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

==== 16.6.3.4 Global Aggregate Impacts (RFC4) ====

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This RFC considers impacts to socio-ecological systems that can be aggregated globally into a single metric, such as monetary damages, lives affected, species lost or ecosystem degradation at a global scale ( [[#Oppenheimer--2014|Oppenheimer et al., 2014]] ; [[#O’Neill--2017|O’Neill et al., 2017]] ). RFC4 shares underlying key risk components with other RFCs (e.g., RFC1 and RFC2, see [[#O’Neill--2017|O’Neill et al., 2017]] ) and thus draws on a similar literature as those assessments; however, this RFC focuses on impacts that reach levels of concern at the global level and also weighs the composite effect of risk elements ranging from economic to biodiversity.

In AR5 Section 19.6.3.5 ( [[#Oppenheimer--2014|Oppenheimer et al., 2014]] ), the transition from undetectable to moderate risk was assessed between 1.6°C and 2.6°C above pre-industrial levels (i.e., 1°C and 2°C above the 1986–2005 level) based on impacts to both Earth’s biodiversity and the overall global economy with ''medium confidence'' . The risk transition between moderate and high risk was set around 3.6°C above pre-industrial levels (i.e., 3°C above the 1986–2005 level), based on literature finding extensive species vulnerability and biodiversity damage with associated loss of ecosystem goods and services at 3.5°C ( [[#Foden--2013|Foden et al., 2013]] ; [[#Warren--2013|Warren et al., 2013]] ). In SR15 [[IPCC:Wg2:Chapter:Chapter-3#3.5.2|Section 3.5.2.4]] ( [[#Hoegh-Guldberg--2018b|Hoegh-Guldberg et al., 2018b]] ), economic literature on potential socioeconomic threshold events and empirical studies of global economic damages, combined with new evidence on biome shifts, extinction risk, species range loss (especially noting the integral role of insects in ecosystem function) and ecosystem degradation, were assessed, and the upper bound of the transition to moderate risk was lowered to 1.5°C warming above pre-industrial levels, and the transition from moderate and high risk was lowered to between 1.5°C and 2.5°C ( ''medium confidence'' ). The boundary between high risk and very high risk was not assessed in either of these reports because the temperature threshold was beyond the scope of the assessment in the case of SR15 and literature available for this highest transition in AR5 was limited.

Since AR5, many new global estimates of the aggregate, economy-wide risks of climate change have been produced, though, as was the case in AR5, these continue to exhibit a low level of agreement, including for today’s level of global warming, due primarily to differences in methods. Cross-Working Group Box ECONOMIC in this chapter includes a more thorough discussion of advancements and limitations of global economic impact estimates and methodologies, finding significant variation in estimates that increases with warming, indicating higher risk in terms of economic costs at higher temperatures ( ''high confidence'' ). Climate change has been found to exacerbate poverty through declines in agricultural productivity, changes in agricultural prices and extreme weather events ( [[#Hertel--2014|Hertel and Lobell, 2014]] ; [[#Hallegatte--2017|Hallegatte and Rozenberg, 2017]] ). In terms of biodiversity risks, the literature indicates that losses in terrestrial and marine ecosystems increase substantially between 1.5°C and 2°C of warming ( [[#Hoegh-Guldberg--2018b|Hoegh-Guldberg et al., 2018b]] ). Since SR15, further evidence of degradation of biodiversity and ecosystem services and ocean acidification at the global aggregate level has continued to accrue due to climate change (see Chapter 2).

For this RFC, the transition from undetectable to moderate risk to global aggregate impacts is assessed with ''medium confidence'' to occur between 1.0°C (start of transition) and 1.5°C (completion of transition) with a median judgement of transition at 1.3°C, based on evidence of a combination of economic consequences, widespread impacts to climate-sensitive livelihoods, changes in biomes, and loss of terrestrial and marine biodiversity. The start of the transition from undetectable to moderate risk is located at recent temperatures based on observed impacts to biodiversity ( [[#16.2.3.1|Section 16.2.3.1]] ). Experts noted aggregate impacts on biodiversity are detectable, with damages that have had global significance (e.g., drought, pine bark beetles, coral reef ecosystems). Consistent with the start of this transition at 1°C, a similar elicitation conducted in [[IPCC:Wg2:Chapter:Chapter-2|Chapter 2]] assessed that risks to biodiversity globally have already transitioned to a moderate level with 1°C warming, while risks of widespread tree mortality are already moderate with 0.9°C warming and moderate risks of ecosystem structure change began with warming of 0.5°C (Table SM2.5, Figure 2.11). Human-induced warming has slowed growth of agricultural productivity over the past 50 years in mid- and low latitudes (Chapter 5; [[#Hurlbert--2019|Hurlbert et al., 2019]] ). Although there is not yet strong evidence of attributable loss of life and livelihoods at the global level (Sections 16.5.2.3.4, 16.5.2.3.5), experts found that regional evidence of such observed impacts was still relevant to defining the beginning of the transition (e.g., Table SM16.22, Chapter 9). Informing the median value and upper bound of the transition to moderate risk, empirical studies and scenario analyses have found that regions with high dependence on climate-sensitive livelihoods like agriculture, fisheries and forestry would be severely impacted even at low levels of warming under conditions of low adaptation (RKR-D, [[#Lobell--2011|Lobell et al., 2011]] ; [[#Hoegh-Guldberg--2018b|Hoegh-Guldberg et al., 2018b]] ).

The transition to high risk is assessed with ''medium confidence'' to occur between 1.5°C (start of transition) and 2.5°C (completion of transition) with a median judgement of transition at 2.0°C. Though economic estimates exhibit wide variation and ''low agreement'' at warming levels above 1.5°C, many estimates are nonlinear, with marginal economic impacts increasing with temperature (see Cross-Working Group Box ECONOMIC in this Chapter). At 1.5°C warming, most aggregate global impacts to GDP are negative across different estimation methods, including bottom-up estimation (e.g., Takakura et al., 2019), meta-analysis (e.g., [[#Howard--2017|Howard and Sterner, 2017]] ) and empirical estimations (e.g., [[#Pretis--2018|Pretis et al., 2018]] ; [[#Kalkuhl--2020|Kalkuhl and Wenz, 2020]] ). At 2°C [[#Watts--2021|Watts et al. (2021)]] estimate a relative decrease in effective labour by 10%, which would have profound economic consequences. [[#Byers--2018|Byers et al. (2018)]] found that global exposure to multi-sector risks approximately doubles between 1.5°C and 2°C, while the percentage of the global population exposed to flooding is projected to rise by 24% with 1.5°C warming and by 30% with 2.0°C warning ( [[#Hirabayashi--2021|Hirabayashi et al., 2021]] ).

[[#16.5.2.3|Section 16.5.2.3.4]] (RKR-D, underlying key risk on poverty) reports that, under medium warming pathways, climate change risks to poverty would become severe if vulnerability is high and adaptation is low ( ''limited evidence'' , ''high agreement'' ). At and beyond 1.5°C, approximately 200 million people with livelihoods derived from small-scale fisheries would face severe risk, given sensitivity to ocean warming, acidification and coral reef loss ( [[#Cheung--2018a|Cheung et al., 2018a]] ; [[#Froehlich--2018|Froehlich et al., 2018]] ; [[#Free--2019|Free et al., 2019]] ). Warming between 1.5°C and 2°C could expose 330–396 million people to lower agricultural yields and associated livelihood impacts ( [[#Byers--2018|Byers et al., 2018]] ; [[#Hoegh-Guldberg--2018a|Hoegh-Guldberg et al., 2018a]] ), due to a high dependency of climate-sensitive livelihoods to agriculture globally ( [[#World%20Bank--2020|World Bank, 2020]] ). Models project that climate change will increase the number of people at risk of hunger in 2050 by 8–80 million people globally, with the range depending on the level of warming (1.5–2.9°C) and SSPs ( [[#Nelson--2018|Nelson et al., 2018]] ; [[#Mbow--2019|Mbow et al., 2019]] ; [[#Janssens--2020|Janssens et al., 2020]] ). Higher atmospheric concentrations of carbon dioxide reduce the nutritional quality of wheat, rice and other major crops, potentially affecting millions of people at a doubling of carbon dioxide relative to pre-industrial ( ''very high confidence'' ) ( [[IPCC:Wg2:Chapter:Chapter-7#7.3.1|Section 7.3.1]] ). Global ocean animal biomass is projected to decrease on average by 5% per 1°C increase; hence, a 2.5°C level of warming is associated with ~13% decline in ocean animal biomass, which would considerably reduce marine food provisioning, fisheries distribution and revenue value, with further consequences for ecosystem functioning (Chapter 5, ''medium confidence'' ).

Losses in terrestrial and marine biodiversity increase substantially beyond 1.5°C of warming ( [[#Hoegh-Guldberg--2018b|Hoegh-Guldberg et al., 2018b]] ). [[#16.5.2.3|Section 16.5.2.3.2]] (RKR-B, risks to terrestrial and marine ecosystems) finds that substantial biodiversity loss globally, abrupt local ecosystem mortality impacts, and ecological species disruption are all projected at global warming levels below 3°C, with insular systems and biodiversity hotspots at risk below 2°C ( ''medium confidence'' ). Insects play a critical role in providing vital ecosystem services that underpin human systems, with major losses of their climatically determined geographic range at 2°C warming implying adverse effects on ecosystem functioning. Consistent with the transitions presented here, a similar burning ember developed in [[IPCC:Wg2:Chapter:Chapter-2|Chapter 2]] assessed a transition from moderate to high risks globally for marine and terrestrial biodiversity (e.g., widespread death of trees, damages to ecosystems, and reduced provision of ecosystem services, and structural change, including biome shifts) beginning between 1.0°C and 2.0°C warming ( Table SM2.5, Figure 2.11).

Though explicit treatment of adaptation is limited in the RFC4 impacts literature (i.e., studies that compare risks for specific adaptation scenarios in terms of globally aggregated impacts with quantified findings), there is evidence of the potential for investments in improved hydro-meteorological information and early-warning systems to avoid some of the most adverse social and economic impacts from extreme weather events in both developed and developing countries, with benefits at a globally significant level ( [[#Hallegatte--2012|Hallegatte, 2012]] ). Studies of adaptation in the agriculture sector (e.g., changing crop variety, timing of crop planting, new types of irrigation, etc.) and infrastructure (e.g., coastal protection, hardening of critical infrastructure, flood and climate-resistant building materials and water storage) show large potential benefits in terms of reduced impacts to lives and livelihoods ( [[#van%20Hooff--2015|van Hooff et al., 2015]] ; [[#Mees--2017|Mees, 2017]] ). At higher warming levels, however, potential adaptations to address biodiversity loss are expected to be limited due to the projected rate and magnitude of change as well as the resources required ( [[#Hannah--2020|Hannah et al., 2020]] ).

The transition to very high risks is assessed to occur within a range of 2.5–4.5°C with ''medium confidence'' over the range, and ''low confidence'' assessed over a narrowed ‘best estimate’ range of 2.7–3.7°C. The lower end of the range reflects the loss of an increasingly large fraction of biodiversity globally. [[IPCC:Wg2:Chapter:Chapter-2|Chapter 2]] has assessed a transition from high to very high risks globally for biodiversity (marine and terrestrial) completing at ~2.5°C warming, noting widespread death of trees, damages to ecosystems, and reduced provision of ecosystem services over the temperature range 2.5–4.5°C ( Table SM2.5, Figure 2.11) and, similarly, a transition from high to very high risks of ecosystem structure change (including biome shifts) between 3°C and 5°C warming ( Table SM2.5, Figure 2.11). A global study of 115,000 common species projects climatically determined geographic range losses of over 50% in 49% of insects, 44% of plants and 26% of vertebrates with global warming of 3.2°C, implying an associated effect on provisional and regulating ecosystem services that support human well-being, including pollination and detritivory ( [[#Warren--2018a|Warren et al., 2018a]] ). The risk of abrupt impacts on ecosystems as multiple species approach tolerance limits simultaneously is projected to threaten up to 15% of ecological communities with 4°C of warming ( [[#Trisos--2020|Trisos et al., 2020]] ). Under a 4°C warming scenario, models project global annual damages associated with SLR of $31,000 billion yr –1 in 2100 ( [[#Brown--2021|Brown et al., 2021]] )

In terms of global economic impact, while an emerging economic literature is addressing many gaps and critiques of previous damage estimates for high warming (e.g., [[#Jensen--2014|Jensen and Traeger, 2014]] ; [[#Burke--2015|Burke et al., 2015]] ; [[#Lontzek--2015|Lontzek et al., 2015]] ; [[#Moore--2015|Moore and Diaz, 2015]] ; [[#Lemoine--2016|Lemoine and Traeger, 2016]] ; [[#Moore--2017a|Moore et al., 2017a]] ; Cai and Lontzek; Takakura et al., 2019, discussed further in Cross-Working Group Box ECONOMIC; [[#Carleton--2020|Carleton et al., 2020]] ; [[#Méjean--2020|Méjean et al., 2020]] ; [[#Rode--2021|Rode et al., 2021]] ), there remains wide variation across disparate methodologies, though the spread of estimates increases with warming in all methodologies, indicating higher risk in terms of economic costs at higher temperatures ( ''high confidence'' ). [[#16.5.2.3|Section 16.5.2.3.4]] (RKR-D) finds that risks to aggregate economic output would become severe at the global scale at high warming (~4.4°C) and minimal adaptation ( ''medium confidence'' ), defining severity as ‘the potential for persistent annual economic losses due to climate change to match or exceed losses during the world’s worst historical economic recessions’. Furthermore, climate change impacts on income inequality could compound risks to living standards ( ''high confidence'' , 16.5.2.3.4). [[IPCC:Wg2:Chapter:Chapter-4|Chapter 4]] finds that, at 4°C, 4 billion people are projected to be exposed to physical water scarcity ( ''medium confidence'' ).

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