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

==== 16.6.3.3 Distribution of Impacts (RFC3) ====

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RFC3 reflects how key risks are distributed unevenly across regions and different population groups, due to the non-uniform spatial distributions of physical climate change hazards, exposure and vulnerability across regions. It addresses how risks disproportionately affect particularly vulnerable societies and socio-ecological systems, including disadvantaged people and communities in countries at all levels of development. AR5 concluded that low-latitude and less developed areas generally face greater risk than higher-latitude and more developed countries, including for food- and health-related risks. This conclusion remains valid and is now supported by greater evidence across a range of sectors and geographic regions.

Note that the assessment here is largely based on the national and regional distribution of impacts, rather than sub-national distribution or explicit consideration of vulnerable elements of society. Climate risks are also strongly related to inequalities, often but not always intersecting with poverty ( [[#16.1|Section 16.1]] ), geographic location, and political and socio-cultural aspects. Thus, countries with high inequality tend to be more vulnerable, and more exposed, to climate hazards ( [[#16.1|Section 16.1]] ). While the literature assessed here tends to be insufficiently granular to resolve local inequalities, it does confirm the AR5 finding that low-latitude and less developed areas generally face greater risk.

AR6 continues to highlight the uneven regional distribution of projected climate change risks. Biodiversity loss is projected to affect a greater number of regions with increasing warming, and to be highest in northern South America, southern Africa, most of Australia, and northern high latitudes ( [[IPCC:Wg2:Chapter:Chapter-2#2.5.1.3|Section 2.5.1.3]] , ''medium confidence'' ). Climate change is projected to increase the number of people at risk of hunger in mid-century, concentrated in Sub-Saharan Africa, South Asia and Central America (Chapter 5, ''high confidence'' ), increasing undernutrition, stunting and related childhood mortality particularly in Africa and Asia and disproportionately affecting children and pregnant women (Chapter 7, ''high confidence'' ), strongly mediated by socioeconomic factors (Sections 7.2.4.4, 7.3.1, ''very high confidence'' ). Strong geographical differences in heat-related mortality are projected to emerge later this century, mainly driven by growth in regions with tropical and subtropical climates ( [[IPCC:Wg2:Chapter:Chapter-7#7.3.1|Section 7.3.1]] , ''very high confidence'' ).

In AR5 and SR15, the transition from undetectable to moderate risk was located below what were at the time ‘recent’ temperatures of between 0.5°C and 0.8°C above pre-industrial levels, with ''medium'' to ''high confidence'' , based on evidence of distributional impacts on crop production and water resources. New literature has continued to confirm this transition has already taken place, including more recent observed impacts for regions and groups within the food and water sectors, strongly linked to Representative Key Risks for health, water and food security (Sections 16.2, 16.5, 5.4.1, 5.5.1, 5.8.1, 5.12; Chapter 7).

In AR6, moderate risks have already been assessed to have occurred in Africa for economic growth and reduced inequality, biodiversity and ecosystems, mortality and morbidity due to heat extremes and infectious disease, and food production in fisheries and crop production (Figure 9.6). In Europe, moderate risks to heat stress, mortality and morbidity have already been reached, as well as for water scarcity in some regions (Figure 13.30, Figure 13.3 1). In Australasia, moderate risks are assessed as present already for heat-related mortality risk as well as cascading effects on cities and settlements, and also very high risks already present in coral reef systems, and high risks to kelp forests and alpine biodiversity (Figure 17.6). In North America, moderate risks have already been reached for freshwater scarcity, water quality (Figure 14.4), agriculture, forestry, tourism, transport, energy and mining, and construction (Figure 14.10).

For this assessment, the transition to moderate risk was assessed to have occurred between 0.7°C and 1.0°C of warming with ''high confidence'' , demonstrating that a moderate level of risk exists at present. The 0.2°C increase in this temperature range as compared with AR5 reflects the fact that AR6 WGI has assessed that the level of global warming reached by 1986–2005 was 0.52–0.82°C (as opposed to 0.55–0.67°C in previous assessments), and also reflects the opportunity for observations to be have made of the observed consequences of the additional rise in temperature that has taken place since the literature underpinning the AR5 assessment was published.

In AR5, the transition from moderate to high risk was assessed to occur between 1.6°C and 2.6°C above the pre-industrial levels with ''medium confidence'' . In SR15, new literature on projected risks allowed this range to be narrowed to 1.5–2°C. There is now substantial literature providing ''robust evidence'' of larger regional risks at 2°C warming than 1.5°C and in a range of systems, including crop production (with risks of simultaneous crop failure) (Thiault et al.; [[#Gaupp--2019|Gaupp et al., 2019]] ), aquaculture and fisheries ( [[#Cheung--2018b|Cheung et al., 2018b]] ; [[#Froehlich--2018|Froehlich et al., 2018]] ; [[#Stewart-Sinclair--2020|Stewart-Sinclair et al., 2020]] ), nutrition-related health ( [[#Springmann--2016|Springmann et al., 2016]] ; [[#Lloyd--2018|Lloyd et al., 2018]] ; [[#Sulser--2021|Sulser et al., 2021]] ) and exposure to stressors such as drought, floods ( [[#Alfieri--2017|Alfieri et al., 2017]] ; [[#Hirabayashi--2021|Hirabayashi et al., 2021]] ) and extreme heat ( [[#Dosio--2018|Dosio et al., 2018]] ; [[#Harrington--2018|Harrington et al., 2018]] ; [[#Sun--2019|Sun et al., 2019]] ). One study ( [[#Gaupp--2019|Gaupp et al., 2019]] ) found that the risk of simultaneous crop failure in maize is estimated to increase from 6% to 40% at 1.5°C relative to the historical baseline climate. In particular, further research on projected regional yield declines of wheat and maize between 1.5°C and 2°C, especially in Africa, has accrued [[#Asseng--2015|Asseng et al. (2015)]] , including in Ethiopia ( [[#Abera--2018|Abera et al., 2018]] ) with associated economic effects ( [[#Wang--2019|Wang et al., 2019]] ). Optimum maize production areas in East Asia are projected to reduce in area by 38% for global warming of 1.5–2.0°C ( [[#He--2019|He et al., 2019]] ). A study of Jamaica also estimated that warming of less than 1.5°C will have an overall negative impact on crop suitability and a general reduction in the range of crops, but above 1.5°C, irreversible changes to Jamaica’s agriculture sector were projected ( [[#Rhiney--2018|Rhiney et al., 2018]] ).

Projections of increasing flood risk associated with global warming of 1.5°C and 2°C continue to highlight regional disparities, with larger-than-average increases projected in Asia and Africa ( [[#Hirabayashi--2021|Hirabayashi et al., 2021]] ), including in China, India and Bangladesh ( [[#Alfieri--2017|Alfieri et al., 2017]] ). Similarly, nearly 80% of the 8–80 million additional people projected to be at risk of hunger owing to climate change are located in Africa and Asia ( [[#Springmann--2016|Springmann et al., 2016]] ; [[#Lloyd--2018|Lloyd and Oreskes, 2018]] ; [[#Nelson--2018|Nelson et al., 2018]] ). [[#Schleussner--2016b|Schleussner et al. (2016b)]] analysed hotspots of multi-sectoral risks with 1.5°C and especially 2°C warming, and highlighted projected crop yield reductions in West Africa, Southeast Asia, and Central and northern South America; a reduction in water availability in the Mediterranean; and widespread bleaching of tropical coral reefs.

High risks to crop production are assessed to occur in Africa with ~1.5–2°C warming (Figure 9.6), to agriculture in North America with ~1.5°C warming (Figure 14.10), and with ~2.8°C in Europe (Figure 13.30). High risks of mortality and morbidity due to heat extremes and infectious disease are assessed to be reached in Africa with ~1.5°C warming (Figure 9.6); heat stress, mortality and morbidity in Europe are assessed to reach a high level of risk at ~2°C (Figure 13.30). Heat-related mortality risk transitions to a high level by ~1.5–2°C warming in Australasia, while cascading effects on cities reach high risk with ~1.2°C warming (Figure 17.6). Risks to water scarcity, forestry, tourism and transportation in North America are projected to reach high levels with ~2°C warming (Figure 14.4, Figure 14.10).

Two complementary multi-sectoral analyses indicate that South Asia and Africa become hotspots of multi-sectoral climate change risk, largely due to changes in water-related indicators which also affect crop production ( [[#Arnell--2018|Arnell et al., 2018]] ; [[#Byers--2018|Byers et al., 2018]] ). For instance, [[#Byers--2018|Byers et al. (2018)]] found that the doubling in global exposure to multi-sector risks that accrues as warming increases from 1.5°C to 2°C is concentrated in Asian and African regions (especially East Africa), which together account for 85–95% of the global exposure.

Considering this evidence, for this assessment, the temperature range for the transition from moderate to high risk is located between 1.5°C and 2°C above pre-industrial levels, with ''high confidence'' in the lower bound of 1.5°C, but ''medium confidence'' in the upper bound of 2°C, because simulation studies do not account for climate variability and therefore risks could be higher.

Very high risk implies limited ability to adapt. Adaptation potential not only differs across sectors and regions, but also occurs on different time scales depending on the nature and implementation level of the adaptation option under consideration and the system in which it is to be deployed. The costs of adaptation actions that would be needed to offset projected climate change impacts for major crop production are projected to rise once global warming reaches 1.5°C ( [[#Iizumi--2020|Iizumi et al., 2020]] ). It has been estimated that the number of additional people at risk of hunger with 2.0°C global warming could be reduced from 40 million to 30 million by raising the level of adaptation action ( [[#Baldos--2014|Baldos and Hertel, 2014]] ), but beyond this level of warming residual impacts are projected to escalate ( [[#Iizumi--2020|Iizumi et al., 2020]] ). [[IPCC:Wg2:Chapter:Chapter-5|Chapter 5]] assessed the potential of existing farm management practices to reduce yield losses, finding an average 8% loss reduction in mid-century and 11% by end-century ( [[IPCC:Wg2:Chapter:Chapter-5#5.4.4.1|Section 5.4.4.1]] ), which is insufficient to offset the negative impacts from climate change, particularly in currently warmer regions ( [[IPCC:Wg2:Chapter:Chapter-5#5.4.3.2|Section 5.4.3.2]] ). The literature indicates that, globally, crop production may be sustained below 2.0°C warming with adaptation, but negative impacts will prevail at 2.0°C warming and above in currently warm regions ( [[IPCC:Wg2:Chapter:Chapter-5#5.4.4.1|Section 5.4.4.1]] ). Importantly, residual damage (that which cannot be avoided despite adaptation) is projected to rise around 2.0°C global warming ( [[#Iizumi--2020|Iizumi et al., 2020]] ). Evidence of constraints and limits for food, fibre and other ecosystem products for the different regions is evident for the various regions ( [[#16.4.3.1|Section 16.4.3.1]] ) indicating limited ability to adapt. Adaptation costs are also higher relative to GDP in low-income countries, for example for the building of sea dikes ( [[#Brown--2021|Brown et al., 2021]] ).

In previous reports, the transition from high to very high risk for the distribution of impacts was not assessed due to limited available literature, but there is now sufficient evidence to do so. A range of literature quantifies the increasing regional probability of drought as compared with the present day, with projected increases in the area exposed to drought ( [[#Carrão--2018|Carrão et al., 2018]] ; [[#Pokhrel--2021|Pokhrel et al., 2021]] ), as well as in the duration ( [[#Naumann--2018|Naumann et al., 2018]] ) and frequency of droughts, with higher warming levels. [[#Naumann--2018|Naumann et al. (2018)]] showed that, for drying areas, drought durations are projected to rise from 2 months per °C below 1.5°C to 4.2 months per °C near 3°C warming. Most of Africa, Australia, southern Europe, southern and central USA, Central America, the Caribbean, northwest China, and parts of Southern America are projected to experience more frequent droughts. Adverse effects of climate change on food production are projected to become much more severe ( [[IPCC:Wg2:Chapter:Chapter-5#5.4.3.2|Section 5.4.3.2]] ) when global temperatures rise more than 2°C globally, but there are predicted to be much more negative impacts experienced sooner on food security in low to mid-latitudes ( [[#Richardson--2018a|Richardson et al., 2018a]] ) ( [[IPCC:Wg2:Chapter:Chapter-5#5.4.1|Section 5.4.1]] ). For instance, climate change by 2050 is projected to increase the number of people at risk of hunger by between 8 and 80 million with 2–3°C warming compared with no-climate-change conditions ( [[#Baldos--2014|Baldos and Hertel, 2014]] ; [[#Hasegawa--2018|Hasegawa et al., 2018]] ; [[#Nelson--2018|Nelson et al., 2018]] ; [[#Janssens--2020|Janssens et al., 2020]] ). In addition to effects upon crop yield, agricultural labour productivity, food access and food-related health are projected to be negatively impacted by 2–3°C warming ( [[#Springmann--2016|Springmann et al., 2016]] ; [[#de%20Lima--2021|de Lima et al., 2021]] ). Regionally, substantial regional disparity in risks to food production is projected to persist at these higher levels of warming. Risks for heat-related morbidity and mortality, ozone-related mortality, malaria, dengue, Lyme disease and West Nile fever are projected to increase regionally and globally (Chapter 7) with potential infestation areas for disease-carrying vectors in multiple geographic regions that could be five times higher at 4°C than at 2°C ( [[#Liu-Helmersson--2019|Liu-Helmersson et al., 2019]] ).

Very high risks to crop production are assessed to occur in Africa above ~2.5°C warming (Figure 9.6) and below 4°C in Europe (Figure 13.29 ). Very high risks of mortality and morbidity due to heat extremes and infectious disease are assessed to occur in Africa with 2.5°C warming (Figure 9.6); heat stress, mortality and morbidity in Europe are assessed to reach a very high level of risk at ~3.2°C (Figure 13.30). Heat-related mortality risk and cascading effects on cities both transition to a very high level by ~2.5°C warming in Australasia (Figure 11.7). Risks to water scarcity in North America are projected to reach very high levels with 3.5°C warming (Figure 14.4). Hence, this assessment concludes with ''medium confidence'' that a transition from high to very high risks, in terms of distribution of impacts, begins at 2°C global warming, with a full transition to very high risks completed by 3.5°C. However, it should be noted that many studies upon which this assessment has been based have not taken into account the impacts of extreme weather events and oscillations in sea surface temperatures; hence, risks at a given level of global warming might be underestimated in the literature.

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