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

=== 16.5.1 Defining Key Risks ===

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A key risk is defined as a potentially severe risk and therefore especially relevant to the interpretation of dangerous anthropogenic interference (DAI) with the climate system, the prevention of which is the ultimate objective of the UNFCCC as stated in its Article 2 ( [[#Oppenheimer--2014|Oppenheimer et al., 2014]] ). Key risks are therefore a relevant lens for the interpretation of this policy framing. The severity of a risk is a context-specific judgement based on a number of criteria discussed below. KRs are ‘potentially’ severe because, while some could already reflect dangerous interference now, more typically they may become severe over time due to changes in the nature of hazards (or, more broadly, climatic impact drivers; [[#IPCC--2021|IPCC, 2021]] ) and/or of the exposure/vulnerability of societies or ecosystems to those hazards. They also may become severe due to the adverse consequences of adaptation or mitigation responses to the risk (on the former, see [[IPCC:Wg2:Chapter:Chapter-17#17.5.1|Section 17.5.1]] ; the latter is not assessed separately here, except as it contributes to risks from climate hazards). Dangerous interferences in this chapter are considered over the course of the 21st century.

KRs may be defined for a wide variety of systems at a range of scales. The broadest definition is for the global human system or planetary ecological system, but KRs may also apply to regions, specific sectors or communities, or to parts of a system rather than to the system as a whole. For example, the population at the lower end of the wealth distribution is often impacted by climate change much more severely than the rest of the population ( [[#Leichenko--2014|Leichenko and Silva, 2014]] ; [[#Hallegatte--2017|Hallegatte and Rozenberg, 2017]] ; [[#Hallegatte--2017|Hallegatte et al., 2017]] ; [[#Pelling--2019|Pelling and Garschagen, 2019]] ).

KRs are determined not just by the nature of hazards, exposure, vulnerability and response options, but also by values, which determine the importance of a risk. Importance is understood here as the degree of relevance to interpreting DAI at a given system’s level or scale, and was an explicit criterion for identifying key vulnerabilities and risks in AR5 ( [[#Oppenheimer--2014|Oppenheimer et al., 2014]] ). Because values can vary across individuals, communities or cultures, as well as over time, what constitutes a KR can vary widely from the perspective of each of these groups, or across individuals. For example, ecosystems providing indirect services and cultural assets such as historic buildings and archaeological sites may be considered very important to preserve by some people but not by others; and some types of infrastructure, such as a commuter rail, may be important to the well-being of some households but less so to others. Therefore, [https://www.ipcc.ch/report/ar6/wg2/chapter/chapter-16 Chapter 16] authors do not make their own judgements about the importance of particular risks. Instead, we highlight importance as an overarching factor but identify and evaluate KRs based on four other criteria for what may be considered potentially severe.

'''Magnitude of adverse consequences.''' Magnitude measures the degree to which particular dimensions of a system are affected, should the risk materialise. Magnitude can include the size or extent of the system, the ''pervasiveness of the consequences'' across the system (geographically or in terms of affected population), as well as the ''degree of consequences'' . Consequences can be measured by a wide range of characteristics. For example, risks to food security can be measured as uncertain consequences for food consumption, access or prices. The magnitude of these consequences would be the degree of change in these measures induced by climate change and accounting for the interaction with exposure and vulnerability. In addition to ''pervasiveness'' and ''degree of change'' , several other aspects can contribute to a judgement of magnitude, although they refer to concepts that are difficult to capture and highly context-specific:

''Irreversibility of consequences'' . Consequences that are irreversible, at least over long time scales, would be considered a higher risk than those that are temporary. For example, changes to the prevailing ecosystem in a given location may not be reversible on the decade to century scale.

''Potential for impact thresholds or tipping points'' . Higher risks are posed by the potential for exceeding a threshold beyond which the magnitude or rate of an impact substantially increases.

''Potential for cascading effects beyond system boundaries'' . Higher risks are posed by those with the potential to generate downstream cascading effects to other ecosystems, sectors or population groups within the affected system and/or to another system, whether neighbouring or distant (Cross-Chapter Box INTEREG in this Chapter).

'''Likelihood of adverse consequences.''' A higher probability of high-magnitude consequences poses a larger risk ''a priori'' , whatever the scale considered. This probability may not be quantifiable, and it may be conditional on assumptions about the hazard, exposure or vulnerability associated with the risk.

'''Temporal characteristics of the risk.''' Risks that occur sooner, or that increase more rapidly over time, present greater challenges to natural and societal adaptation. A persistent risk (due to the persistence of the hazard, exposure and vulnerability) may also pose a higher threat than a temporary risk due, for example, to a short-term increase in the vulnerability of a population (e.g., due to conflict or an economic downturn).

'''Ability to respond to the risk''' . Risks are more severe if the affected ecosystems or societies have limited ability to reduce hazards (e.g., for human systems, through mitigation, ecosystem management and possibly solar radiation management); to reduce exposure or vulnerability through various human or ecological adaptation options; or to cope with or respond to the consequences, should they occur.

The relative influence of these different criteria is case-specific and left to author judgement in the identification of KRs (groups of authors in regional and sectoral chapters) and the assessment of representative key risks (author teams, see SM16.4). But in general, the more criteria are met, the higher is the risk.

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