Editing IPCC:AR6/WGII/Chapter-13 (section)

==== 13.10.2.1 KR1: Risks of Human Mortality and Heat Stress, and of Ecosystem Disruptions Due to Heat Extremes and Increases in Average Temperatures ====

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Key risk 1 has cut across humans and ecosystems, and severe consequences are mainly driven by an increasing frequency, intensity and duration of heat extremes and increasing average temperatures ( ''high confidence'' ) ( [[#Urban--2015|Urban, 2015]] ; [[#Forzieri--2017|Forzieri et al., 2017]] ; [[#Feyen--2020|Feyen et al., 2020]] ; [[#Naumann--2020|Naumann et al., 2020]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ). The risk of human heat stress and mortality is largely influenced by underlying socioeconomic pathways, with consequences being more severe under SSP3, SSP4 and SSP5 scenarios than SSP1 ( ''very high confidence'' ) (Figure 13.22; Sections 13.6.1.5.2, 13.7.1.1; [[#Hunt--2017|Hunt et al., 2017]] ; [[#Kendrovski--2017|Kendrovski et al., 2017]] ; [[#Rohat--2019|Rohat et al., 2019]] ; [[#Casanueva--2020|Casanueva et al., 2020]] ). The SSPs impact natural systems as well but are not yet well studied. The impact of warming in marine systems are often synergistic with SLR in coastal systems and ocean acidification driven by the rise in CO 2 , while habitat fragmentation and land use have important synergies in terrestrial systems ( ''high confidence'' ) (Sections 13.3.1.2, 13.4.1.2). More intense heatwaves on land and in the ocean, particularly in Mediterranean Europe ( [[#13.4|Section 13.4]] ; Cross-Chapter Paper 4; [[#Darmaraki--2019b|Darmaraki et al., 2019b]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ), are expected to cause mass mortalities of vulnerable species, and species extinction, altering the provision of important ecosystem goods and services ( [[#Marbà--2010|Marbà and Duarte, 2010]] ).

The burning embers on risks for humans (Figure 13.29a) differentiate between present and medium adaptation conditions, drawing on SSP2 and SSP4 (and to a lesser extent SSP3), and high adaptation conditions, drawing on SSP1 and papers using various temperature adjustment methods (Table SM13.25). There is ''high confidence'' that the risk is already moderate now because it has been detected and attributed with ''high confidence'' ( [[#13.10.1|Section 13.10.1]] ). The transition from moderate to high risk for human health is assessed to happen after 1.5°C GWL in a scenario with present to medium adaptation and implies a two- to threefold increase (compared with moderate risk levels) in magnitude of consequences such as mortality, morbidity, heat stress and thermal discomfort ( [[#Rohat--2019|Rohat et al., 2019]] ; [[#Casanueva--2020|Casanueva et al., 2020]] ; [[#Naumann--2020|Naumann et al., 2020]] ). At this level, the risk will also become more persistent across the continent due to increase in heat events exceeding critical thresholds for health ( ''high confidence'' on the direction of change and temperature transition, but ''medium confidence'' on the magnitude) ( [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ).

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'''Figure 13.29 |''' '''Burning embers and illustrative adaptation pathways for risks to human health from heat (Key Risk 1)'''

'''(a)''' Burning ember diagrams for the risk to human health from heat are shown. The low to medium adaptation scenario corresponds to present, SSP2 and SSP4 socioeconomic conditions. The high adaptation includes SSP1 and adaptation needed to maintain current risk levels.

'''(b,c)''' Illustrative adaptation pathways for NEU (top) and SEU (bottom), and key messages based on the feasibility and effectiveness assessment in Figures 13.20 and 13.24. Grey shading means long lead time and dotted lines signal reduced effectiveness. The circles imply transfer to another measure and the bars imply that the measure has reached a tipping point (Tables SM13.24, SM13.25).

The burning embers on risk for terrestrial and marine ecosystems, and some of their services, are shown in Figure 13.28 (second and third ember from the left) (Tables SM13.26, SM13.27). The transition to moderate risk is currently happening as warming already results in changes in timing of development, species migration northward and upwards, and desynchronisation of species interactions, especially at the range limits, with cascading and cumulative impacts through ecosystems and food webs ( ''high confidence'' ) (Sections 13.3, 13.4; Figures 13.8, 13.12). While some terrestrial ecosystems are already impacted today, such as Alpine, cryosphere and peatlands, the impacts are not widespread and severe yet across a wide range of terrestrial systems. Around 2°C GWL, losses accelerate in marine ecosystem and appear across systems, including habitat losses especially in coastal wetlands ( [[#Roebeling--2013|Roebeling et al., 2013]] ; [[#Clark--2020|Clark et al., 2020]] ), biodiversity and biomass losses ( [[#Bryndum-Buchholz--2019|Bryndum-Buchholz et al., 2019]] ; [[#Lotze--2019|Lotze et al., 2019]] ) and ecosystem services such as fishing ( ''high confidence'' on the direction of change, but ''medium confidence'' on the local and regional magnitude) ( [[#Raybaud--2017|Raybaud et al., 2017]] ). The transition is happening at slightly higher warming in terrestrial systems due to a higher number of thermal refugia in terrestrial systems causing relocation but not already severe impacts ( ''medium confidence'' ) (Chapter 2).

There is ''medium confidence'' that high adaptation or conditions posing low challenges for adaptation (e.g., SSP1) in the context of human health can delay the transition from moderate to high risk ( [[#Åström--2017|Åström et al., 2017]] ; [[#Ebi--2021|Ebi et al., 2021]] ). The illustrative adaptation pathways in Figure 13.29b,c show the sequencing of options to a high adaptation future for NEU and SEU. Whether or not adaptation measures are effective to reduce risk severity for people’s health depends on local context ( ''high confidence'' ) (Figure 13.29; Sections 13.6.2, 13.7.2). Some adaptation options are found to be highly effective across Europe irrespective of warming levels, including air conditioning and urban planning ( ''high confidence'' ) (Sections 13.6.2, 13.7.2; [[#Jenkins--2014b|Jenkins et al., 2014b]] ; [[#Donner--2015|Donner et al., 2015]] ; [[#Dodoo--2016|Dodoo and Gustavsson, 2016]] ; [[#Åström--2017|Åström et al., 2017]] ; [[#Dino--2019|Dino and Meral Akgül, 2019]] ; [[#Venter--2020|Venter et al., 2020]] ), although air conditioning increasingly faces some feasibility constraints (Figure 13.20). Building interventions alone have low to medium effectiveness independent of the region. Many behavioural changes, such as personal and home heat protection, have already been implemented in SEU ( [[#13.7.2|Section 13.7.2]] ; [[#Martinez--2019|Martinez et al., 2019]] ). To reach high adaptation, a combination of low, medium and high effectiveness measures in different sectors and sub-regions is needed, many of which entail systems’ transformations (e.g., heat-proof land management) (Chapter 16) and remain effective at higher warming levels ( ''medium confidence'' ) ( [[#Díaz--2019|Díaz et al., 2019]] ). These transformations have long lead times, thereby requiring timely start of implementation including regions that are not yet experiencing high heat stress (e.g., NEU) ( ''high agreement, medium evidence'' ).

Autonomous adaptation of species via migration in response to climate change is well documented in contemporary, historical and geological records (Chapter 2; Cross-Chapter Box PALEO in Chapter 1); however, the projected rate of climate change can exceed migration potential, leading to evolutionary adaptation or increased extinction risk (Chapters 2, 3; Sections 13.3, 13.4). A reduction of non-climatic stressors, such as nutrient loads, resource extraction, habitat fragmentation or pesticides on land, are considered important adaptation options to increase the resilience to climate-change impacts ( ''high confidence'' ) (Sections 13.3, 13.4; [[#Ramírez--2018|Ramírez et al., 2018]] ). A major governance tool to reduce climatic and non-climatic impacts is the establishment of networks of protected areas (Sections 13.3.2, 13.4.2) especially when aggregated, zoned or linked with corridors for migration ( ''high confidence'' ), as well as a cost-effective adaptation strategy with multiple additional co-benefits ( [[#Berry--2015|Berry et al., 2015]] ; [[#Roberts--2017|Roberts et al., 2017]] ). Reforestation, rewilding and habitat restoration are long-term strategies for reducing risk for biodiversity loss supported by assisted migration and evolution ( [[#13.3.2|Section 13.3.2]] , 13.4), though current laws and regulations do not include species migration ( ''high confidence'' ) ( [[#Prober--2019|Prober et al., 2019]] ; [[#Fernandez-Anez--2021|Fernandez-Anez et al., 2021]] ).

Very high risks are expected beyond 3°C GWL due to the magnitude and increased likelihood of serious consequences, as well as to the limited ability of humans and ecosystems to cope with these impacts. There is ''high confidence'' that even under high adaptation scenarios for human systems or autonomous adaptation of natural systems, the risk will still be high at 3°C GWL and beyond ( [[#13.7.2|Section 13.7.2]] ; [[#Hanna--2015|Hanna and Tait, 2015]] ; [[#Spencer--2016|Spencer et al., 2016]] ) with ''medium confidence'' on the temperature range of the transition. Projected SLR will strongly impact coastal ecosystems ( ''high confidence'' ), minimising their contribution to shoreline protection ( [[#13.10.2.4|Section 13.10.2.4]] ).

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