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Information on social aspects of climate change intervention, such as behavioral choices and public acceptance, are often not included in global climate models. As a result, they have been critiqued for not adequately reflecting ‘real world’ conditions. At the same time, these models are important and influential policy tools. To improve these models, calls are being made for more interaction – or integration – between the social science and modelling research communities. Yet, it remains unclear how to achieve this. Responding to this gap, we explore what kind of integration is currently taking place, how, and opportunities for further development.

Carbon dioxide removal is key to climate change mitigation, yet implications of its deployment remain unclear. Recent exponential growth in literature is rapidly filling this gap but makes the synthesis of the evidence on carbon dioxide removal side effects increasingly challenging. Here we address this issue by mapping this literature and proposing a taxonomy to synthesize and compare evidence on carbon dioxide removal side effects. The expansive evidence warrants the use of machine learning to systematically select relevant research and provide an inventory of nearly 400 co-benefits, challenges, and limits. We find rich evidence in Europe but little information for Africa, South America, and Oceania, where large-scale carbon dioxide removal is nevertheless projected. There is a predominance of articles discussing negative effects compared to positive ones. Starting from the limitations of our analysis and literature gaps, we provide entry points for future studies that can build on our literature-based taxonomy.

Commitments to net-zero carbon dioxide (CO2) or greenhouse gas (GHG) emissions targets now cover 88% of countries’ emissions. Underlying the accounting behind net-zero frameworks is the assumption that emissions can be balanced with removals such that their net climate effect is zero. However, when considering the full climate impacts of CO2 emissions and removals, there are reasons to expect that the two are not equivalent in terms of their climate outcomes. We identify potential contributors to non-equivalence, including impermanence, biogeophysical and non-CO2 GHG effects, and argue that these non-equivalencies need to be accounted for to achieve climate goals. Given key uncertainties about the full climate impact of CO2 removal, it is prudent to prioritize emission reductions over removals.

Non-technical summary We identify a set of essential recent advances in climate change research with high policy relevance, across natural and social sciences: (1) looming inevitability and implications of overshooting the 1.5°C warming limit, (2) urgent need for a rapid and managed fossil fuel phase-out, (3) challenges for scaling carbon dioxide removal, (4) uncertainties regarding the future contribution of natural carbon sinks, (5) intertwinedness of the crises of biodiversity loss and climate change, (6) compound events, (7) mountain glacier loss, (8) human immobility in the face of climate risks, (9) adaptation justice, and (10) just transitions in food systems. Technical summary The Intergovernmental Panel on Climate Change Assessment Reports provides the scientific foundation for international climate negotiations and constitutes an unmatched resource for researchers. However, the assessment cycles take multiple years. As a contribution to cross- and interdisciplinary understanding of climate change across diverse research communities, we have streamlined an annual process to identify and synthesize significant research advances. We collected input from experts on various fields using an online questionnaire and prioritized a set of 10 key research insights with high policy relevance. This year, we focus on: (1) the looming overshoot of the 1.5°C warming limit, (2) the urgency of fossil fuel phase-out, (3) challenges to scale-up carbon dioxide removal, (4) uncertainties regarding future natural carbon sinks, (5) the need for joint governance of biodiversity loss and climate change, (6) advances in understanding compound events, (7) accelerated mountain glacier loss, (8) human immobility amidst climate risks, (9) adaptation justice, and (10) just transitions in food systems. We present a succinct account of these insights, reflect on their policy implications, and offer an integrated set of policy-relevant messages. This science synthesis and science communica

Carbon dioxide removal (CDR) features prominently in the 1.5 °C compatible and high overshoot pathways in the IPCC's Sixth Assessment Report (AR6, WGIII). However, the amount of CDR varies considerably among scenarios. We analyze the range in CDR volumes in AR6 WGIII pathways by exploring relationships between variables as potential driving forces, focusing on CDR in 2050 and scenario properties linked to reaching net-zero CO2. It is also shown how the relative and absolute contribution of CDR to total mitigation up until reaching net-zero CO2 substantially differs across scenarios. The volumes of CDR in 2050 and 2100 and the cumulative amount throughout the 21st century were most strongly correlated to the degree to which CO2 emissions are reduced as a means of reaching net-zero CO2. CDR in 2050 is also substantially correlated to the timing of net-zero CO2. The robustness of the analyzed relationships was evaluated by comparing different scenario filtering and data-cleaning approaches. Beyond filtering and cleaning, additional factors that influence CDR deployment in scenarios, such as discount rates, carbon price trajectories, and scenario design choices, were discussed.

Carbon dioxide removal (CDR) figures prominently in modelled pathways to achieve the Paris Agreement's goal of limiting global warming to 1.5-2°C compared to pre-industrial levels. However, national roles and responsibilities to deliver CDR have been informed with CDR quota analyses that focus on developed economies and global major emitters. This study extends the discussion to implications for developing countries. For that purpose, we employ a diverse set of allocation methods on a wide range of global emissions scenarios to address equitability and uncertainty in sharing the burden of climate change mitigation. We further focus on tropical developing countries due to their large potential for natural climate solutions (NCS) that deliver CDR. Our analysis indicates the potential for stringent CDR quotas for the top seven countries that contribute ∼60% of pantropical cost-effective NCS potential, with median national quotas across emissions scenarios ranging from 0.1-29 GtCO2. However, the results reveal strong heterogeneity of quotas and inherent bias across allocation methods making agreement on an ‘equitable’ quota unlikely. Competition among NCS and non-NCS CDR options may arise when ambitious CDR quotas are implemented in countries with vast forest areas or large potential for expansion of tree cover. Therefore, it is important to not use CDR quotas to evaluate national climate actions or to inform climate targets that could exacerbate trade-offs between emissions reduction, biodiversity and ecosystem services in these NCS-rich countries. Instead, results from burden-sharing exercises could foster higher ambition if used to inform voluntary cooperation mechanisms. Discrepancy between perceived fairness and CDR quotas should be critically and transparently embraced to encourage acknowledgment of socio-ecological co-benefits as compensation. Such an approach will allow tropical developing economies to prioritise protection and restoration of nature in their cli

A global, independent scientific assessment of Carbon Dioxide Removal

The restoration of tropical forests has become a popular nature-based solution for climate change mitigation, protection of biodiversity, and improving the livelihoods of local populations. The Bonn Challenge and the UN Decade on Ecosystem Restoration underscore the international momentum of the restoration movement, with many countries committing to restore millions of hectares of deforested and degraded land in the next decade. Brazil and Indonesia are among the ones with the most ambitious restoration commitments globally. Since both their economies are highly dependent on the export of agricultural commodities, reconciling economic growth with environmental sustainability will be a major policy challenge. In this paper, we (a) identify the main restoration targets and the policies supporting their implementation in both countries, (b) provide a descriptive overview of these restoration-supportive policies, and (c) discuss the main challenges that Brazil and Indonesia face in the implementation of their restoration commitments. We find that Brazil has an explicit and dedicated strategy to achieve its restoration target, but that recent political developments have weakened environmental governance in the country, affecting the implementation of its restoration commitment. In the case of Indonesia, we find that the government has rather focused and progressed on the restoration of peatlands and mangroves, whereas its commitment to restore forestlands has yet to benefit from a dedicated plan that allows to coordinate policies and agencies’ efforts towards the achievement of its restoration target.

Non-technical summary We summarize some of the past year's most important findings within climate change-related research. New research has improved our understanding about the remaining options to achieve the Paris Agreement goals, through overcoming political barriers to carbon pricing, taking into account non-CO2factors, a well-designed implementation of demand-side and nature-based solutions, resilience building of ecosystems and the recognition that climate change mitigation costs can be justified by benefits to the health of humans and nature alone. We consider new insights about what to expect if we fail to include a new dimension of fire extremes and the prospect of cascading climate tipping elements. Technical summary A synthesis is made of 10 topics within climate research, where there have been significant advances since January 2020. The insights are based on input from an international open call with broad disciplinary scope. Findings include: (1) the options to still keep global warming below 1.5 °C; (2) the impact of non-CO2factors in global warming; (3) a new dimension of fire extremes forced by climate change; (4) the increasing pressure on interconnected climate tipping elements; (5) the dimensions of climate justice; (6) political challenges impeding the effectiveness of carbon pricing; (7) demand-side solutions as vehicles of climate mitigation; (8) the potentials and caveats of nature-based solutions; (9) how building resilience of marine ecosystems is possible; and (10) that the costs of climate change mitigation policies can be more than justified by the benefits to the health of humans and nature. Social media summary How do we limit global warming to 1.5 °C and why is it crucial? See highlights of latest climate science.

The extent to which societies will globally be able to adapt to climate change is not well understood. Here we analyze socioeconomic dimensions of adaptive capacity of populations to deal with heat stress and find income, urbanization and income inequality to be important factors in explaining adaptation to heat stress with air conditioning (AC). Using the scenario framework of the Shared Socioeconomic Pathways (SSPs), we estimate the future cooling gap, which represents the difference between the population exposed to heat stress and the population able to protect against heat stress with AC. Depending on the scenario of socioeconomic development, total population affected by the cooling gap may vary between 2 billion and 5 billion people in 2050, with the scenario-dependent range widening further towards the end of the century. Our analysis shows vast regional inequalities in adaptive capacity for one of the most universal manifestations of climate change, underscoring the need to account for the different potential levels of adaptive capacity in assessments of climate change impacts.

In 1955, after the Korean War, only 35% of the national land area in South Korea was covered by forests. In the 1960s, the Korean Government implemented the national forestation program in order to increase the extent of the forest surface and thereby counteract the negative ecological consequences from deforestation, such as erosion and ground instability. According to previous studies, this led to an increase in carbon (C) accumulated in the forest biomass of 1.48 Gt CO2 (0.40 Gt C) in the period 1954–2012. However, these studies did not take into account the amount of soil organic carbon (SOC) that was accumulated during that period and the influence of management practices on soil ecosystem services. Currently, South Korean authorities are considering the idea of implementing some forest management practices in order to increase timber extraction (e.g., by reducing the cutting age of the trees or by applying thinning and tending measures). In this study, we assess the influence of these management regimes on SOC dynamics and propose a theoretical framework to assess the influence of forest management practices on three ecosystem services, namely, C sequestration, water supply, and biomass production, while considering soil functioning, and especially SOC, as a group of supporting services underpinning the three named ecosystem services. We find that, in terms of SOC sequestration, reducing the cutting age from 80 to 40 years would be suitable only in the case of high biomass production forests, whereas in the case of lower biomass production forests reducing the cutting age would achieve very low SOC levels. However, we propose that increasing tree species diversity, even though it would not lead to a direct increase in the SOC content, could help to lessen the negative effects of reducing the cutting age by improving other soil properties, which in turn positively affect soil functioning (e.g., soil biodiversity, nutrient availability) and the resilience of the

Land-management options for greenhouse gas removal (GGR) include afforestation or reforestation (AR), wetland restoration, soil carbon sequestration (SCS), biochar, terrestrial enhanced weathering (TEW), and bioenergy with carbon capture and storage (BECCS). We assess the opportunities and risks associated with these options through the lens of their potential impacts on ecosystem services (Nature's Contributions to People; NCPs) and the United Nations Sustainable Development Goals (SDGs). We find that all land-based GGR options contribute positively to at least some NCPs and SDGs. Wetland restoration and SCS almost exclusively deliver positive impacts. A few GGR options, such as afforestation, BECCS, and biochar potentially impact negatively some NCPs and SDGs, particularly when implemented at scale, largely through competition for land. For those that present risks or are least understood, more research is required, and demonstration projects need to proceed with caution. For options that present low risks and provide cobenefits, implementation can proceed more rapidly following no-regrets principles.

In a modeling study, optimizing the transformation of the US coal sector to achieve emissions reductions consistent with the 2°C target, we include all current coal-fired power plants of the US fleet, a large part of which will need to be replaced due to their high age. Coal-fired power plants can either be (1) replaced by higher efficiency coal plants or (2) natural gas plants while units are not yet at the end of their lifetime and can be (3) retrofitted with carbon capture and storage (CCS) or (4) retrofitted to cofire coal and biomass coupled with CCS (BECCS) thereby achieving negative emissions. Our results show that if the 2°C emissions mitigation target should be achieved, the cost-optimal way of doing so is through an early implementation of BECCS. This strategy also helps to address the US Administrations’ concern for coal workers: there is a more gradual phaseout of coal, which allows to retain 40,000 jobs that would be loss due to the fleet retirement for aging. In addition, 22,000 new workers would be permanently employed in the coal sector by the end of midcentury, especially in areas where the deployments of BECCS would start already by 2030. Our modeling results indicate the Great Lakes area and the southeast United States as the greatest winners of this negative emissions strategy. If planned in an integrated and forward-looking way, climate change mitigation can boost employment and competitiveness.

The global response to warming of 1.5oC comprises transitions in land and ecosystem, energy, urban and infrastructure, and industrial systems. The feasibility of mitigation and adaptation options, and the enabling conditions for strengthening and implementing the systemic changes, are assessed in this chapter.

With the Paris Agreement's ambition of limiting climate change to well below 2 °C, negative emission technologies (NETs) have moved into the limelight of discussions in climate science and policy. Despite several assessments, the current knowledge on NETs is still diffuse and incomplete, but also growing fast. Here, we synthesize a comprehensive body of NETs literature, using scientometric tools and performing an in-depth assessment of the quantitative and qualitative evidence therein. We clarify the role of NETs in climate change mitigation scenarios, their ethical implications, as well as the challenges involved in bringing the various NETs to the market and scaling them up in time. There are six major findings arising from our assessment: first, keeping warming below 1.5 °C requires the large-scale deployment of NETs, but this dependency can still be kept to a minimum for the 2 °C warming limit. Second, accounting for economic and biophysical limits, we identify relevant potentials for all NETs except ocean fertilization. Third, any single NET is unlikely to sustainably achieve the large NETs deployment observed in many 1.5 °C and 2 °C mitigation scenarios. Yet, portfolios of multiple NETs, each deployed at modest scales, could be invaluable for reaching the climate goals. Fourth, a substantial gap exists between the upscaling and rapid diffusion of NETs implied in scenarios and progress in actual innovation and deployment. If NETs are required at the scales currently discussed, the resulting urgency of implementation is currently neither reflected in science nor policy. Fifth, NETs face severe barriers to implementation and are only weakly incentivized so far. Finally, we identify distinct ethical discourses relevant for NETs, but highlight the need to root them firmly in the available evidence in order to render such discussions relevant in practice.

Concerns have been voiced that implementing climate change mitigation measures could come at the cost of employment, especially in the context of the US coal sector. However, repurposing US coal plants presents an opportunity to address emission mitigation and job creation, if the right technology change is adopted. In this study, the transformation of the US coal sector until 2050 is modeled to achieve ambitious climate targets. Results show that the cost-optimal strategy for meeting 2050 emission reductions consistent with 2°C stabilization pathways is through the early deployment of BECCS and by replacing 50% of aging coal plants with natural gas plants. This strategy addresses the concerns surrounding employment for coal workers by retaining 40,000 jobs, and creating 22,000 additional jobs by mid-century. Climate change mitigation does not have to come at the cost of employment, and policymakers could seek to take advantage of the social co-benefits of mitigation.

This study analyzes the Russian forest biomass-based bioenergy sector. It is shown that presently - although given abundant resources - the share of heat and electricity from biomass is very minor. With the help of two IIASA models (G4 M and BeWhere), future green-field bioenergy plants are identified in a geographically explicit way. Results indicate that by using 3.78 Mt (or 6.16 M m3), twice as much heat and electricity than is presently available from forest biomass could be generated. This amount corresponds to 3.3% of the total annual wood removals or 12% of the annually harvested firewood, or about 11% of illegal logging. With this amount of wood, it is possible to provide an additional 444 thousand households with heat and 1.8 M households with electricity; and at the same time to replace 2.7 Mt of coal or 1.7 Mt of oil or 1.8 G m3 of natural gas, reducing emissions of greenhouse gases from burning fossil fuels by 716 Mt of CO2-equivalent per year. A multitude of co-benefits can be quantified for the socioeconomic sector such as green jobs linked to bioenergy. The sustainable sourcing of woody biomass for bioenergy is possible as shown with the help of an online crowdsourcing tool Geo-Wiki.org for forest certification.

The current state of the art in modeling forest cover change is to combine a detailed representation of biophysical processes with economic decision-making principles. Yet, there is an increasing consensus that the quality of political institutions is another relevant component in determining forest cover change patterns. In this paper, the Global Forest Model is used to analyze whether including an index, measuring the capacity of political institutions to guarantee sustainable natural resource management, allows to improve the precision of the modeled forest cover trend. The analysis shows that incorporating the index indeed allows reducing the gap between the estimated and observed forest cover trends for the 2000 to 2010 calibration period.

Climate policy uncertainty significantly hinders investments in low-carbon technologies, and the global community is behind schedule to curb carbon emissions. Strong actions will be necessary to limit the increase in global temperatures, and continued delays create risks of escalating climate change damages and future policy costs. These risks are system-wide, long-term and large-scale and thus hard to diversify across firms. Because of its unique scale, cost structure and near-term availability, Reducing Emissions from Deforestation and forest Degradation in developing countries (REDD+) has significant potential to help manage climate policy risks and facilitate the transition to lower greenhouse gas emissions. ‘Call’ options contracts in the form of the right but not the obligation to buy high-quality emissions reduction credits from jurisdictional REDD+ programmes at a predetermined price per ton of CO2 could help unlock this potential despite the current lack of carbon markets that accept REDD+ for compliance. This approach could provide a globally important cost-containment mechanism and insurance for firms against higher future carbon prices, while channelling finance to avoid deforestation until policy uncertainties decline and carbon markets scale up.

There are currently no spatially explicit, openly accessible data available on forest certification below national level, so understanding the drivers of certification in the past, examining the scope for further certification and using this information for development of future sustainable forest management strategies is challenging. Hence, this paper presents a methodology for the development of a global map of certified forest areas at 1 km resolution in order to satisfy this information need. Validation of the map with certified areas in Russia showed reasonable results, but the lack of openly accessible data requires broadening the strategy for improving the global certification map in the future. Thus, the second aim of the paper is to present an online tool for visualization and interactive improvement of the global forest certification product through collaborative mapping, aiming at a range of stakeholders including third-party certifiers, green NGOs, forestry organizations, decision-makers, scientists and local experts. Such an approach can help to make more accurate information on forest certification available, promote the sharing of data and encourage more transparent and sustainable forest management, i.e. both producers and users can benefit from this online tool.

Current emission pledges to the Paris Agreement appear insufficient to hold the global average temperature increase to well below 2 °C above pre-industrial levels. Yet, details are missing on how to track progress towards the ‘Paris goal’, inform the five-yearly ‘global stocktake’, and increase the ambition of Nationally Determined Contributions (NDCs). We develop a nested structure of key indicators to track progress through time. Global emissions track aggregated progress, country-level decompositions track emerging trends that link directly to NDCs, and technology diffusion indicates future reductions. We find the recent slowdown in global emissions growth11 is due to reduced growth in coal use since 2011, primarily in China and secondarily in the United States. The slowdown is projected to continue in 2016, with global CO2 emissions from fossil fuels and industry similar to the 2015 level of 36 GtCO2. Explosive and policy-driven growth in wind and solar has contributed to the global emissions slowdown, but has been less important than economic factors and energy efficiency. We show that many key indicators are currently broadly consistent with emission scenarios that keep temperatures below 2 °C, but the continued lack of large-scale carbon capture and storage threatens 2030 targets and the longer-term Paris ambition of net-zero emissions.

Generating negative emissions by removing carbon dioxide from the atmosphere is a key requirement for limiting global warming to well below 2 °C, or even 1.5 °C, and therefore for achieving the long-term climate goals of the recent Paris Agreement. Despite being a relatively young topic, negative emission technologies (NETs) have attracted growing attention in climate change research over the last decade. A sizeable body of evidence on NETs has accumulated across different fields that is by today too large and too diverse to be comprehensively tracked by individuals. Yet, understanding the size, composition and thematic structure of this literature corpus is a crucial pre-condition for effective scientific assessments of NETs as, for example, required for the new special report on the 1.5 °C by the Intergovernmental Panel on Climate Change (IPCC). In this paper we use scientometric methods and topic modelling to identify and characterize the available evidence on NETs as recorded in the Web of Science. We find that the development of the literature on NETs has started later than for climate change as a whole, but proceeds more quickly by now. A total number of about 2900 studies have accumulated between 1991 and 2016 with almost 500 new publications in 2016. The discourse on NETs takes place in distinct communities around energy systems, forests as well as biochar and other soil carbon options. Integrated analysis of NET portfolios—though crucial for understanding how much NETs are possible at what costs and risks—are still in their infancy and do not feature as a theme across the literature corpus. Overall, our analysis suggests that NETs research is relatively marginal in the wider climate change discourse despite its importance for global climate policy.

A global crediting mechanism would enable developing countries without binding emissions reduction targets to participate in the international carbon market. Linking the framework on Reducing Emissions from Deforestation and Forest Degradation (REDD) as an offset program to major cap-and-trade programs is a particularly promising approach to increase both climate finance and cost-efficiency. However, the coexistence of permits and offsets also creates a classic case of interaction effects. In this paper, we explore how the availability of multiple compliance instruments affects energy investment incentives. Alternative trading and linkage schemes are compared using a real options model of firm-level investment decisions under stochastic prices and the ability to delay investments. We first isolate the critical design factors that drive private investments in the energy sector. We then identify policy regimes that balance the different concerns in the polarized debate for and against the inclusion of forest carbon offsets.

Humankind has never been so populous, technically equipped, and economically and culturally integrated as it is today. In the twenty-first century, societies are confronted with a multitude of challenges in their efforts to manage the Earth system.