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

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

<div id="h2-2-siblings" class="h2-siblings"></div>

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.

<div id="1.3" class="h1-container"></div>

<span id="the-multilateral-context-emissions-trends-and-key-developments"></span>