Editing IPCC:AR6/SYR/Longer-Report (section)

==== 3.1.2 Impacts and Related Risks ====

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'''For a given level of warming, many climate-related risks are assessed to be higher than in AR5 (''' '''''high confidence).''''' Levels of risk '''[[#footnote-037|120]]''' for all Reasons for Concern '''[[#footnote-036|121]]''' (RFCs) are assessed to become high to very high at lower global warming levels compared to what was assessed in AR5 ( ''high confidence'' ). This is based upon recent evidence of observed impacts, improved process understanding, and new knowledge on exposure and vulnerability of human and natural systems, including limits to adaptation. Depending on the level of global warming, the assessed long-term impacts will be up to multiple times higher than currently observed ( ''high confidence'' ) for 127 identified key risks, e.g., in terms of the number of affected people and species. Risks, including cascading risks (see 3.1.3) and risks from overshoot (see 3.3.4), are projected to become increasingly severe with every increment of global warming ( ''veryhigh confidence'' ). { ''WGII SPM B.3. 3, WGII SPM B.4, WGII SPM B.5, WGII 16.6.3; SRCCL SPM A5.3'' } . ( ''Figure 3.2, Figure 3.3'' )

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'''Figure 3.2: Projected''' '''risks and''' '''impacts''' '''of''' '''climate change on natural and human systems at different''' '''global''' '''warming levels (GWLs) relative to 1850-1900 levels''' '''.''' Projected risks and impacts shown on the maps are based on outputs from different subsets of Earth system models that were used to project each impact indicator without additional adaptation. WGII provides further assessment of the impacts on human and natural systems using these projections and additional lines of evidence. '''(a)''' Risks of species losses as indicated by the percentage of assessed species exposed to potentially dangerous temperature conditions, as defined by conditions beyond the estimated historical (1850 – 2005) maximum mean annual temperature experienced by each species, at GWLs of 1.5°C, 2°C, 3°C and 4°C. Underpinning projections of temperature are from 21 Earth system models and do not consider extreme events impacting ecosystems such as the Arctic. '''(b)''' Risk to human health as indicated by the days per year of population exposure to hypothermic conditions that pose a risk of mortality from surface air temperature and humidity conditions for historical period (1991 – 2005) and at GWLs of 1.7°C to 2.3°C (mean = 1.9°C; 13 climate models), 2.4°C to 3.1°C (2.7°C; 16 climate models) and 4.2°C to 5.4°C (4.7°C; 15 climate models). Interquartile ranges of WGLs by 2081 – 2100 under RCP2.6, RCP4.5 and RCP8.5. The presented index is consistent with common features found in many indices included within WGI and WGII assessments. '''(c)''' Impacts on food production: (c1) Changes in maize yield at projected GWLs of 1.6°C to 2.4°C (2.0°C), 3.3°C to 4.8°C (4.1°C) and 3.9°C to 6.0°C (4.9°C). Median yield changes from an ensemble of 12 crop models, each driven by bias-adjusted outputs from 5 Earth system models from the Agricultural Model Intercomparison and Improvement Project (AgMIP) and the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP). Maps depict 2080 – 2099 compared to 1986 – 2005 for current growing regions (&gt;10 ha), with the corresponding range of future global warming levels shown under SSP1-2.6, SSP3-7.0 and SSP5-8.5, respectively. Hatching indicates areas where &lt;70% of the climate-crop model combinations agree on the sign of impact. (c2) Changes in maximum fisheries catch potential by 2081 – 2099 relative to 1986-2005 at projected GWLs of 0.9°C to 2.0°C (1.5°C) and 3.4°C to 5.2°C (4.3°C). GWLs by 2081 – 2100 under RCP2.6 and RCP8.5. Hatching indicates where the two climate-fisheries models disagree in the direction of change. Large relative changes in low yielding regions may correspond to small absolute changes. Biodiversity and fisheries in Antarctica were not analysed due to data limitations. Food security is also affected by crop and fishery failures not presented here. { ''WGII Fig. TS.5, WGII Fig TS.9, WGII Annex I: Global to Regional Atlas Figure AI.1'' ''5, Figure AI.22, Figure AI.23, Figure AI.29; WGII 7.3.1.2, 7.2.4.1, SROCC Figure SPM.3'' } ( ''3.1.2, Cross-Section Box.2'' )

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Climate-related risks for natural and human systems are higher for global warming of 1.5°C than at present (1.1°C) but lower than at 2°C ( ''high confidence'' ). (see Section 2.1.2). Climate-related risks to health, livelihoods, food security, water supply, human security, and economic growth are projected to increase with global warming of 1.5°C. In terrestrial ecosystems, 3 to 14% of the tens of thousands of species assessed will ''likely'' face a very high risk of extinction at a GWL of 1.5°C. Coral reefs are projected to decline by a further 70–90% at 1.5°C of global warming ( ''high confidence'' ). At this GWL, many low-elevation and small glaciers around the world would lose most of their mass or disappear within decades to centuries. ( ''high confidence'' ). Regions at disproportionately higher risk include Arctic ecosystems, dryland regions, small island developing states and Least Developed Countries ( ''high confidence'' ). { ''WGII SPM B.3, WGII SPM B.4.1, WGII TS.C.4.2; SR1.5 SPM A.3, SR1.5 SPM B.4.2, SR1.5 SPM B.5, SR1.5 SPM B.5.1'' } . ( ''Figure 3.3'' )

At 2°C of global warming, overall risk levels associated with the unequal distribution of impacts (RFC3), global aggregate impacts (RFC4) and large-scale singular events (RFC5) would be transitioning to high ( ''medium confidence'' ), those associated with extreme weather events (RFC2) would be transitioning to very high ( ''medium confidence'' ), and those associated with unique and threatened systems (RFC1) would be very high ( ''high confidence'' ). (Figure 3.3, panel a). With about 2°C warming, climate-related changes in food availability and diet quality are estimated to increase nutrition-related diseases and the number of undernourished people, affecting tens (under low vulnerability and low warming) to hundreds of millions of people (under high vulnerability and high warming), particularly among low-income households in low- and middle-income countries in sub-Saharan Africa, South Asia and Central America ( ''high confidence'' ). For example, snowmelt water availability for irrigation is projected to decline in some snowmelt dependent river basins by up to 20%. ( ''medium confidence'' ). Climate change risks to cities, settlements and key infrastructure will rise sharply in the mid and long term with further global warming, especially in places already exposed to high temperatures, along coastlines, or with high vulnerabilities ( ''high confidence'' ). { ''WGII SPM B.3. 3, WGII SPM B.4.2, WGII SPM B.4.5, WGII TS C.3.3, WGII TS.C.12.2'' } ( ''Figure 3.3'' )

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[[File:62a284d1415b19b9fa25d90ec4fd452b IPCC_AR6_SYR_Figure_3_3_1.png]] [[File:b4a013ed03c21fdb14a08c7345e5ed9b IPCC_AR6_SYR_Figure_3_3_2.png]]
'''Figure 3.3: Synthetic risk diagrams of''' '''global and''' '''sectoral assessments and examples of regional key''' '''risks.''' The burning embers result from a literature based expert elicitation. '''Panel (a): Left''' - Global surface temperature changes in °C relative to 1850 – 1900. These changes were obtained by combining CMIP6 model simulations with observational constraints based on past simulated warming, as well as an updated assessment of equilibrium climate sensitivity. ''Very likely'' ranges are shown for the low and high GHG emissions scenarios (SSP1-2.6 and SSP3-7.0). '''Right''' - Global Reasons for Concern, comparing AR6 (thick embers) and AR5 (thin embers) assessments. Diagrams are shown for each RFC, assuming low to no adaptation (i.e., adaptation is fragmented, localised and comprises incremental adjustments to existing practices). However, the transition to a very high-risk level has an emphasis on irreversibility and adaptation limits. The horizontal line denotes the present global warming of 1.1°C which is used to separate the observed, past impacts below the line from the future projected risks above it. Lines connect the midpoints of the transition from moderate to high risk across AR5 and AR6. '''Panel (b)''' : Risks for land-based systems and ocean/coastal ecosystems. Diagrams shown for each risk assume low to no adaptation. Text bubbles indicate examples of impacts at a given warming level. '''Panel (c): Left -''' Global mean sea level change in centimetres, relative to 1900. The historical changes (black) are observed by tide gauges before 1992 and altimeters afterwards. The future changes to 2100 (coloured lines and shading) are assessed consistently with observational constraints based on emulation of CMIP, ice-sheet, and glacier models, and ''likely'' ranges are shown for SSP1-2.6 and SSP3-7.0. '''Right''' - Assessment of the combined risk of coastal flooding, erosion and salinization for four illustrative coastal geographies in 2100, due to changing mean and extreme sea levels, under two response scenarios, with respect to the SROCC baseline period (1986 – 2005) and indicating the IPCC AR6 baseline period (1995 – 2014). The assessment does not account for changes in extreme sea level beyond those directly induced by mean sea level rise; risk levels could increase if other changes in extreme sea levels were considered (e.g., due to changes in cyclone intensity). “No-to-moderate response” describes efforts as of today (i.e., no further significant action or new types of actions). “Maximum potential response” represents a combination of responses implemented to their full extent and thus significant additional efforts compared to today, assuming minimal financial, social and political barriers. The assessment criteria include exposure and vulnerability (density of assets, level of degradation of terrestrial and marine buffer ecosystems), coastal hazards (flooding, shoreline erosion, salinization), in-situ responses (hard engineered coastal defences, ecosystem restoration or creation of new natural buffers areas, and subsidence management) and planned relocation. Planned relocation refers to managed retreat or resettlement. Forced displacement is not considered in this assessment. The term response is used here instead of adaptation because some responses, such as retreat, may or may not be considered to be adaptation. '''Panel (d): Left''' - Heat-sensitive human health outcomes under three scenarios of adaptation effectiveness. The diagrams are truncated at the nearest whole ºC within the range of temperature change in 2100 under three SSP scenarios. '''Right''' - Risks associated with food security due to climate change and patterns of socio-economic development. Risks to food security include availability and access to food, including population at risk of hunger, food price increases and increases in disability adjusted life years attributable to childhood underweight. Risks are assessed for two contrasted socio-economic pathways (SSP1 and SSP3) excluding the effects of targeted mitigation and adaptation policies. '''Panel (e)''' : Examples of regional key risks. Risks identified are of at least ''medium confidence'' level. Key risks are identified based on the magnitude of adverse consequences (pervasiveness of the consequences, degree of change, irreversibility of consequences, potential for impact thresholds or tipping points, potential for cascading effects beyond system boundaries); likelihood of adverse consequences; temporal characteristics of the risk; and ability to respond to the risk, e.g., by adaptation. { ''WGI Figure SPM.8; WGII SPM B.3.3, WGII Figure SPM.3, WGII SM 16.6, WGII SM 16.7.4; SROCC Figure SPM.3d, SROCC SPM.5a, SROCC 4SM; SRCCL Figure SPM.2, SRCCL 7.3.1, SRCCL 7 SM'' } ( ''Cross-Section Box.2'' )

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At global warming of 3°C, additional risks in many sectors and regions reach high or very high levels, implying widespread systemic impacts, irreversible change and many additional adaptation limits (see [[#3.2|Section 3.2]] ) ( ''high confidence'' ). For example, very high extinction risk for endemic species in biodiversity hotspots is projected to increase at least tenfold if warming rises from 1.5°C to 3°C ( ''medium confidence'' ). Projected increases in direct flood damages are higher by 1.4 to 2 times at 2°C and 2.5 to 3.9 times at 3°C, compared to 1.5°C global warming without adaptation ( ''medium confidence'' ). { ''WGII SPM B.4.1, WGII SPM B.4.2, WGII Figure SPM.3, WGII TS Appendix AII, WGII Appendix I Global to Regional Atlas Figure AI.46'' } ( ''Figure 3.2, Figure 3.3'' )

Global warming of 4°C and above is projected to lead to far-reaching impacts on natural and human systems ( ''high confidence'' ). Beyond 4°C of warming, projected impacts on natural systems include local extinction of ~50% of tropical marine species ( ''medium confidence'' ) and biome shifts across 35% of global land area. ( ''medium confidence'' ). At this level of warming, approximately 10% of the global land area is projected to face both increasing high and decreasing low extreme streamflow, affecting, without additional adaptation, over 2.1 billion people ( ''medium confidence'' ).and about 4 billion people are projected to experience water scarcity ( ''medium confidence'' ). At 4°C of warming, the global burned area is projected to increase by 50 to 70% and the fire frequency by ~30% compared to today ( ''medium confidence'' ). { ''WGII SPM B.4.1, WGII SPM B.4.2, WGII TS.C.1.2, WGII TS.C.2.3, WGII TS.C.4.1, WGII TS.C.4.4'' } . ( ''Figure 3.2, Figure 3.3'' )

'''Projected adverse impacts and related losses and damages from climate change escalate with every increment of global warming (''' '''''very''''' '''''high confidence)''''' ''', but they will also strongly depend on socio-economic development trajectories and adaptation actions to reduce vulnerability and exposure (''' '''''high confidence).''''' For example, development pathways with higher demand for food, animal feed, and water, more resource-intensive consumption and production, and limited technological improvements result in higher risks from water scarcity in drylands, land degradation and food insecurity ( ''high confidence'' ). Changes in, for example, demography or investments in health systems have effect on a variety of health-related outcomes including heat-related morbidity and mortality (Figure 3.3 Panel d). { ''WGII SPM B.3, WGII SPM B.4, WGII Figure SPM.3; SRCCL SPM A.6'' }

'''With every increment of warming, climate change impacts and risks will become increasingly complex and more difficult to manage.''' Many regions are projected to experience an increase in the probability of compound events with higher global warming, such as concurrent heatwaves and droughts, compound flooding and fire weather. In addition, multiple climatic and non-climatic risk drivers such as biodiversity loss or violent conflict will interact, resulting in compounding overall risk and risks cascading across sectors and regions. Furthermore, risks can arise from some responses that are intended to reduce the risks of climate change, e.g., adverse side effects of some emission reduction and carbon dioxide removal (CDR) measures (see 3.4.1). ( ''high confidence'' ) { ''WGI SPM C.2.7, WGI Figure SPM.6, WGI TS.4.3; WGII SPM B.1.7, WGII B.2.2, WGII SPM B.5, WGII SPM B.5.4, WGII SPM C.4.2, WGII SPM B.5, WGII CCB2'' }

'''Solar Radiation Modification (SRM) approaches, if they were to be implemented, introduce a widespread range of new risks to people and ecosystems, which are not well understood.''' SRM has the potential to offset warming within one or two decades and ameliorate some climate hazards but would not restore climate to a previous state, and substantial residual or overcompensating climate change would occur at regional and seasonal scales ( ''high confidence'' ). Effects of SRM would depend on the specific approach used '''[[#footnote-035|122]]''' , and a sudden and sustained termination of SRM in a high CO 2 emissions scenario would cause rapid climate change ( ''high confidence'' ). SRM would not stop atmospheric CO 2 concentrations from increasing nor reduce resulting ocean acidification under continued anthropogenic emissions ( ''high confidence'' ). Large uncertainties and knowledge gaps are associated with the potential of SRM approaches to reduce climate change risks. Lack of robust and formal SRM governance poses risks as deployment by a limited number of states could create international tensions. { ''WGI 4.6; WGII SPM B.5.5; WGIII 14.4.5.1; WGIII 14 Cross-Working Group Box Solar Radiation Modification; SR1.5 SPM C.1.4'' }

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