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

==== 16.5.3.1 Warming Level, Including Risks Avoided by Mitigation ====

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Studies illustrating sensitivity to warming level typically do so by contrasting projected impacts for the same socioeconomic conditions but different climate pathways or temperature levels, often based on Representative Concentration Pathways (RCPs) ( [[#van%20Vuuren--2014|van Vuuren and Carter, 2014]] ). We refer to future climate conditions either based on their global average warming level or as a ‘high warming’ scenario (based on RCP8.5), medium warming (RCP4.5 or RCP6.0) or low warming (RCP2.6 or 1.5°C scenarios). Because some of these scenarios assume no or minimal mitigation (RCP8.5, RCP6.0) while others do (RCP4.5, RCP2.6), differences in outcomes between them reflect risks avoided by mitigation (assuming consistent socioeconomic assumptions).

Some ecological risks (Chapter 2) are particularly sensitive to warming. For example, warm-water coral reefs are already experiencing High risk levels and are expected to face Very High risks under 1.5°C of global warming ( [[#Hoegh-Guldberg--2018a|Hoegh-Guldberg et al., 2018a]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ). Some societal risks, such as human mortality due to extreme heat, also are sensitive to warming. A medium-warming scenario (relative to high warming) reduces projected global average mortality due to heat from seven deaths per 10,000 people yr –1 (7/10,000 yr −1 ) by 2100 to ~1/10,000 yr −1 , assuming high-vulnerability societal conditions ( [[#Carleton--2020|Carleton et al., 2020]] ). At the national level, without considering adaptation, reductions in a broader measure of mortality are projected across a range of countries including Colombia, the Philippines, and several in Europe ( [[#Guo--2018|Guo et al., 2018]] ), and exposure of the US population to high-mortality heatwaves is reduced by nearly half ( [[#Anderson--2018a|Anderson et al., 2018a]] ). Without considering changes in exposure or vulnerability, warming of 1.5–2°C (compared with 4–5°C) reduces global mortality impacts from an increase of 2.1–13.0% to 0.1–2.2% ( [[#Gasparrini--2017|Gasparrini et al., 2017]] ; [[#Vicedo-Cabrera--2018a|Vicedo-Cabrera et al., 2018a]] ) and impacts in China from up to 4/10,000 yr −1 ( [[#Weinberger--2017|Weinberger et al., 2017]] ) to 0.3–0.5/10,000 yr −1 ( [[#Wang--2019|Wang and Hijmans, 2019]] ).

A low-warming scenario (relative to high warming) reduces aggregate economic impacts from around 7% of global GDP to less than 1% (Takakura et al., 2019), and changes impacts on the number of people suffering from hunger from an increase (by 7–55 million) to a decrease (by up to 6 million) ( [[#Janssens--2020|Janssens et al., 2020]] ). Low versus high warming also reduces the coastal population at risk of flooding due to SLR from tripling by 2100 (relative to today) to doubling ( [[#Kulp--2019|Kulp and Strauss, 2019]] , [[#16.5.2.3|Section 16.5.2.3.2]] ). The SROCC estimates that SLR risks are reduced from Moderate-to-High to Moderate for large tropical agricultural deltas and resource-rich megacities, and from High and Very High to Moderate-to-High for Arctic human communities and urban atoll islands, respectively ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ).

Higher levels of warming are projected to also generate higher income inequality between countries (e.g., [[#Pretis--2018|Pretis et al., 2018]] ; Takakura et al., 2019) as well as within them ( [[#Hallegatte--2016|Hallegatte et al., 2016]] ) even though other drivers will be more important ( [[#16.5.2.3.5|Section 16.5.2.3.5]] ). Similarly, climate and weather events are expected to play an increasing role in shaping risks to peace ( ''limited evidence, medium agreement)'' and migration ( ''medium evidence, high agreement'' ) in the future, but uncertainty is high due to complex causal pathways and non-climate factors likely dominate outcomes ( [[#16.5.2.3.8|Section 16.5.2.3.8]] ). There is ''high agreement'' that future SLR will amplify levels of forced migration from small islands and low-lying coastal areas in the absence of appropriate adaptive responses ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ).

A synthesis of risk assessments in the recent IPCC Special Reports ( [[#Magnan--2021|Magnan et al., 2021]] ) concludes that an integrated measure of today’s global climate risk level will increase by the end of this century by two- to four-fold under low and high warming, respectively (based on aggregated scores developed in the study). An additional comparison of risk levels under +1.5°C and +2°C suggests that every additional 0.5°C of global warming will increase the risk level by about a third.

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