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Abstract The 6th Assessment Report from the Intergovernmental Panel on Climate Change lacked sufficient land-sector scenario information to estimate total carbon dioxide removal deployment. Here, using a dataset of land-based carbon dioxide removal based on the scenarios assessed by the Intergovernmental Panel on Climate Change, we show that removals via afforestation and reforestation play a critical near-term role in mitigation, accounting for around 10% (median) of the net greenhouse gas emission reductions between 2020 and 2030 in scenarios that limit warming to 1.5 °C with limited overshoot. Novel carbon dioxide removal technologies such as direct air carbon capture and storage scale to multi-gigatonne levels by 2050 and beyond to balance residual emissions and draw down warming. We show that reducing fossil fuel and deforestation emissions (gross emissions) accounts for over 80% of net greenhouse gas reductions until global net zero carbon dioxide (CO2) independent of climate objective stringency. We explore the regional distributions of gross emissions and total carbon dioxide removal in cost-effective mitigation pathways and highlight the importance of incorporating fairness and broader sustainability considerations in future assessments of mitigation pathways with carbon dioxide removal.

IntroductionFor limiting global warming to well below 2°C rapid and stringent GHG emissions reductions are required. In addition, we also need to actively remove CO2 from the atmosphere via carbon dioxide removal (CDR). This will require advances in policymaking and governance to incentivise, coordinate and regulate CDR, including strict monitoring to ensure durable, additional removals that do not compete with emission reduction efforts. While it is critical to learn from the existing evidence on CDR policy and governance, there is no overview of this dispersed body of literature right now. IPCC and other science assessments have therefore treated the subject very selectively. This work addresses this lack of overview by systematically mapping the literature assessing policy and governance dimensions of CDR.MethodsSystematic mapping provides a comprehensive view of a research field by analysing the state of evidence, i.e. how much research is available at any point in time on which topics and geographies studied by whom, when and where. We use an AI-enhanced approach to systematic mapping, trimming down an initial set of about 30,000 documents on CDR to a set of 876 that deal with governance and policy issues.ResultsOur findings show sharply growing attention to CDR policies and governance issues over time, but with limited coverage of the Global South. Long established conventional CDR methods such as afforestation dominate the literatureparticularly in ex-post studies -with little coverage of many novel CDR methods, such as biochar or direct air carbon capture and storage. We observe a shift from an initial discussion on CDR in international agreements towards the planning and implementation phase of national and sub-national policies.DiscussionOur map can help to inform upcoming science assessments with critical information around CDR policies and governance and might serve as a starting point for generating a rigorous knowledge base on the topic in the future.

Carbon Dioxide Removal (CDR) is a key element of any mitigation strategy aiming to achieve the long-term temperature goal of the Paris Agreement, as well as national net-zero and net-negative greenhouse gas emissions targets. For robust CDR policy, the credibility of certification schemes is essential.

The climate crisis requires nations to achieve human well-being with low national levels of carbon emissions. Countries vary from one another dramatically in how effectively they convert resources into well-being, and some nations with low levels of emissions have relatively high objective and subjective well-being. We identify urgent research and policy agendas for four groups of countries with either low or high emissions and well-being indicators. Least studied are those with low well-being and high emissions. Understanding social and political barriers to switching from high-carbon to lower-carbon modes of production and consumption, and ways to overcome them, will be fundamental.

‘Discourses of climate delay’ pervade current debates on climate action. These discourses accept the existence of climate change, but justify inaction or inadequate efforts. In contemporary discussions on what actions should be taken, by whom and how fast, proponents of climate delay would argue for minimal action or action taken by others. They focus attention on the negative social effects of climate policies and raise doubt that mitigation is possible. Here, we outline the common features of climate delay discourses and provide a guide to identifying them.

Manhattan, Berlin and New Delhi all need to take action to adapt to climate change and to reduce greenhouse gas emissions. While case studies on these cities provide valuable insights, comparability and scalability remain sidelined. It is therefore timely to review the state-of-the-art in data infrastructures, including earth observations, social media data, and how they could be better integrated to advance climate change science in cities and urban areas. We present three routes for expanding knowledge on global urban areas: mainstreaming data collections, amplifying the use of big data and taking further advantage of computational methods to analyse qualitative data to gain new insights. These data-based approaches have the potential to upscale urban climate solutions and effect change at the global scale.

A rapid phase-out of unabated coal use is essential to limit global warming to below 2 °C. This review presents a comprehensive assessment of coal transitions in mitigation scenarios consistent with the Paris Agreement, using data from more than 1500 publicly available scenarios generated by more than 30 integrated assessment models. Our ensemble analysis uses clustering techniques to categorize coal transition pathways in models and bridges evidence on technological learning and innovation with historical data of energy systems. Six key findings emerge: First, we identify three archetypal coal transitions within Paris-consistent mitigation pathways. About 38% of scenarios are ‘coal phase out’ trajectories and rapidly reduce coal consumption to near zero. ‘Coal persistence’ pathways (42%) reduce coal consumption much more gradually and incompletely. The remaining 20% follow ‘coal resurgence’ pathways, characterized by increased coal consumption in the second half of the century. Second, coal persistence and resurgence archetypes rely on the widespread availability and rapid scale-up of carbon capture and storage technology (CCS). Third, coal-transition archetypes spread across all levels of climate policy ambition and scenario cycles, reflecting their dependence on model structures and assumptions. Fourth, most baseline scenarios—including the shared socio-economic pathways (SSPs)—show much higher coal dependency compared to historical observations over the last 60 years. Fifth, coal-transition scenarios consistently incorporate very optimistic assumptions about the cost and scalability of CCS technologies, while being pessimistic about the cost and scalability of renewable energy technologies. Sixth, evaluation against coal-dependent baseline scenarios suggests that many mitigation scenarios overestimate the technical difficulty and costs of coal phase-outs. To improve future research, we recommend using up-to-date cost data and evidence about innovation and diffusion dynamics of different groups of zero or low-carbon technologies. Revised SSP quantifications need to incorporate projected technology learning and consistent cost structures, while reflecting recent trends in coal consumption.

Residual emissions are an important category of analysis in climate targets and scenarios, describing the emissions that would need to be compensated by carbon dioxide removal to reach net zero CO _2 . This article sheds light on the size and composition of residual emissions in integrated assessment modelling (IAM) scenarios at net-zero CO _2 , using the Intergovernmental Panel on Climate Change 6th assessment report (AR6) scenario database. I find that IAMs depict residual greenhouse gas emissions levels of 16 [12–26] GtCO _2 e/yr across below 1.5 °C, below 1.5 °C with overshoot, and below 2 °C scenarios—representing gross emissions reductions of 72 [55–78] % from 2020 to the point of net zero CO _2 . Emissions from livestock, agriculture, transport, waste and industrial processes remain most prevalent at net zero, with scenario averages tending to depict significantly lower reduction rates and higher residual emissions than those recently published in corporate net zero standards. Nonetheless, scenarios depict a wide range of outcomes across all of these ‘hard-to-abate’ sectors, with lower values characterised by demand-side shifts in the food and energy system, alongside the rapid electrification of end-use sectors and limits on CDR scaling. Current scenario reporting does not easily facilitate the calculation of gross emissions from the AR6 database, which would be a prerequisite to a more systematic exploration of the residual emissions frontier and its implications for climate policy.

The food system, spanning from pre-production processes to post-production stages, is responsible for about one third of global greenhouse gas emissions and requires significant mitigation efforts to prevent dangerous levels of global warming. This article summarises trends and drivers of global food system emissions from 1990 to 2018. We highlight regional diversity in patterns of food system emissions and identify the highest global emitters. While food system emissions have stabilised in some regions and countries, global emissions are increasing, with growth in certain sectors and countries outweighing the handful of cases where sustained emissions reductions have been realised. Emissions from livestock rearing account for a large portion of global emissions, and the contribution of post-production emissions is steadily increasing in all regions. We also provide an overview of food system policies at the national level, mapping them to each emissions segment. This highlights the significant shortfall in policy activity required to address the challenge of climate change mitigation in general, and the impacts of livestock and post-production emissions in particular. Our work lays the groundwork for addressing specific country-level questions on optimal policy pathways to achieve emission reductions.

As current action remains insufficient to meet the goals of the Paris agreement let alone to stabilize the climate, there is increasing hope that solutions related to demand, services and social aspects of climate change mitigation can close the gap. However, given these topics are not investigated by a single epistemic community, the literature base underpinning the associated research continues to be undefined. Here, we aim to delineate a plausible body of literature capturing a comprehensive spectrum of demand, services and social aspects of climate change mitigation. As method we use a novel double-stacked expert—machine learning research architecture and expert evaluation to develop a typology and map key messages relevant for climate change mitigation within this body of literature. First, relying on the official key words provided to the Intergovernmental Panel on Climate Change by governments (across 17 queries), and on specific investigations of domain experts (27 queries), we identify 121 165 non-unique and 99 065 unique academic publications covering issues relevant for demand-side mitigation. Second, we identify a literature typology with four key clusters: policy, housing, mobility, and food/consumption. Third, we systematically extract key content-based insights finding that the housing literature emphasizes social and collective action, whereas the food/consumption literatures highlight behavioral change, but insights also demonstrate the dynamic relationship between behavioral change and social norms. All clusters point to the possibility of improved public health as a result of demand-side solutions. The centrality of the policy cluster suggests that political actions are what bring the different specific approaches together. Fourth, by mapping the underlying epistemic communities we find that researchers are already highly interconnected, glued together by common interests in sustainability and energy demand. We conclude by outlining avenues for interdisciplinary collaboration, synthetic analysis, community building, and by suggesting next steps for evaluating this body of literature.

A rapid coal phase-out is needed to meet the goals of the Paris Agreement, but is hindered by serious challenges ranging from vested interests to the risks of social disruption. To understand how to organize a global coal phase-out, it is crucial to go beyond cost-effective climate mitigation scenarios and learn from the experience of previous coal transitions. Despite the relevance of the topic, evidence remains fragmented throughout different research fields, and not easily accessible. To address this gap, this paper provides a systematic map and comprehensive review of the literature on historical coal transitions. We use computer-assisted systematic mapping and review methods to chart and evaluate the available evidence on historical declines in coal production and consumption. We extracted a dataset of 278 case studies from 194 publications, covering coal transitions in 44 countries and ranging from the end of the 19th century until 2021. We find a relatively recent and rapidly expanding body of literature reflecting the growing importance of an early coal phase-out in scientific and political debates. Previous evidence has primarily focused on the United Kingdom, the United States, and Germany, while other countries that experienced large coal declines, like those in Eastern Europe, are strongly underrepresented. An increasing number of studies, mostly published in the last 5 years, has been focusing on China. Most of the countries successfully reducing coal dependency have undergone both demand-side and supply-side transitions. This supports the use of policy approaches targeting both demand and supply to achieve a complete coal phase-out. From a political economy perspective, our dataset highlights that most transitions are driven by rising production costs for coal, falling prices for alternative energies, or local environmental concerns, especially regarding air pollution. The main challenges for coal-dependent regions are structural change transformations, in particular for industry and labor. Rising unemployment is the most largely documented outcome in the sample. Policymakers at multiple levels are instrumental in facilitating coal transitions. They rely mainly on regulatory instruments to foster the transitions and compensation schemes or investment plans to deal with their transformative processes. Even though many models suggest that coal phase-outs are among the low-hanging fruits on the way to climate neutrality and meeting the international climate goals, our case studies analysis highlights the intricate political economy at work that needs to be addressed through well-designed and just policies.

It is critical to ensure climate and energy policies are just, equitable and beneficial for communities, both to sustain public support for decarbonisation and address multifaceted societal challenges. Our objective in this article is to examine the diverse social outcomes that have resulted from climate policies, in varying contexts worldwide, over the past few decades. We review 203 ex-post climate policy assessments that analyse social outcomes in the literature. We systematically and comprehensively map out this work, identifying articles on carbon, energy and transport taxes, feed-in-tariffs, subsidies, direct procurement policies, large renewable deployment projects, and other regulatory and market-based interventions. We code each article in terms of their studied social outcomes and effects, with a focus on electricity access, energy affordability, community cohesion, employment, distributional and equity issues, livelihoods and poverty, procedural justice, subjective well-being and drudgery. Our analysis finds that climate and energy policies often fall short of delivering positive social outcomes. Nonetheless, across country contexts and policy types there are manifold examples of climate policymaking that does deliver on both social and climate goals. This requires attending to distributive and procedural justice in policy design, and making use of appropriate mechanisms to ensure that policy costs and benefits are fairly shared. We emphasize the need to further advance ex-post policy assessments and learn about what policies work for a just transition.

The relationship between human health and well-being, energy use and carbon emissions is a foremost concern in sustainable development. If past advances in well-being have been accomplished only through increases in energy use, there may be significant trade-offs between achieving universal human development and mitigating climate change. We test the explanatory power of economic, dietary and modern energy factors in accounting for past improvements in life expectancy, using a simple novel method, functional dynamic decomposition. We elucidate the paradox that a strong correlation between emissions and human development at one point in time does not imply that their dynamics are coupled in the long term. Increases in primary energy and carbon emissions can account for only a quarter of improvements in life expectancy, but are closely tied to growth in income. Facing this carbon-development paradox requires prioritizing human well-being over economic growth.

The Paris Agreement (PA) sets out to strengthen the global response to climate change, setting targets for mitigation, adaptation, and finance, and establishing mechanisms through which to achieve these targets. The effectiveness of the PA’s mechanisms in achieving its targets, however, has been questioned. This review systematically maps the peer-reviewed literature on the PA, categorizing the available evidence on whether or not the ‘Paris Regime’ can be effective. We split our analysis into three methodologically distinct sections: first we categorize the literature according to the mechanisms being studied. We find a diverse body of literature, albeit with a clear focus on mitigation, and identify adaptation and capacity building to be clear gaps. Second, we carry out a content analysis, identifying common drivers of, barriers to, and recommendations for effectiveness. Here we find mixed evidence, with potential drivers often qualified by more concrete barriers. Thirdly, we use scientometrics to identify six research clusters. These cover loss and damage, finance, legal issues, international politics, experimental evidence, and studies on tracking progress on the PA’s targets. We conclude with a narrative discussion of our findings, presenting three central themes. First, transparency is widely considered a precondition for the PA to be institutionally effective. However, a lack of clear reporting standards and comparable information renders the PA’s transparency provisions ineffective. Second, environmental effectiveness relies on national ambition, of which there is currently too little. It remains unclear to which extent the Paris Regime structure itself can induce significant ratcheting-up of ambition. Finally, the PA facilitates the diffusion of norms, enables learning and the sharing of best practices. This production of shared norms provides the most promising avenue for overcoming the current lack of ambition. One of the primary successes of the PA is in providing a platform for the exchange of experiences and ideas.

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.

We assess the literature on innovation and upscaling for negative emissions technologies (NETs) using a systematic and reproducible literature coding procedure. To structure our review, we employ the framework of sequential stages in the innovation process, with which we code each NETs article in innovation space. We find that while there is a growing body of innovation literature on NETs, 59% of the articles are focused on the earliest stages of the innovation process, ‘research and development’ (R&D). The subsequent stages of innovation are also represented in the literature, but at much lower levels of activity than R&D. Distinguishing between innovation stages that are related to the supply of the technology (R&D, demonstrations, scale up) and demand for the technology (demand pull, niche markets, public acceptance), we find an overwhelming emphasis (83%) on the supply side. BECCS articles have an above average share of demand-side articles while direct air carbon capture and storage has a very low share. Innovation in NETs has much to learn from successfully diffused technologies; appealing to heterogeneous users, managing policy risk, as well as understanding and addressing public concerns are all crucial yet not well represented in the extant literature. Results from integrated assessment models show that while NETs play a key role in the second half of the 21st century for 1.5 °C and 2 °C scenarios, the major period of new NETs deployment is between 2030 and 2050. Given that the broader innovation literature consistently finds long time periods involved in scaling up and deploying novel technologies, there is an urgency to developing NETs that is largely unappreciated. This challenge is exacerbated by the thousands to millions of actors that potentially need to adopt these technologies for them to achieve planetary scale. This urgency is reflected neither in the Paris Agreement nor in most of the literature we review here. If NETs are to be deployed at the levels required to meet 1.5 °C and 2 °C targets, then important post-R&D issues will need to be addressed in the literature, including incentives for early deployment, niche markets, scale-up, demand, and—particularly if deployment is to be hastened—public acceptance.

Meeting the 1.5 °C goal will require a rapid scale-up of zero-carbon energy supply, fuel switching to electricity, efficiency and demand-reduction in all sectors, and the replenishment of natural carbon sinks. These transformations will have immediate impacts on various of the sustainable development goals. As goals such as affordable and clean energy and zero hunger are more immediate to great parts of global population, these impacts are central for societal acceptability of climate policies. Yet, little is known about how the achievement of other social and environmental sustainability objectives can be directly managed through emission reduction policies. In addition, the integrated assessment literature has so far emphasized a single, global (cost-minimizing) carbon price as the optimal mechanism to achieve emissions reductions. In this paper we introduce a broader suite of policies—including direct sector-level regulation, early mitigation action, and lifestyle changes—into the integrated energy-economy-land-use modeling system REMIND-MAgPIE. We examine their impact on non-climate sustainability issues when mean warming is to be kept well below 2 °C or 1.5 °C. We find that a combination of these policies can alleviate air pollution, water extraction, uranium extraction, food and energy price hikes, and dependence on negative emissions technologies, thus resulting in substantially reduced sustainability risks associated with mitigating climate change. Importantly, we find that these targeted policies can more than compensate for most sustainability risks of increasing climate ambition from 2 °C to 1.5 °C.

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.