Editing IPCC:AR6/WGIII/Chapter-1 (section)

== 1.2 Previous Assessments ==

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=== 1.2.1 Key Findings from Previous Assessment Reports ===

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Successive WGIII IPCC assessments have emphasised the importance of climate mitigation along with the need to consider broader societal goals especially sustainable development. Key insights from AR5 and the subsequent three Special Reports ( [[#IPCC--2018b|IPCC 2018b]] , 2019b, 2019c) are summarised below.

The AR5 projected that in baseline scenarios (i.e., based on prevailing trends without explicit additional mitigation efforts), agriculture, forestry and other land use (AFOLU) would be the only sector where emissions could fall by 2100, with some CO 2 removal ( [[#IPCC--2014b|IPCC 2014b]] , p. 17). Direct CO 2 emissions from energy were projected to double or even triple by 2050 ( [[#IPCC--2014b|IPCC 2014b]] , p. 20) due to global population and economic growth, resulting in global mean surface temperature increases in 2100 from 3.7°C to 4.8°C compared to pre-industrial levels. The AR5 noted that mitigation effort and the costs associated with ambitious mitigation differ significantly across countries, and in ‘globally cost-effective’ scenarios, the biggest reductions (relative to projections) occur in the countries with the highest future emissions in the baseline scenarios ( [[#IPCC--2014b|IPCC 2014b]] , p. 17). Since most physical capital (e.g., power plants, buildings, transport infrastructure) involved in GHG emissions is long-lived, the timing of the shift in investments and strategies will be crucial ( [[#IPCC--2014b|IPCC 2014b]] , p. 18).

A key message from recent Special Reports is the urgency to mitigate GHG emissions in order to avoid rapid and potentially irreversible changes in natural and human systems ( [[#IPCC--2018b|IPCC 2018b]] , 2019b, 2019c). Successive IPCC reports have drawn upon increasing sophistication of modelling tools to project emissions in the absence of ambitious decarbonisation action, as well as the emission pathways that meet long-term temperature targets. The SR1.5 examined pathways limiting warming to 1.5°C, compared to the historical baseline of 1850–1900, finding that ‘in pathways with no or limited overshoot of 1.5°C, global net anthropogenic CO 2 emissions decline by about 45% from 2010 levels by 2030, reaching net zero around 2050’ (2045–2055 interquartile range); with ‘overshoot’ referring to higher temperatures, then brought down by 2100 through ‘net negative’ emissions. It found this would require rapid and far-reaching transitions in energy, land, urban and infrastructure (including transport and buildings), and industrial systems ( ''high confidence'' ) ( [[#IPCC--2018b|IPCC 2018b]] ).

The SR1.5 found that the Nationally Determined Contributions (NDCs) as declared under the Paris Agreement (PA) would not limit warming to 1.5°C; despite significant updates to NDCs in 2020/21, this remains the case, although delivery of these more ambitious NDCs would somewhat enhance the prospects for staying below 2°C ( [[#1.3.3|Section 1.3.3]] ).

The AR5 WGIII and the Special Reports analysed economic costs associated with climate action. The estimates vary widely depending on the assumptions made as to how ordered the transition is, temperature target, technology availability, and the metric or model used, among others (Chapter 6). Modelled direct mitigation costs of pathways to 1.5°C, with no/limited overshoot, span a wide range, but were typically three to four times higher than in pathways to 2°C ( ''high confidence'' ), before taking account of benefits, including significant reduction in loss of life and livelihoods, and avoided climate impacts ( [[#IPCC--2018b|IPCC 2018b]] ).

Successive IPCC reports highlight a strong connection between climate mitigation and sustainable development. Climate mitigation and adaptation goals have synergies and trade-offs with efforts to achieve sustainable development, including poverty eradication. A comprehensive assessment of climate policy therefore involves going beyond a narrow focus on specific mitigation and adaptation options to incorporate climate issues into the design of comprehensive strategies for equitable sustainable development. At the same time, some climate mitigation policies can run counter to sustainable development and eradicating poverty, which highlights the need to consider trade-offs alongside benefits. Examples include synergies between climate policy and improved air quality, reducing premature deaths and morbidity ( [[#IPCC--2014b|IPCC 2014b]] , Figure SPM.6) (AR6 WGI Sections 6.6.3 and 6.7.3), but there would be trade-offs if policy raises net energy bills, with distributional implications. The Special Report on Climate Change and Land (SRCCL) also emphasises important synergies and trade-offs, bringing new light on the link between healthy and sustainable food consumption and emissions caused by the agricultural sector. Land-related responses that contribute to climate change adaptation and mitigation can also combat desertification and land degradation, and enhance food security ( [[#IPCC--2019a|IPCC 2019a]] ).

Previous Assessment Reports (ARs) have detailed the contribution of various sectors and activities to global GHG emissions. When indirect emissions (mainly from electricity, heat and other energy conversions) are included, the four main consumption (end-use) drivers are industry, AFOLU, buildings and transport (Figure 2.14), though the magnitude of these emissions can vary widely between countries. These – together with the energy and urban systems which feed and shape end-use sectors – define the sectoral chapters in this AR6 WGIII report.

Estimates of emissions associated with production and transport of internationally traded goods were first presented in AR5 WGIII, which estimated the ‘embodied emission transfers’ from upper-middle-income countries to industrialised countries through trade at about 10% of CO 2 emissions in each of these groups ( [[#IPCC--2014a|IPCC 2014a]] , Figure TS.5). The literature on this and discussion on their accounting has grown substantially since then (Chapters 2 and 8).

The atmosphere is a shared global resource and an integral part of the ‘global commons’. In the depletion/restoration of this resource, myriad actors at various scales are involved, for instance, individuals, communities, firms and states. ''Inter alia'' , international cooperation to tackle ozone depletion and acid rain offer useful examples. The AR5 noted that greater cooperation would ensue if policies are perceived as fair and equitable by all countries along the spectrum of economic development – implying a need for equitable sharing of the effort. A key takeaway from AR5 is that climate policy involves value judgement and ethics. ( [[#IPCC--2014a|IPCC 2014a]] Box TS.1: ‘People and countries have rights and owe duties towards each other. These are matters of justice, equity, or fairness. They fall within the subject matter of moral and political philosophy, jurisprudence, and economics.’ p. 37). International cooperation and collective action on climate change alongside local, national, regional and global policies will be crucial to solve the problem, and this report notes cooperative approaches beyond simple ‘global commons’ framings (Chapters 13 and 14).

The AR5 (all Working Group reports) also underlined that climate policy inherently involves risk and uncertainty (in nature, economy, society and individuals). To help evaluate responses, there exists a rich suite of analytical tools, for example, cost-benefit analysis, cost-effectiveness analysis, multi-criteria analysis, expected utility theory, and catastrophe and risk models. All have pros and cons, and have been further developed in subsequent literature and in AR6 (Sections 1.2.2 and 1.7).

Recent assessments ( [[#IPCC--2014a|IPCC 2014a]] , 2018b) began to consider the role of individual behavioural choices and cultural norms in driving energy and food patterns. Notably, SR1.5 ( [[IPCC:Wg3:Chapter:Chapter-4#4.4.3|Section 4.4.3]] ) outlined emerging evidence on the potential for changes in behaviour, lifestyle and culture to contribute to decarbonisation (and lower the cost); for the first time, AR6 devotes a whole chapter (Chapter 5) to consider these and other underlying drivers of energy demand, food choices and social aspects.

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=== 1.2.2 Developments in Climate Science, Impacts and Risk ===

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The assessment of the Physical Science Basis (IPCC AR6 WGI) documents sustained and widespread changes in the atmosphere, cryosphere, biosphere and ocean, providing unequivocal evidence of a world that has warmed, associated with rising atmospheric CO 2 concentrations reaching levels not experienced in at least the last 2 million years. Aside from temperature, other clearly discernible, human-induced changes beyond natural variations include declines in Arctic Sea ice and glaciers, thawing of permafrost, and a strengthening of the global water cycle (AR6 WGI SPM A.2, B.3 and B.4). Oceanic changes include rising sea level, acidification, deoxygenation, and changing salinity (WGI SPM B.3). Over land, in recent decades, both frequency and severity have increased for hot extremes but decreased for cold extremes; intensification of heavy precipitation is observed in parallel with a decrease in available water in dry seasons, along with an increased occurrence of weather conditions that promote wildfires.

In defining the objective of international climate negotiations as being to ‘prevent dangerous anthropogenic interference’ ( [[#UNFCCC--1992|UNFCCC 1992]] , Art. 2), the UNFCCC underlines the centrality of risk framing in considering the threats of climate change and potential response measures. Against the background of ‘unequivocal’ (AR4) evidence of human-induced climate change, and the growing experience of direct impacts, the IPCC has sought to systematise a robust approach to risk and risk management.

In AR6 the IPCC employs a common risk framing across all three working groups and provides guidance for more consistent and transparent usage (AR6 WGI Cross-Chapter Box 3 in Chapter 1; AR6 WGII [[#1.4.1|Section 1.4.1]] ; IPCC risk guidance). AR6 defines risk as ‘the potential for adverse consequences for human or ecological systems, recognising the diversity of values and objectives associated with such systems’ (Annex I), encompassing risks from both potential impacts of climate change and human responses to it ( [[#Reisinger--2020|Reisinger et al. 2020]] ). The risk framing includes steps for identifying, evaluating, and prioritising current and future risks; for understanding the interactions among different sources of risk; for distributing effort and equitable sharing of risks; for monitoring and adjusting actions over time while continuing to assess changing circumstances; and for communications among analysts, decision-makers, and the public.

Climatechange risk assessments face challenges including a tendency to mischaracterise risks and pay insufficient attention to the potential for surprises ( [[#Weitzman--2011|Weitzman 2011]] ; [[#Aven--2015|Aven and Renn 2015]] ; [[#Stoerk--2018|Stoerk et al. 2018]] ). Concepts of resilience and vulnerability provide overlapping, alternative entry points to understanding and addressing the societal challenges caused and exacerbated by climate change (AR6 WGII, [[#1.2.1|Section 1.2.1]] ).

The AR6 WGII devotes a full chapter (Chapter 17) to ‘Decision-Making Options for Managing Risk’, detailing the analytic approaches and drawing upon the ''Cynefin'' classification of ''known, knowable, complex'' and ''chaotic'' systems ( [[#17.3.1|Section 17.3.1]] ). With deep uncertainty, risk management often aims to identify specific combinations of response actions and enabling institutions that increase the potential for favourable outcomes despite irreducible uncertainties (AR6 WGII [https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-17 Chapter 17] Cross-Chapter Box DEEP; also [[#Marchau--2019|Marchau et al. (2019)]] ; [[#Doukas--2020|Doukas and Nikas (2020)]] ).

Literature trying to quantify the cost of climate damages has continued to develop. Different methodologies systematically affect outcomes, with recent estimates based on empirical approaches – econometric measurements based on actual impacts – ‘categorically higher than estimates from other approaches’ (AR6 WGII, Cross-Working Group Box ECONOMIC in Chapter 16, and [[#16.6.2|Section 16.6.2]] ). This, along with other developments strengthen foundations for calculating a ‘social cost of carbon’. This informs a common metric for comparing different risks and estimating benefits compared to the costs of GHG reductions and other risk-reducing options ( [[#1.7.1|Section 1.7.1]] ); emissions mitigation itself also involves multiple uncertainties, which alongside risks can also involve potential opportunities ( [[#1.7.3|Section 1.7.3]] ).

Simultaneously, the literature increasingly emphasises the importance of multi-objective risk assessment and management (e.g., representative key risks in AR6 WGII Chapter 16), which may or may not correlate with any single estimate of economic value (AR6 WGII, [[#1.4.1|Section 1.4.1]] ; IPCC risk guidance). Given the deep uncertainties and risks, the goals established (notably in the Paris Agreement and SDGs) reflect negotiated outcomes informed by the scientific assessment of risks.

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