on sustainability and climate risk management. She has extensive experience in risk, capital and regulation gained from a wide variety of roles across firms including Head of Wholesale Credit Analytics at HSBC. She also worked at the U.K. Financial Services Authority, where she was responsible for counterparty credit risk during the last financial crisis.

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ISATECH 2022: Novel Techniques in Maintenance, Repair, and Overhaul pp 255–262 Cite as

Aviation Carbon Accounting for Climate Change Mitigation: The Case of Turkey

• Orhan Yucel 11 ,
• Alper Dalkiran 12 ,
• Seval Kardes Selimoglu 13 &
• T. Hikmet Karakoc 14 , 15  
• Conference paper
• First Online: 23 November 2023

Part of the Sustainable Aviation book series (SA)

Global warming has become one of the biggest challenges of our world. Aviation industry, as a major contributor, is expected to play its own part in fighting the global warming by accounting for its carbon emissions. This study examines the approaches and strategies of airline companies for carbon accounting and reducing their contribution to global warming. Turkish Airlines, Turkey’s largest airline, is studied to reflect how Turkey’s air transport industry addresses the challenges of global warming and accounts for its carbon emissions. The study aims to set an example for other countries and businesses by showcasing the approaches and strategies of Turkish airline companies on carbon accounting.

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Abbreviations

Air Transport Action Group

International Air Transport Association

European Union Emission Trading System

Carbon Offsetting and Reduction Scheme for International Aviation

International Civil Aviation Organization

General Directorate for Civil Aviation

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Cappadocia Vocational Collage, Cappadocia University, Nevsehir, Türkiye

Orhan Yucel

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Alper Dalkiran

Faculty of Economics and Administrative Sciences, Anadolu University, Eskişehir, Türkiye

Seval Kardes Selimoglu

Department of Airframe and Powerplant Maintenance, Institute of Graduate Programs, Eskisehir Technical University, Eskişehir, Türkiye

T. Hikmet Karakoc

Information Technology Research and Application Center, Istanbul Ticaret University, Istanbul, Türkiye

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Jelena Svorcan

School of Aviation, Süleyman Demirel University, Keciborlu, Isparta, Türkiye

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Yucel, O., Dalkiran, A., Selimoglu, S.K., Karakoc, T.H. (2024). Aviation Carbon Accounting for Climate Change Mitigation: The Case of Turkey. In: Karakoc, T.H., et al. Novel Techniques in Maintenance, Repair, and Overhaul. ISATECH 2022. Sustainable Aviation. Springer, Cham. https://doi.org/10.1007/978-3-031-42041-2_32

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Forthcoming water study on climate mitigation measures

Work to evaluate the water requirements for implementing various climate mitigation measures is underway. The final study, by UN-Water in partnership with the International Universities Climate Alliance, is planned for the 2024 Bonn Climate Conference in June 2024. 

The UN Water Expert Group on Water and Climate Change is undertaking a study on the water requirements of climate mitigation measures, in partnership with the International Universities Climate Alliance (IUCA) . This follows the Technical Workshop on Water and Climate Change Mitigation , held in Bonn on 13 June 2023, which set out to identify what is known and not known about the dependency of Paris Agreement targets on the sustainable management of water resources. 

The study is being led by United Nations Economic Commission for Europe (UNECE), United Nations Educational, Scientific and Cultural Organization (UNESCO) and World Meteorological Organization (WMO) as the co-coordinators of the Expert Group, and based on the terms of reference . 

A progress report of the study will be presented at the 28th meeting of the Conference of the Parties (COP 28) that will convene from 30 November to 12 December 2023 in Dubai, United Arab Emirates. The final report will be used to prepare a UN Water Analytical Brief on this topic to be available for the 2024 Bonn Climate Conference in June 2024. 

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Study on options for mitigating climate change in agriculture by putting a price on emissions and rewarding carbon farming

A new study investigates ways to price GHG emissions from agricultural activities along the agri-food value chain and how this can be accompanied by providing farmers and other landowners with financial incentives for climate action.

Agriculture has a positive and important role to play in climate change mitigation: crops, hedgerows and trees sequester carbon from the atmosphere and properly managed soils provide carbon storage. However, agriculture contributes more than 10% of the EU’s total greenhouse gas (GHG) emissions, chiefly through the release of methane and nitrous oxide.

The EU is committed to become climate-neutral by 2050, and that means significant efforts to reduce emissions are necessary across all sectors. This includes agriculture, which is also one of the sectors most affected by climate change. As a contribution to identifying possible policy approaches to reducing emissions in this sector, the European Commission has today published a new independent study on “ Pricing agricultural emissions and rewarding climate action in the agri-food value chain ”.

The study, commissioned by the Commission’s Directorate General for Climate Action, was undertaken by Trinomics and its partners the Institute for European Environmental Policy (IEEP) and Ecologic, together with the Austrian Environment Agency (Umweltbundesamt) and consultancy Carbon Counts. It responds to a 2021 report by the European Court of Auditors, which recommended that the Commission should “assess the potential of applying the polluter-pays principle to agricultural emissions, and reward farmers for long-term carbon removals”.  The study aims to investigate possible ways to price GHG emissions from agricultural activities along the agri-food value chain and to explore how this can be accompanied by providing farmers and other landowners with financial incentives for climate action and carbon farming.

The first part of the study presents five policy options as workable examples for an emission trading system (separate from the already existing EU ETS ) that could incentivise climate mitigation action in the agri-food sector (AgETS). Here the authors outline the design aspects of five AgETS options covering different GHGs and targeting different actors in the agri-food value chain (not only farmers but also input providers and food processors), and assessed these options according to their effectiveness, efficiency, relevance, coherence and added value.

The study then investigates how a future AgETS could financially reward carbon removals from Land Use, Land Use Change, and Forestry (LULUCF). Here, the authors provide an overview of LULUCF removals options in the EU, put forward key considerations when combining an AgETS with incentives for removals, and assessed the strengths and weaknesses of five potential policy models for linking removals to an AgETS.

In preparing this report, the consortium consulted relevant stakeholders through an expert roundtable, a public workshop , a stakeholder survey and interviews with experts. The feedback from these consultation activities informed the findings presented in the study and is summarised in three separate annexes .

The study recognises benefits in combining design aspects of the different AgETS policy options and establishing a harmonised GHG reporting tool in the EU. In addition, the study stresses the importance of continuously monitoring and regularly evaluating any policy that would link carbon removals in an AgETS market and notes that the simplest policy models that involve the smallest administrative costs and pose less risks to environmental integrity could be an appropriate starting point. Finally, the authors underline that any policy establishing an AgETS policy and linking it to carbon removal support must be accompanied by wider sectoral and economy-wide changes (e.g., transitional aid through the form of subsidies, grants, and loans for farms) to fully enable the land sector to strengthen its sustainability.

Today’s study provides first insights on possible pricing and carbon farming measures in the agri-food value chain on how the agriculture and LULUCF sectors could further reduce greenhouse gas emissions and contribute to the EU’s long-term goal of net-zero GHG emissions by 2050. Together with other inputs considering a range of measures, the study will inform the policy debate that will follow the publication in the first quarter of 2024 of the European Commission’s Communication on an EU Climate target for 2040. That Communication, based on a detailed impact assessment, will prepare the ground for a legislative proposal for an EU 2040 climate target through an amendment of the European Climate Law . Any subsequent sectoral legislation to deliver on the 2040 target would follow at a later date, and be subject to the usual preparatory and legislative processes.    

Read More Study: Pricing agricultural emissions and rewarding climate action in the agri-food value chain

Annexed reports summarising the expert roundtable, technical workshop and stakeholder survey

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News | November 11, 2015

Seven case studies in carbon and climate.

By Carol Rasmussen, NASA's Earth Science News Team, and Kate Ramsayer, NASA's Goddard Space Flight Center

Every part of the mosaic of Earth's surface — ocean and land, Arctic and tropics, forest and grassland — absorbs and releases carbon in a different way. Wild-card events such as massive wildfires and drought complicate the global picture even more. To better predict future climate, we need to understand how Earth's ecosystems will change as the climate warms and how extreme events will shape and interact with the future environment. Here are seven pressing concerns.

The Far North is warming twice as fast as the rest of Earth, on average. With a 5-year Arctic airborne observing campaign just wrapping up and a 10-year campaign just starting that will integrate airborne, satellite and surface measurements, NASA is using unprecedented resources to discover how the drastic changes in Arctic carbon are likely to influence our climatic future.

Wildfires have become common in the North. Because firefighting is so difficult in remote areas, many of these fires burn unchecked for months, throwing huge plumes of carbon into the atmosphere. A recent report found a nearly 10-fold increase in the number of large fires in the Arctic region over the last 50 years, and the total area burned by fires is increasing annually.

Organic carbon from plant and animal remains is preserved for millennia in frozen Arctic soil, too cold to decompose. Arctic soils known as permafrost contain more carbon than there is in Earth's atmosphere today. As the frozen landscape continues to thaw, the likelihood increases that not only fires but decomposition will create Arctic atmospheric emissions rivaling those of fossil fuels. The chemical form these emissions take — carbon dioxide or methane — will make a big difference in how much greenhouse warming they create.

Initial results from NASA's Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE) airborne campaign have allayed concerns that large bursts of methane, a more potent greenhouse gas, are already being released from thawing Arctic soils. CARVE principal investigator Charles Miller of NASA's Jet Propulsion Laboratory (JPL), Pasadena, California, is looking forward to NASA's ABoVE field campaign (Arctic Boreal Vulnerability Experiment) to gain more insight. "CARVE just scratched the surface, compared to what ABoVE will do," Miller said.

Methane is the Billy the Kid of carbon-containing greenhouse gases: it does a lot of damage in a short life. There's much less of it in Earth's atmosphere than there is carbon dioxide, but molecule for molecule, it causes far more greenhouse warming than CO 2 does over its average 10-year life span in the atmosphere.

Methane is produced by bacteria that decompose organic material in damp places with little or no oxygen, such as freshwater marshes and the stomachs of cows. Currently, over half of atmospheric methane comes from human-related sources, such as livestock, rice farming, landfills and leaks of natural gas. Natural sources include termites and wetlands. Because of increasing human sources, the atmospheric concentration of methane has doubled in the last 200 years to a level not seen on our planet for 650,000 years.

Locating and measuring human emissions of methane are significant challenges. NASA's Carbon Monitoring System is funding several projects testing new technologies and techniques to improve our ability to monitor the colorless gas and help decision makers pinpoint sources of emissions. One project, led by Daniel Jacob of Harvard University, used satellite observations of methane to infer emissions over North America. The research found that human methane emissions in eastern Texas were 50 to 100 percent higher than previous estimates. "This study shows the potential of satellite observations to assess how methane emissions are changing," said Kevin Bowman, a JPL research scientist who was a coauthor of the study.

Tropical forests

Tropical forests are carbon storage heavyweights. The Amazon in South America alone absorbs a quarter of all carbon dioxide that ends up on land. Forests in Asia and Africa also do their part in "breathing in" as much carbon dioxide as possible and using it to grow.

However, there is evidence that tropical forests may be reaching some kind of limit to growth. While growth rates in temperate and boreal forests continue to increase, trees in the Amazon have been growing more slowly in recent years. They've also been dying sooner. That's partly because the forest was stressed by two severe droughts in 2005 and 2010 — so severe that the Amazon emitted more carbon overall than it absorbed during those years, due to increased fires and reduced growth. Those unprecedented droughts may have been only a foretaste of what is ahead, because models predict that droughts will increase in frequency and severity in the future.

In the past 40-50 years, the greatest threat to tropical rainforests has been not climate but humans, and here the news from the Amazon is better. Brazil has reduced Amazon deforestation in its territory by 60 to 70 percent since 2004, despite troubling increases in the last three years. According to Doug Morton, a scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, further reductions may not make a marked difference in the global carbon budget. "No one wants to abandon efforts to preserve and protect the tropical forests," he said. "But doing that with the expectation that [it] is a meaningful way to address global greenhouse gas emissions has become less defensible."

In the last few years, Brazil's progress has left Indonesia the distinction of being the nation with the highest deforestation rate and also with the largest overall area of forest cleared in the world. Although Indonesia's forests are only a quarter to a fifth the extent of the Amazon, fires there emit massive amounts of carbon, because about half of the Indonesian forests grow on carbon-rich peat. A recent study estimated that this fall, daily greenhouse gas emissions from recent Indonesian fires regularly surpassed daily emissions from the entire United States.

Wildfires are natural and necessary for some forest ecosystems, keeping them healthy by fertilizing soil, clearing ground for young plants, and allowing species to germinate and reproduce. Like the carbon cycle itself, fires are being pushed out of their normal roles by climate change. Shorter winters and higher temperatures during the other seasons lead to drier vegetation and soils. Globally, fire seasons are almost 20 percent longer today, on average, than they were 35 years ago.

Currently, wildfires are estimated to spew 2 to 4 billion tons of carbon into the atmosphere each year on average — about half as much as is emitted by fossil fuel burning. Large as that number is, it's just the beginning of the impact of fires on the carbon cycle. As a burned forest regrows, decades will pass before it reaches its former levels of carbon absorption. If the area is cleared for agriculture, the croplands will never absorb as much carbon as the forest did.

As atmospheric carbon dioxide continues to increase and global temperatures warm, climate models show the threat of wildfires increasing throughout this century. In Earth's more arid regions like the U.S. West, rising temperatures will continue to dry out vegetation so fires start and burn more easily. In Arctic and boreal ecosystems, intense wildfires are burning not just the trees, but also the carbon-rich soil itself, accelerating the thaw of permafrost, and dumping even more carbon dioxide and methane into the atmosphere.

North American forests

With decades of Landsat satellite imagery at their fingertips, researchers can track changes to North American forests since the mid-1980s. A warming climate is making its presence known.

Through the North American Forest Dynamics project, and a dataset based on Landsat imagery released this earlier this month, researchers can track where tree cover is disappearing through logging, wildfires, windstorms, insect outbreaks, drought, mountaintop mining, and people clearing land for development and agriculture. Equally, they can see where forests are growing back over past logging projects, abandoned croplands and other previously disturbed areas.

"One takeaway from the project is how active U.S. forests are, and how young American forests are," said Jeff Masek of Goddard, one of the project’s principal investigators along with researchers from the University of Maryland and the U.S. Forest Service. In the Southeast, fast-growing tree farms illustrate a human influence on the forest life cycle. In the West, however, much of the forest disturbance is directly or indirectly tied to climate. Wildfires stretched across more acres in Alaska this year than they have in any other year in the satellite record. Insects and drought have turned green forests brown in the Rocky Mountains. In the Southwest, pinyon-juniper forests have died back due to drought.

Scientists are studying North American forests and the carbon they store with other remote sensing instruments. With radars and lidars, which measure height of vegetation from satellite or airborne platforms, they can calculate how much biomass — the total amount of plant material, like trunks, stems and leaves — these forests contain. Then, models looking at how fast forests are growing or shrinking can calculate carbon uptake and release into the atmosphere. An instrument planned to fly on the International Space Station (ISS), called the Global Ecosystem Dynamics Investigation (GEDI) lidar, will measure tree height from orbit, and a second ISS mission called the Ecosystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) will monitor how forests are using water, an indicator of their carbon uptake during growth. Two other upcoming radar satellite missions (the NASA-ISRO SAR radar, or NISAR, and the European Space Agency’s BIOMASS radar) will provide even more complementary, comprehensive information on vegetation.

Ocean carbon absorption

When carbon-dioxide-rich air meets seawater containing less carbon dioxide, the greenhouse gas diffuses from the atmosphere into the ocean as irresistibly as a ball rolls downhill. Today, about a quarter of human-produced carbon dioxide emissions get absorbed into the ocean. Once the carbon is in the water, it can stay there for hundreds of years.

Warm, CO 2 -rich surface water flows in ocean currents to colder parts of the globe, releasing its heat along the way. In the polar regions, the now-cool water sinks several miles deep, carrying its carbon burden to the depths. Eventually, that same water wells up far away and returns carbon to the surface; but the entire trip is thought to take about a thousand years. In other words, water upwelling today dates from the Middle Ages – long before fossil fuel emissions.

That's good for the atmosphere, but the ocean pays a heavy price for absorbing so much carbon: acidification. Carbon dioxide reacts chemically with seawater to make the water more acidic. This fundamental change threatens many marine creatures. The chain of chemical reactions ends up reducing the amount of a particular form of carbon — the carbonate ion — that these organisms need to make shells and skeletons. Dubbed the “other carbon dioxide problem,” ocean acidification has potential impacts on millions of people who depend on the ocean for food and resources.

Phytoplankton

Microscopic, aquatic plants called phytoplankton are another way that ocean ecosystems absorb carbon dioxide emissions. Phytoplankton float with currents, consuming carbon dioxide as they grow. They are at the base of the ocean's food chain, eaten by tiny animals called zooplankton that are then consumed by larger species. When phytoplankton and zooplankton die, they may sink to the ocean floor, taking the carbon stored in their bodies with them.

Satellite instruments like the Moderate resolution Imaging Spectroradiometer (MODIS) on NASA's Terra and Aqua let us observe ocean color, which researchers can use to estimate abundance — more green equals more phytoplankton. But not all phytoplankton are equal. Some bigger species, like diatoms, need more nutrients in the surface waters. The bigger species also are generally heavier so more readily sink to the ocean floor.

As ocean currents change, however, the layers of surface water that have the right mix of sunlight, temperature and nutrients for phytoplankton to thrive are changing as well. “In the Northern Hemisphere, there’s a declining trend in phytoplankton,” said Cecile Rousseaux, an oceanographer with the Global Modeling and Assimilation Office at Goddard. She used models to determine that the decline at the highest latitudes was due to a decrease in abundance of diatoms. One future mission, the Pre-Aerosol, Clouds, and ocean Ecosystem (PACE) satellite, will use instruments designed to see shades of color in the ocean — and through that, allow scientists to better quantify different phytoplankton species.

In the Arctic, however, phytoplankton may be increasing due to climate change. The NASA-sponsored Impacts of Climate on the Eco-Systems and Chemistry of the Arctic Pacific Environment (ICESCAPE) expedition on a U.S. Coast Guard icebreaker in 2010 and 2011 found unprecedented phytoplankton blooms under about three feet (a meter) of sea ice off Alaska. Scientists think this unusually thin ice allows sunlight to filter down to the water, catalyzing plant blooms where they had never been observed before.

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