Editing IPCC:AR6/WGII/Chapter-16 (section)

=== 16.1.3 Storyline of the Chapter, and What’s New Compared with Previous Assessments ===

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Figure 16.1 illustrates the elements covered by the chapter, which can be summarised as four key questions.

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'''Figure 16.1 |''' '''Illustrative storyline of the chapter highlighting the central questions addressed in the various sections, from realised risks (observed impacts) to future risks (key risks and reasons for concern), informed by adaptation-related responses and the limits to adaptation.''' The arrows illustrate actions to reduce hazard, exposure and vulnerability, which shape risks over time. Accordingly, the green areas at the centre of the propeller diagrams indicate the ability for such solutions to reduce risk, up to certain adaptation limits, leaving the white residual risk (or observed impacts) in the centre. The shading of the right-hand-side propeller diagram compared with the non-shaded one on the left reflects some degree of uncertainty about future risks. The figure builds on the conceptual framework of risk–adaptation relationships used in SROCC ( [[#Garschagen--2019|Garschagen et al., 2019]] ).

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==== 16.1.3.1 What Impacts Are Being Experienced? ====

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This assessment of climate-related impacts that are already taking place is covered in [[#16.2|Section 16.2]] , which aims to differentiate between observed changes in climate hazards (also called ‘climate impact drivers’ in IPCC Working Group I) and the exposure and vulnerability of human and ecological systems.

Observed impacts of climate change were synthesised in the TAR, AR4 and AR5. The TAR found that recent regional climate changes had already affected many physical and biological systems, with preliminary indications that some human systems had been affected, primarily through floods and droughts. AR4 found ''likely'' [[#footnote-001|2]] discernible impacts on many physical and biological systems, and more ''limited evidence'' for impacts on human environments. AR5 devoted a separate chapter to observed impacts, which found growing evidence of impacts on human and ecological systems on all continents and across oceans ( [[#Cramer--2014|Cramer et al., 2014]] ).

[[#16.2|Section 16.2]] reports on the expanded literature since then, generally reflecting a growing and more certain impact of climate change on humans and ecological systems.

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==== 16.1.3.2 What Responses Are Being Undertaken? ====

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[[#16.3|Section 16.3]] provides, for the first time, a comprehensive synthesis of observed adaptation-related responses to the rising risks.

Such adaptation responses were first covered in the TAR, and further developed in the AR4 and AR5. For instance, AR5 [[IPCC:Wg2:Chapter:Chapter-15|Chapter 15]] notes that adaptation to climate change was transitioning from a phase of awareness to the construction of actual strategies and plans in societies ( [[#Mimura--2014|Mimura et al., 2014]] ) but did not include a comprehensive mapping of responses.

Based on such a comprehensive mapping, [[#16.3|Section 16.3]] finds growing evidence of adaptation-related responses, although these are dominated by minor modifications to usual practices or measures for dealing with extreme weather events, and there is ''limited evidence'' for the extent to which they reduce climate risk.

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==== 16.1.3.3 What are the limits to adaptation? ====

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The literature on limits to adaptation, which is covered in [[#16.4|Section 16.4]] , has strongly evolved since AR5, including links to discussions on loss and damage in the United Nations Framework Convention on Climate Change (UNFCCC). While the Summary for Policymakers (SPM) of AR4 noted that there was no clear picture of the limits to adaptation, or the cost, AR5 [https://www.ipcc.ch/report/ar6/wg2/chapter/chapter-16 Chapter 16] ( [[#Klein--2014|Klein et al., 2014]] ) reported increasing insights emerging from the interactions between climate change and biophysical and socioeconomic constraints, and highlighted the fact that limits could be both hard and soft. It also noted that residual losses and damages will occur from climate change despite adaptation and mitigation action. However, AR5 [https://www.ipcc.ch/report/ar6/wg2/chapter/chapter-16 Chapter 16] still found that the empirical evidence needed to identify limits to adaptation of specific sectors, regions, ecosystems or species that can be avoided with different greenhouse gas (GHG) mitigation pathways was lacking.

[[#16.4|Section 16.4]] provides a more comprehensive assessment of limits to adaptation, highlighting again that limits to adaptation are not fixed, but are properties of dynamic socio-ecological systems. They are shaped not only by the magnitude of the climate hazards (e.g., the amount of sea level rise in low-lying coasts and islands) and the exposure and vulnerability to those hazards (e.g., people and assets in those areas), but also by physical, infrastructural and social tolerance thresholds and adaptation choices of actors in societies (e.g., the decision to migrate from locations strongly impacted by climate change). The evolution of such socioeconomic systems over time, including their interaction with the changing physical climate, determines the evolution of limits to adaptation.

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==== 16.1.3.4 What Future Risks Are of Greatest Concern? ====

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The fourth and final element of the chapter is the question about the risks we face, and which ones we should be most concerned about. This is addressed in Sections 16.5 and 16.6.

[[#16.5.1|Section 16.5.1]] presents a full discussion of ‘key risks’, synthesised from across all chapters, defined as those risks that are potentially severe and therefore especially relevant to the interpretation of ‘dangerous anthropogenic interference with the climate system’ in the terminology of UNFCCC Article 2.

In 2015, the Paris Agreement established the goal of ‘holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels’. However, assessment of key risks across a range of future warming levels remains a high priority for several reasons: (1) understanding risks at higher levels of warming can help prepare for them, should efforts to limit warming be unsuccessful ( [[#UNEP--2017|UNEP, 2017]] ); (2) understanding risks at higher levels can inform the benefits of limiting warming to lower levels; (3) in addition, there is continued debate about whether warming limits should be at or rather somewhere below 2°C (in particular at 1.5°C); and (4) there is a more explicit recognition that key risks can result not only from increased warming, but also from changes in the exposure and vulnerability of society, and from a lack of ambitious adaptation efforts. Thus, relatively limited warming does not automatically imply that key risks will not occur. In assessing key risks, we have applied four criteria: magnitude of adverse consequences, likelihood of adverse consequences, temporal characteristics of the risk, and ability to respond. Of course, this is an aggregated approach to what is dangerous; it should be noted that in practice, ‘dangerous’ will occur at a myriad of temperature levels depending on who or what is at risk (and their circumstances), geographic scale and time scale.

A new element is that we particularly look at a set of eight ‘representative key risks’ that exemplify the underlying set of key risks identified in the earlier chapters: risk to the integrity of low-lying coastal socio-ecological systems, risk to terrestrial and ocean ecosystems, risk to critical physical infrastructure and networks, risk to living standards (including economic impacts, poverty and inequality), risk to human health, risk to food security, risk to water security, and risk to peace and human mobility ( [[#16.5.2.3|Section 16.5.2.3]] ).

Another increased focus relates to the issue of compound risks. This includes risks associated with compound hazards (Working Group I AR6 Chapter 11, [[#Seneviratne--2021|Seneviratne et al., 2021]] ), but also implications for future risk when repeated impacts erode vulnerability, as well as through transboundary effects (including effects both from one system to a neighbouring one, as well as from one system to a distant one), also discussed in the cross-chapter box on inter-regional risks and adaptation (Cross-Chapter Box INTEREG in this Chapter).

[[#16.6|Section 16.6]] maps the representative key risks in [[#16.5|Section 16.5]] to the SDGs, noting both direct and indirect implications for climate resilient development as assessed in Chapter 18.

Finally, [[#16.6|Section 16.6]] presents an updated assessment of the so-called Reasons for Concern (RFC): risks related to unique and threatened systems, extreme events, distribution of impacts, aggregate impacts (including the cross-chapter box on the global economic impacts of climate change and the social cost of carbon, Cross-Working Group Box ECONOMIC) and the risk of irreversible and abrupt transitions.

The AR4 and AR5 each also evaluated the most important climate risks, framed firstly in terms of the state of knowledge relevant to Article 2 of the UNFCCC. The TAR first synthesised this knowledge in five RFCs. AR4 identified a set of ‘key vulnerabilities’ and provided an update of the RFCs. AR5 further refined a new risk framework developed in the IPCC Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX), and used it to assess ‘key risks’ and provide another update of the overarching RFCs, drawing as well on Cramer et al.’s (2014) assessment of observed changes.

Our risk assessment also further builds on risk assessments from the Special Reports that are part of the AR6 cycle, that is, SR15, SRCCL and SROCC. While since AR4 the RFC assessment framework has remained largely consistent, refinements in methodology have included the consideration of different risks, the role of adaptation, use of confidence statements, more formalised protocols and standardised metrics ( [[#Zommers--2020|Zommers et al., 2020]] ). In subsequent assessment cycles, the risk level at a given temperature has generally increased, reflecting accumulating scientific evidence ( [[#Zommers--2020|Zommers et al., 2020]] ).

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