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

== Section 3: Long-Term Climate and Development Futures ==

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=== 3.1 Long-Term Climate Change, Impacts and Related Risks ===

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'''Future warming will be driven by future emissions and will affect all major climate system components, with every region experiencing multiple and co-occurring changes. Many climate-related risks are assessed to be higher than in previous assessments, and projected long-term impacts are up to multiple times higher than currently observed. Multiple climatic and non-climatic risks will interact, resulting in compounding and cascading risks across sectors and regions. Sea level rise, as well as other irreversible changes, will continue for thousands of years, at rates depending on future emissions. ( '''''high confidence''''' ) .'''
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==== 3.1.1. Long-term Climate Change ====

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'''The uncertainty range on assessed''' '''future changes in global surface temperature is narrower than in the AR5.''' For the first time in an IPCC assessment cycle, multi-model projections of global surface temperature, ocean warming and sea level are constrained using observations and the assessed climate sensitivity. The ''likely'' range of equilibrium climate sensitivity has been narrowed to 2.5°C to 4.0°C (with a best estimate of 3.0°C) based on multiple lines of evidence '''[[#footnote-045|112]]''' , including improved understanding of cloud feedbacks. For related emissions scenarios, this leads to narrower uncertainty ranges for long-term projected global temperature change than in AR5. { ''WGI A.4, WGI Box SPM.1, WGI TS.3.2, WGI 4.3'' }

'''Future warming depends on future GHG emissions, with cumulative net CO''' '''2''' dominating. The assessed best estimates and ''very likely'' ranges of warming for 2081-2100 with respect to 1850–1900 vary from 1.4 [1.0 to 1.8]°C in the very low GHG emissions scenario (SSP1-1.9) to 2.7 [2.1 to 3.5]°C in the intermediate GHG emissions scenario (SSP2-4.5) and 4.4 [3.3 to 5.7]°C in the very high GHG emissions scenario (SSP5-8.5) '''[[#footnote-044|113]]''' . { ''WGI SPM B.1.1, WGI Table SPM.1, WGI Figure SPM.4'' } . ( ''Cross-Section Box.2 Figure 1'' )

'''Modelled pathways consistent with the continuation of policies implemented by the end of 2020 lead to global warming of 3.2 [2.2 to 3.5]°C (5–95% range) by 2100 (''' '''''medium confidence)''''' '''(see also Section 2.3.1).''' Pathways of &gt;4°C (≥50%) by 2100 would imply a reversal of current technology and/or mitigation policy trends ( ''medium confidence'' ). However, such warming could occur in emissions pathways consistent with policies implemented by the end of 2020 if climate sensitivity or carbon cycle feedbacks are higher than the best estimate ( ''high confidence'' ). { ''WGIII SPM C.1.3'' }

'''Global warming will continue to increase in the near term in nearly all considered scenarios and modelled pathways. Deep, rapid, and sustained GHG emissions reductions, reaching net zero CO''' '''2''' emissions and including strong emissions reductions of other GHGs, in particular CH '''4 , are necessary to limit warming to 1.5°C (&gt;50%) or less than 2°C (&gt;67%) by the end of century (''' '''''high confidence).''''' The best estimate of reaching 1.5°C of global warming lies in the first half of the 2030s in most of the considered scenarios and modelled pathways '''[[#footnote-043|114]]''' . In the very low GHG emissions scenario (SSP1-1.9), CO 2 emissions reach net zero around 2050 and the best-estimate end-of-century warming is 1.4°C, after a temporary overshoot (see Section 3.3.4) of no more than 0.1°C above 1.5°C global warming. Global warming of 2°C will be exceeded during the 21st century unless deep reductions in CO 2 and other GHG emissions occur in the coming decades. Deep, rapid, and sustained reductions in GHG emissions would lead to improvements in air quality within a few years, to reductions in trends of global surface temperature discernible after around 20 years, and over longer time periods for many other climate impact-drivers '''[[#footnote-042|115]]''' ( ''high confidence'' ). Targeted reductions of air pollutant emissions lead to more rapid improvements in air quality compared to reductions in GHG emissions only, but in the long term, further improvements are projected in scenarios that combine efforts to reduce air pollutants as well as GHG emissions ( ''high confidence'' ) '''[[#footnote-041|116]]''' . { ''WGI SPMB.1, WGI SPM B.1.3, WGI SPM D.1, WGI SPM D.2, WGI Figure SPM.4, WGI Table SPM.1,'' . ''WGI Cross-Section Box TS.1; WGIII SPM C.3, WGIII Table SPM.2, WGIII Figure SPM.5, WGIII Box SPM.1 Figure 1, WGIII Table 3.2'' } ( ''Table 3.1, Cross-Section Box.2 Figure 1'' )

'''Changes in short-lived climate forcers (SLCF) resulting from the five considered scenarios lead to an additional net global warming in the near and long term (''' '''''high confidence)''''' '''. Simultaneous stringent climate change mitigation and air pollution control policies limit this additional warming and lead to strong benefits for air quality (''' '''''high confidence)''''' '''.''' In high and very high GHG emissions scenarios (SSP3-7.0 and SSP5-8.5), combined changes in SLCF emissions, such as CH 4 , aerosol and ozone precursors, lead to a net global warming by 2100 of ''likely'' 0.4°C to 0.9°C relative to 2019. This is due to projected increases in atmospheric concentration of CH 4 , tropospheric ozone, hydrofluorocarbons and, when strong air pollution control is considered, reductions of cooling aerosols. In low and very low GHG emissions scenarios (SSP1-1.9 and SSP1-2.6), air pollution control policies, reductions in CH 4 and other ozone precursors lead to a net cooling, whereas reductions in anthropogenic cooling aerosols lead to a net warming ( ''high confidence'' ). Altogether, this causes a ''likely'' net warming of 0.0°C to 0.3°C due to SLCF changes in 2100 relative to 2019 and strong reductions in global surface ozone and particulate matter ( ''high confidence'' ). { ''WGI SPMD.1.7, WGI Box TS.7'' } . ( ''Cross-Section Box.2'' )

'''Continued GHG emissions will further affect all major climate system components, and many changes will be irreversible on centennial to millennial time scales.''' Many changes in the climate system become larger in direct relation to increasing global warming. With every additional increment of global warming, changes in extremes continue to become larger. Additional warming will lead to more frequent and intense marine heatwaves and is projected to further amplify permafrost thawing and loss of seasonal snow cover, glaciers, land ice and Arctic sea ice ( ''high confidence'' ). Continued global warming is projected to further intensify the global water cycle, including its variability, global monsoon precipitation '''[[#footnote-040|117]]''' , and very wet and very dry weather and climate events and seasons ( ''high confidence'' ). The portion of global land experiencing detectable changes in seasonal mean precipitation is projected to increase ( ''medium confidence'' ) with more variable precipitation and surface water flows over most land regions within seasons ( ''high confidence'' ).and from year to year ( ''medium confidence'' ). Many changes due to past and future GHG emissions are irreversible '''[[#footnote-039|118]]''' on centennial to millennial time scales, especially in the ocean, ice sheets and global sea level (see 3.1.3). Ocean acidification ( ''virtually certain'' ), ocean deoxygenation ( ''high confidence'' ).and global mean sea level ( ''virtually certain'' ).will continue to increase in the 21st century, at rates dependent on future emissions. { ''WGI SPM B.2, WGI SPM B.2.2, WGI SPM B.2.3, WGI SPM B.2.5, WGI SPM B.3, WGI SPM B.3.1,'' . ''WGI SPM B.3.2, WGI SPM B.4, WGI SPM B.5, WGI SPM B.5.1, WGI SPM B.5.3, WGI Figure SPM.8'' } . ( ''Figure 3.1'' )

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[[File:25de7726501494cfe4fb4973aee3bda3 IPCC_AR6_SYR_Figure_3_1.png]]
'''Figure 3.1: Projected changes of annual maximum daily''' '''temperature, annual mean total column soil moisture CMIP and annual maximum daily''' '''precipitation at''' '''global''' '''warming levels of 1.5°C, 2°C, 3°C, and 4°C relative to 1850-1900. Simulated (a)''' annual maximum temperature change (°C), '''(b)''' annual mean total column soil moisture (standard deviation), '''(c)''' annual maximum daily precipitation change (%). Changes correspond to CMIP6 multi-model median changes. In panels (b) and (c), large positive relative changes in dry regions may correspond to small absolute changes. In panel (b), the unit is the standard deviation of interannual variability in soil moisture during 1850-1900. Standard deviation is a widely used metric in characterising drought severity. A projected reduction in mean soil moisture by one standard deviation corresponds to soil moisture conditions typical of droughts that occurred about once every six years during 1850-1900. The WGI Interactive Atlas ( https://interactive-atlas.ipcc.ch/ ) can be used to explore additional changes in the climate system across the range of global warming levels presented in this figure. ''WGI Figure SPM.5, WGI Figure TS.5, WGI Figure 11.11, WGI Figure 11.16, WGI Figure 11.19'' ( ''Cross-Section Box.2'' )

[https://www.ipcc.ch/figures/figure-3-1 ]

'''With further global warming, every region is projected to increasingly experience concurrent and multiple changes in climatic impact-drivers.''' Increases in hot and decreases in cold climatic impact-drivers, such as temperature extremes, are projected in all regions ( ''high confidence'' ). At 1.5°C global warming, heavy precipitation and flooding events are projected to intensify and become more frequent in most regions in Africa, Asia ( ''high confidence'' ), North America ( ''medium'' to ''high confidence'' ).and Europe. ( ''medium confidence'' ). At 2°C or above, these changes expand to more regions and/or become more significant ( ''high confidence'' ), and more frequent and/or severe agricultural and ecological droughts are projected in Europe, Africa, Australasia and North, Central and South America ( ''medium'' to ''high confidence'' ). Other projected regional changes include intensification of tropical cyclones and/or extratropical storms ( ''medium confidence'' ), and increases in aridity and fire weather '''[[#footnote-038|119]]''' ( ''medium'' to ''high confidence'' ). Compound heatwaves and droughts become ''likely'' more frequent, including concurrently at multiple locations ( ''high confidence'' ). { ''WGI SPMC.2, WGI SPM C.2.1, WGI SPM C.2.2, WGI SPM C.2.3, WGI SPM C.2.4, WGI SPM C.2.7'' }

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==== 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'' )

[https://www.ipcc.ch/figures/figure-3-2 ]

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'' )

[https://www.ipcc.ch/figures/figure-3-3 ]

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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==== 3.1.3 The Likelihood and Risks of Abrupt and Irreversible Change ====

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'''The likelihood of abrupt and irreversible changes and their impacts increase with higher global warming levels (''' '''''high confidence).''''' As warming levels increase, so do the risks of species extinction or irreversible loss of biodiversity in ecosystems such as forests ( ''medium confidence'' ), coral reefs ( ''very high confidence'' ) and in Arctic regions ( ''high confidence'' ). Risks associated with large-scale singular events or tipping points, such as ice sheet instability or ecosystem loss from tropical forests, transition to high risk between 1.5°C to 2.5°C ( ''medium confidence'' ) and to very high risk between 2.5°C to 4°C ( ''low confidence'' ). The response of biogeochemical cycles to anthropogenic perturbations can be abrupt at regional scales and irreversible on decadal to century time scales ( ''high confidence'' ). The probability of crossing uncertain regional thresholds increases with further warming ( ''high confidence'' ). { ''WGI SPMC.3.2, WGI Box TS.9, WGI TS.2.6; WGII Figure SPM.3, WGII SPM B.3.1, WGII SPM B.4.1, WGII SPM B.5.2, WGII Table TS.1, WGII TS.C.1, WGII TS.C.13.3; SROCC SPM B.4'' }

'''Sea level rise is unavoidable for centuries to millennia due to continuing deep ocean warming and ice sheet melt, and sea levels will remain elevated for thousands of years (''' '''''high confidence).''''' Global mean sea level rise will continue in the 21st century ( ''virtually certain'' ), with projected regional relative sea level rise within 20% of the global mean along two-thirds of the global coastline ( ''medium confidence'' ). The magnitude, the rate, the timing of threshold exceedances, and the long-term commitment of sea level rise depend on emissions, with higher emissions leading to greater and faster rates of sea level rise. Due to relative sea level rise, extreme sea level events that occurred once per century in the recent past are projected to occur at least annually at more than half of all tide gauge locations by 2100 and risks for coastal ecosystems, people and infrastructure will continue to increase beyond 2100 ( ''high confidence'' ). At sustained warming levels between 2°C and 3°C, the Greenland and West Antarctic ice sheets will be lost almost completely and irreversibly over multiple millennia ( ''limited evidence'' ). The probability and rate of ice mass loss increase with higher global surface temperatures ( ''high confidence'' ). Over the next 2000 years, global mean sea level will rise by about 2 to 3 m if warming is limited to 1.5°C and 2 to 6 m if limited to 2°C ( ''low confidence'' ). Projections of multi-millennial global mean sea level rise are consistent with reconstructed levels during past warm climate periods: global mean sea level was ''very likely'' 5 to 25 m higher than today roughly 3 million years ago, when global temperatures were 2.5°C to 4°C higher than 1850–1900 ( ''medium confidence'' ). Further examples of unavoidable changes in the climate system due to multi-decadal or longer response timescales include continued glacier melt ( ''very high confidence'' ) and permafrost carbon loss ( ''high confidence'' ). { ''WGI SPM B.5.2, WGI SPM B.5.3, WGI SPM B.5.4, WGI SPM C.2.5, WGI Box TS.4, WGI Box TS.9, WGI 9.5.1; WGII TS C.5; SROCC SPM B.3, SROCC SPM B.6, SROCC SPM B.9'' } . ( ''Figure 3.4'' )

'''The probability of low- likelihood outcomes associated with potentially very large impacts increases with higher global warming levels (''' '''''high confidence).''''' Warming substantially above the assessed ''very likely'' range for a given scenario cannot be ruled out, and there is ''high confidence'' this would lead to regional changes greater than assessed in many aspects of the climate system. Low-likelihood, high-impact outcomes could occur at regional scales even for global warming within the ''very likely'' assessed range for a given GHG emissions scenario. Global mean sea level rise above the ''likely'' range – approaching 2 m by 2100 and in excess of 15 m by 2300 under a very high GHG emissions scenario (SSP5-8.5) ( ''lowconfidence'' ) – cannot be ruled out due to deep uncertainty in ice-sheet processes '''[[#footnote-034|123]]''' and would have severe impacts on populations in low elevation coastal zones. If global warming increases, some compound extreme events '''[[#footnote-033|124]]''' will become more frequent, with higher likelihood of unprecedented intensities, durations or spatial extent ( ''high confidence'' ). The Atlantic Meridional Overturning Circulation is ''very likely'' to weaken over the 21st century for all considered scenarios ( ''high confidence'' ), however an abrupt collapse is not expected before 2100 ( ''medium confidence'' ). If such a low probability event were to occur, it would ''very likely'' cause abrupt shifts in regional weather patterns and water cycle, such as a southward shift in the tropical rain belt, and large impacts on ecosystems and human activities. A sequence of large explosive volcanic eruptions within decades, as have occurred in the past, is a low-likelihood high-impact event that would lead to substantial cooling globally and regional climate perturbations over several decades. { ''WGI SPM B.5.3, WGI SPM C.3, WGI SPM C.3.1, WGI SPM C.3.2, WGI SPM C.3.3, WGI SPM C.3.4, WGI SPM C.3.5, WGI Figure SPM.8, WGI Box TS.3, WGI Figure TS.6, WGI Box 9.4; WGII SPM B.4.5, WGII SPM C.2.8; SROCC SPM B.2.7'' } . ( ''Figure 3.4, Cross-SectionBox.2'' )

<div id="3.2" class="h2-container"></div>

<span id="long-term-adaptation-options-and-limits"></span>
=== 3.2 Long-term Adaptation Options and Limits ===

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'''With increasing warming, adaptation options will become more constrained and less effective. At higher levels of warming, losses and damages will increase, and additional human and natural systems will reach adaptation limits. Integrated, cross-cutting multi-sectoral solutions increase the effectiveness of adaptation. Maladaptation can create lock-ins of vulnerability, exposure and risks but can be avoided by long-term planning and the implementation of adaptation actions that are flexible, multi-sectoral and inclusive. (''high confidence'')'''

'''The effectiveness of adaptation to reduce climate risk is documented for specific contexts, sectors and regions and will decrease with increasing warming (''' '''''high confidence)''''' '''[[#footnote-032|125]] .''' For example, common adaptation responses in agriculture – adopting improved cultivars and agronomic practices, and changes in cropping patterns and crop systems – will become less effective from 2°C to higher levels of warming ( ''high confidence'' ). The effectiveness of most water-related adaptation options to reduce projected risks declines with increasing warming ( ''high confidence'' ). Adaptations for hydropower and thermo-electric power generation are effective in most regions up to 1.5°C to 2°C, with decreasing effectiveness at higher levels of warming ( ''medium confidence'' ). Ecosystem-based Adaptation is vulnerable to climate change impacts, with effectiveness declining with increasing global warming ( ''high confidence'' ). Globally, adaptation options related to agroforestry and forestry have a sharp decline in effectiveness at 3°C, with a substantial increase in residual risk ( ''medium confidence'' ). { ''WGII SPM C.2, WGII SPM C.2.1, WGII SPM C.2.5, WGII SPM C.2.10, WGII Figure TS.6 Panel (e), 4.7.2'' } .

'''With increasing global warming, more limits to adaptation will be reached and losses and damages, strongly concentrated among the poorest vulnerable populations, will increase (''' '''''high confidence).''''' Already below 1.5°C, autonomous and evolutionary adaptation responses by terrestrial and aquatic ecosystems will increasingly face hard limits ( ''high confidence'' ) ( [[#2.1|Section 2.1]] .2). Above 1.5°C, some ecosystem-based adaptation measures will lose their effectiveness in providing benefits to people as these ecosystems will reach hard adaptation limits ( ''high confidence'' ). Adaptation to address the risks of heat stress, heat mortality and reduced capacities for outdoor work for humans face soft and hard limits across regions that become significantly more severe at 1.5°C, and are particularly relevant for regions with warm climates ( ''high confidence'' ). Above 1.5°C global warming level, limited freshwater resources pose potential hard limits for small islands and for regions dependent on glacier and snow melt ( ''medium confidence'' ). By 2°C, soft limits are projected for multiple staple crops, particularly in tropical regions ( ''high confidence'' ). By 3°C, soft limits are projected for some water management measures for many regions, with hard limits projected for parts of Europe ( ''medium confidence'' ). { ''WGII SPM C.3, WGII SPM C.3.3, WGII SPM C.3.4, WGII SPM C.3.5, WGII TS.D.2.2, WGII TS.D.2.3; SR1.5 SPM B.6; SROCC SPM C.1'' }

'''Integrated, cross-cutting multi- sectoral solutions increase the effectiveness of adaptation.''' For example, inclusive, integrated and long-term planning at local, municipal, sub-national and national scales, together with effective regulation and monitoring systems and financial and technological resources and capabilities foster urban and rural system transition. There are a range of cross-cutting adaptation options, such as disaster risk management, early warning systems, climate services and risk spreading and sharing that have broad applicability across sectors and provide greater benefits to other adaptation options when combined. Transitioning from incremental to transformational adaptation, and addressing a range of constraints, primarily in the financial, governance, institutional and policy domains, can help overcome soft adaptation limits. However, adaptation does not prevent all losses and damages, even with effective adaptation and before reaching soft and hard limits. ( ''high confidence'' ). { ''WGII SPM C.2, WGII SPM C.2.6, WGII SPM.C.2.13, WGII SPM C.3.1, WGII SPM.C.3.4, WGII SPM C.3.5, WGII Figure TS.6 Panel (e)'' }

'''Maladaptive responses to climate change can create lock-ins of vulnerability, exposure and risks that are difficult and expensive to change and exacerbate existing inequalities.''' Actions that focus on sectors and risks in isolation and on short-term gains often lead to maladaptation. Adaptation options can become maladaptive due to their environmental impacts that constrain ecosystem services and decrease biodiversity and ecosystem resilience to climate change or by causing adverse outcomes for different groups, exacerbating inequity. Maladaptation can be avoided by flexible, multi-sectoral, inclusive and long-term planning and implementation of adaptation actions with benefits to many sectors and systems. ( ''high confidence'' ). { ''WGII SPM C.4, WGII SPM.C.4.1, WGII SPM C.4.2, WGII SPM C.4.3'' }

'''Sea level rise poses a distinctive and severe adaptation challenge as it implies both dealing with slow onset changes and increases in the frequency and magnitude of extreme sea level events (''' '''''high confidence).''''' Such adaptation challenges would occur much earlier under high rates of sea level rise ( ''high confidence'' ). Responses to ongoing sea level rise and land subsidence include protection, accommodation, advance and planned relocation ( ''high confidence'' ). These responses are more effective if combined and/or sequenced, planned well ahead, aligned with sociocultural values and underpinned by inclusive community engagement processes ( ''high confidence'' ). Ecosystem-based solutions such as wetlands provide co-benefits for the environment and climate mitigation, and reduce costs for flood defences ( ''medium confidence)'' , but have site-specific physical limits, at least above 1.5ºC of global warming ( ''high confidence'' ) and lose effectiveness at high rates of sea level rise beyond 0.5 to 1 cm yr ''-1'' ( ''medium confidence'' ). Seawalls can be maladaptive as they effectively reduce impacts in the short term but can also result in lock-ins and increase exposure to climate risks in the long term unless they are integrated into a long-term adaptive plan ( ''high confidence'' ). { ''WGI SPM C.2.5; WGII SPM C.2.8, WGII SPM C.4.1; WGII 13.10, WGII Cross-Chapter Box SLR; SROCC SPM B. 9, SROCC SPM C.3.2, SROCC Figure SPM.4, SROCC Figure SPM.5c'' } . ( ''Figure 3.4'' )

<div id="figure-3-4" class="_idGenObjectLayout-1 figure-cont"></div>

[[File:345f715021e1870ec756d6a4637cbcaa IPCC_AR6_SYR_Figure_3_4.png]]
'''Figure 3.4:''' '''Observed and''' '''projected global mean''' '''sea level change and its''' '''impacts, and time scales of coastal''' '''risk''' '''management. Panel (a):''' Global mean sea level change in metres relative to 1900. The historical changes (black) are observed by tide gauges before 1992 and altimeters afterwards. The future changes to 2100 and for 2150 (coloured lines and shading) are assessed consistently with observational constraints based on emulation of CMIP, ice-sheet, and glacier models, and median values and. ''likely'' ranges are shown for the considered scenarios. Relative to 1995-2014, the ''likely'' global mean sea level rise by 2050 is between 0.15 to 0.23 m in the very low GHG emissions scenario (SSP1-1.9) and 0.20 to 0.29 m in the very high GHG emissions scenario (SSP5-8.5); by 2100 between 0.28 to 0.55 m under SSP1-1.9 and 0.63 to 1.01 m under SSP5-8.5; and by 2150 between 0.37 to 0.86 m under SSP1-1.9 and 0.98 to 1.88 m under SSP5-8.5 ''(medium confidence)'' . Changes relative to 1900 are calculated by adding 0.158 m (observed global mean sea level rise from 1900 to 1995-2014) to simulated changes relative to 1995-2014. The future changes to 2300 (bars) are based on literature assessment, representing the 17th–83rd percentile range for SSP1-2.6 (0.3 to 3.1 m) and SSP5-8.5 (1.7 to 6.8 m). Red dashed lines: Low-likelihood, high-impact storyline, including ice sheet instability processes. These indicate the potential impact of deeply uncertain processes, and show the 83rd percentile of SSP5-8.5 projections that include low-likelihood, high-impact processes that cannot be ruled out; because of ''low confidence'' in projections of these processes, this is not part of a ''likely'' range. IPCC AR6 global and regional sea level projections are hosted at https://sealevel.nasa.gov/ipcc-ar6-sea-level-projection-tool . The low-lying coastal zone is currently home to around 896 million people (nearly 11% of the 2020 global population), projected to reach more than one billion by 2050 across all five SSPs. '''Panel (b):''' Typical time scales for the planning, implementation (dashed bars) and operational lifetime of current coastal risk-management measures (blue bars). Higher rates of sea level rise demand earlier and stronger responses and reduce the lifetime of measures (inset). As the scale and pace of sea level rise accelerates beyond 2050, long-term adjustments may in some locations be beyond the limits of current adaptation options and for some small islands and low-lying coasts could be an existential risk. { ''WGI SPM B.5, WGI C.2.5, WGI Figure SPM.8, WGI 9.6; WGII SPM B.4.5, WGII B.5.2, WGII C.2.8, WGII D.3.3, WGII TS.D.7,'' ''WGII Cross-Chapter Box SLR'' } ( ''Cross-Section Box.2'' )

[https://www.ipcc.ch/figures/figure-3-4 ]

<div id="3.3" class="h2-container"></div>

<span id="mitigation-pathways"></span>
=== 3.3 Mitigation Pathways ===

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'''Limiting human-caused global warming requires net zero anthropogenic CO 2 emissions. Pathways consistent with 1.5°C and 2°C carbon budgets imply rapid, deep, and in most cases immediate GHG emission reductions in all sectors ( '''''high confidence''''' ) . Exceeding a warming level and returning (i.e. overshoot) implies increased risks and potential irreversible impacts; achieving and sustaining global net negative CO 2 emissions would reduce warming ( '''''high confidence''''' ) .'''
<div id="3.3.1" class="h3-container"></div>

<span id="remaining-carbon-budgets"></span>
==== 3.3.1 Remaining Carbon Budgets ====

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'''Limiting global temperature increase to a specific level requires limiting cumulative net CO''' '''2''' emissions to within a finite carbon budget '''[[#footnote-031|126]] , along with strong reductions in other GHGs.''' For every 1000 GtCO 2 emitted by human activity, global mean temperature rises by ''likely'' 0.27°C to 0.63°C (best estimate of 0.45°C). This relationship implies that there is a finite carbon budget that cannot be exceeded in order to limit warming to any given level. { ''WGI SPM D.1, WGI SPM D.1.1; SR1.5 SPM C.1.3'' } . ( ''Figure 3.5'' )

<div id="figure-3-5" class="_idGenObjectLayout-1 figure-cont"></div>

[[File:09c7d038fb8108fbff99643186928951 IPCC_AR6_SYR_Figure_3_5.png]]
'''Figure 3.5: Cumulative past,''' '''projected, and committed''' '''emissions, and associated global''' '''temperature changes. Panel (a)''' Assessed remaining carbon budgets to limit warming ''more likely than not'' to 1.5°C, to 2°C with a 83% and 67% likelihood, compared to cumulative emissions corresponding to constant 2019 emissions until 2030, existing and planned fossil fuel infrastructures (in GtCO 2 ). For remaining carbon budgets, thin lines indicate the uncertainty due to the contribution of non-CO 2 warming. For lifetime emissions from fossil fuel infrastructure, thin lines indicate the assessed sensitivity range. '''Panel (b)''' Relationship between cumulative CO 2 emissions and the increase in global surface temperature. Historical data (thin black line) shows historical CO 2 emissions versus observed global surface temperature increase relative to the period 1850-1900. The grey range with its central line shows a corresponding estimate of the human-caused share of historical warming. Coloured areas show the assessed ''very likely'' range of global surface temperature projections, and thick coloured central lines show the median estimate as a function of cumulative CO 2 emissions for the selected scenarios SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5. Projections until 2050 use the cumulative CO 2 emissions of each respective scenario, and the projected global warming includes the contribution from all anthropogenic forcers. { ''WGI SPM D.1, WGI Figure SPM.10, WGI Table SPM.2; WGIII SPM B.1, WGIII SPM B.7, WGIII 2.7; SR1.5 SPM C.1.3'' }

[https://www.ipcc.ch/figures/figure-3-5 ]

'''The best estimates of the remaining carbon budget (RCB) from the beginning of 2020 for limiting warming to 1.5°C with a 50% likelihood''' '''[[#footnote-030|127]] is estimated to be 500 GtCO''' '''2''' ; for 2°C (67% likelihood) this is 1150 GtCO '''2''' . '''[[#footnote-029|128]]''' Remaining carbon budgets have been quantified based on the assessed value of TCRE and its uncertainty, estimates of historical warming, climate system feedbacks such as emissions from thawing permafrost, and the global surface temperature change after global anthropogenic CO 2 emissions reach net zero, as well as variations in projected warming from non-CO 2 emissions due in part to mitigation action. The stronger the reductions in non-CO 2 emissions the lower the resulting temperatures are for a given RCB or the larger RCB for the same level of temperature change. For instance, the RCB for limiting warming to 1.5°C with a 50% likelihood could vary between 300 to 600 GtCO 2 depending on non-CO 2 warming '''[[#footnote-028|129]]''' . Limiting warming to 2°C with a 67% (or 83%) likelihood would imply a RCB of 1150 (900) GtCO 2 from the beginning of 2020. To stay below 2°C with a 50% likelihood, the RCB is higher, i.e., 1350 GtCO 2 '''[[#footnote-027|130]]''' . { ''WGI SPM D.1.2, WGI Table SPM.2; WGIII Box SPM.1, WGIII Box 3.4; SR1.5 SPM C.1.3'' }

If the annual CO 2 emissions between 2020–2030 stayed, on average, at the same level as 2019, the resulting cumulative emissions would almost exhaust the remaining carbon budget for 1.5°C (50%), and exhaust more than a third of the remaining carbon budget for 2°C (67%) (Figure 3.5). Based on central estimates only, historical cumulative net CO 2 emissions between 1850 and 2019 (2400 ±240 GtCO 2 ) amount to about four-fifths '''[[#footnote-026|131]]''' of the total carbon budget for a 50% probability of limiting global warming to 1.5°C (central estimate about 2900 GtCO 2 ) and to about two-thirds '''[[#footnote-025|132]]''' of the total carbon budget for a 67% probability to limit global warming to 2°C (central estimate about 3550 GtCO 2 ). { ''WGI Table SPM.2; WGIII SPM B.1.3, WGIII Table 2.1'' }

<div id="Table 3.1" class="_idGenObjectStyleOverride-2 figure-cont"></div>

'''Table 3.1: Key''' '''characteristics of the''' '''modelled global''' '''emissions''' '''pathways.''' Summary of projected CO 2 and GHG emissions, projected net zero timings and the resulting global warming outcomes. Pathways are categorised (columns), according to their likelihood of limiting warming to different peak warming levels (if peak temperature occurs before 2100) and 2100 warming levels. Values shown are for the median [p50] and 5–95th percentiles [p5–p95], noting that not all pathways achieve net zero CO 2 or GHGs. ''WGIII Table SPM.2''

[[File:2d779f5398d89c701823620ff6102e1c IPCC_AR6_SYR_Table_3_1.png]]
'''1''' Detailed explanations on the Table are provided in WGIII Box SPM.1 and WGIII Table SPM.2. The relationship between the temperature categories and SSP/RCPs is discussed in Cross-Section Box.2. Values in the table refer to the 50th and [5–95th] percentile values across the pathways falling within a given category as defined in WGIII Box SPM.1. The three dots (…) sign denotes that the value cannot be given (as the value is after 2100 or, for net zero, net zero is not reached). Based on the assessment of climate emulators in AR6 WG I (Chapter 7, Box 7.1), two climate emulators were used for the probabilistic assessment of the resulting warming of the pathways. For the ‘Temperature Change’ and ‘Likelihood’ columns, the non-bracketed values represent the 50th percentile across the pathways in that category and the median [50th percentile] across the warming estimates of the probabilistic MAGICC climate model emulator. For the bracketed ranges in the “ likelihood” column, the median warming for every pathway in that category is calculated for each of the two climate model emulators (MAGICC and FaIR). These ranges cover both the uncertainty of the emissions pathways as well as the climate emulators’ uncertainty. All global warming levels are relative to 1850-1900.

'''2''' C3 pathways are sub-categorised according to the timing of policy action to match the emissions pathways in WGIII Figure SPM.4.

'''3''' Global emission reductions in mitigation pathways are reported on a pathway-by-pathway basis relative to harmonised modelled global emissions in 2019 rather than the global emissions reported in WGIII SPM Section B and WGIII Chapter 2; this ensures internal consistency in assumptions about emission sources and activities, as well as consistency with temperature projections based on the physical climate science assessment by WGI (see WGIII SPM Footnote 49). Negative values (e.g., in C5, C6) represent an increase in emissions. The modelled GHG emissions in 2019 are 55 [53–58] GtCO 2 -eq, thus within the uncertainty ranges of estimates for 2019 emissions [53-66] GtCO 2 -eq (see 2.1.1).

'''4''' Emissions milestones are provided for 5-year intervals in order to be consistent with the underlying 5-year time-step data of the modelled pathways. Ranges in square brackets underneath refer to the range across the pathways, comprising the lower bound of the 5th percentile 5-year interval and the upper bound of the 95th percentile 5-year interval. Numbers in round brackets signify the fraction of pathways that reach specific milestones over the 21st century. Percentiles reported across all pathways in that category include those that do not reach net zero before 2100.

'''5''' For cases where models do not report all GHGs, missing GHG species are infilled and aggregated into a Kyoto basket of GHG emissions in CO 2 -eq defined by the 100-year global warming potential. For each pathway, reporting of CO 2 , CH 4 , and N 2 O emissions was the minimum required for the assessment of the climate response and the assignment to a climate category. Emissions pathways without climate assessment are not included in the ranges presented here. See WGIII Annex III.II.5.

'''6''' Cumulative emissions are calculated from the start of 2020 to the time of net zero and 2100, respectively. They are based on harmonised net CO 2 emissions, ensuring consistency with the WG I assessment of the remaining carbon budget. { ''WGIII Box 3.4, WGIII SPM Footnote 50'' } .

'''In scenarios with increasing CO''' '''2''' '''emissions, the land and ocean carbon sinks are projected to be less effective at slowing the accumulation of CO''' '''2''' '''in the atmosphere''' '''''(''''' '''''high confidence).''''' While natural land and ocean carbon sinks are projected to take up, in absolute terms, a progressively larger amount of CO 2 under higher compared to lower CO 2 emissions scenarios, they become less effective, that is, the proportion of emissions taken up by land and ocean decreases with increasing cumulative net CO 2 emissions ( ''high confidence'' ). Additional ecosystem responses to warming not yet fully included in climate models, such as GHG fluxes from wetlands, permafrost thaw, and wildfires, would further increase concentrations of these gases in the atmosphere ( ''high confidence'' ). In scenarios where CO 2 concentrations peak and decline during the 21st century, the land and ocean begin to take up less carbon in response to declining atmospheric CO 2 concentrations ( ''high confidence'' ) and turn into a weak net source by 2100 in the very low GHG emissions scenario. ( ''medium confidence'' ) '''[[#footnote-024|133]]''' . { ''WGI SPM B.4, WGI SPM B.4.1, WGI SPM B.4.2, WGI SPM B.4.3'' }

<div id="3.3.2" class="h3-container"></div>

<span id="net-zero-emissions-timing-and-implications"></span>
==== 3.3.2 Net Zero Emissions: Timing and Implications ====

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'''From a physical science perspective, limiting human-caused global warming to a specific level requires limiting cumulative CO''' '''2''' '''emissions, reaching net zero or net negative CO''' '''2''' '''emissions, along with strong reductions of other GHG emissions (see Cross-Section Box.1). Global modelled pathways that reach and sustain net zero GHG emissions are projected to result in a gradual decline in surface temperature (''' '''''high confidence).''''' Reaching net zero GHG emissions primarily requires deep reductions in CO 2 , methane, and other GHG emissions, and implies net negative CO 2 emissions. '''[[#footnote-023|134]]''' Carbon dioxide removal (CDR) will be necessary to achieve net negative CO 2 emissions '''[[#footnote-022|135]]''' . Achieving global net zero CO 2 emissions, with remaining anthropogenic CO 2 emissions balanced by durably stored CO 2 from anthropogenic removal, is a requirement to stabilise CO 2 -induced global surface temperature increase (see 3.3.3). ( ''high confidence'' ). This is different from achieving net zero GHG emissions, where metric-weighted anthropogenic GHG emissions (see Cross-Section Box.1) equal CO 2 removal ( ''high confidence'' ). Emissions pathways that reach and sustain net zero GHG emissions defined by the 100-year global warming potential imply net negative CO 2 emissions and are projected to result in a gradual decline in surface temperature after an earlier peak ( ''high confidence'' ). While reaching net zero CO 2 or net zero GHG emissions requires deep and rapid reductions in gross emissions, the deployment of CDR to counterbalance hard-to-abate residual emissions (e.g., some emissions from agriculture, aviation, shipping, and industrial processes) is unavoidable ( ''high confidence'' ). { ''WGI SPM D.1,'' . ''WGI SPM D.1.1, WGI SPM D.1.8; WGIII SPM C.2, WGIII SPM C.3, WGIII SPM C.11, WGIII Box TS.6; SR1.5 SPM A.2.2'' }

'''In modelled pathways, the timing of net zero CO '''2''' emissions, followed by net zero GHG emissions, depends on several variables, including the desired climate outcome, the mitigation strategy and the gases covered''' '''''(''''' '''''high confidence).''''' Global net zero CO 2 emissions are reached in the early 2050s in pathways that limit warming to 1.5°C (&gt;50%) with no or limited overshoot, and around the early 2070s in pathways that limit warming to 2°C (&gt;67%). While non-CO 2 GHG emissions are strongly reduced in all pathways that limit warming to 2°C (&gt;67%) or lower, residual emissions of CH 4 and N 2 O and F-gases of about 8 [5–11] GtCO 2 -eq yr ''-1'' remain at the time of net zero GHG, counterbalanced by net negative CO 2 emissions. As a result, net zero CO 2 would be reached before net zero GHGs. ( ''high confidence'' ). { ''WGIII SPM C.2, WGIII SPM C.2.3, WGIII SPM C.2.4, WGIII Table SPM.2, WGIII 3.3'' } (Figure 3.6)

<div id="figure-3-6" class="_idGenObjectLayout-1 figure-cont"></div>

[[File:e98ad538bc980b5fbaf4f010b3d6eec1 IPCC_AR6_SYR_Figure_3_6.png]]
'''Figure 3.6: Total GHG, CO''' '''2''' '''and CH''' '''4''' '''emissions and timing of reaching net zero in different''' '''mitigation''' '''pathways. Top row:''' GHG, CO 2 and CH 4 emissions over time (in GtCO 2 eq) with historical emissions, projected emissions in line with policies implemented until the end of 2020 (grey), and pathways consistent with temperature goals in colour (blue, purple, and brown, respectively). '''Panel (a) (left)''' shows pathways that limit warming to 1.5°C (&gt;50%) with no or limited overshoot (C1) and '''Panel (b) (right)''' shows pathways that limit warming to 2°C (&gt;67%) (C3). '''Bottom row: Panel (c)''' shows median (vertical line), ''likely'' (bar) and ''very likely'' (thin lines) timing of reaching net zero GHG and CO 2 emissions for global modelled pathways that limit warming to 1.5°C (&gt;50%) with no or limited overshoot (C1) (left) or 2°C (&gt;67%) (C3) (right). { ''WGIII Figure SPM.5'' }

[https://www.ipcc.ch/figures/figure-3-6 ]

<div id="3.3.3" class="h3-container"></div>

<span id="sectoral-contributions-to-mitigation"></span>
==== 3.3.3 Sectoral Contributions to Mitigation ====

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'''All global modelled pathways that limit warming to 2°C (&gt;67%)''' '''or lower by 2100 involve rapid and deep and in most cases immediate GHG emissions reductions in all sectors (see also 4. 1, 4.5).''' Reductions in GHG emissions in industry, transport, buildings, and urban areas can be achieved through a combination of energy efficiency and conservation and a transition to low-GHG technologies and energy carriers (see also 4.5, Figure 4.4). Socio-cultural options and behavioural change can reduce global GHG emissions of end-use sectors, with most of the potential in developed countries, if combined with improved infrastructure design and access. ( ''high confidence'' ). { ''WGIII SPM C.3, WGIII SPM C.5, WGIII SPM C.6, WGIII SPM C.7.3, WGIII SPM C.8, WGIII SPM C.10.2'' }

'''Global modelled mitigation pathways reaching net zero CO''' '''2''' '''and GHG emissions include transitioning from fossil fuels without carbon capture and storage (CCS) to very low- or zero-carbon energy sources, such as renewables or fossil fuels with CCS, demand-side measures and improving efficiency, reducing non-CO''' '''2''' '''GHG emissions, and CDR''' '''[[#footnote-021|136]]''' . In global modelled pathways that limit warming to 2°C or below, almost all electricity is supplied from zero or low-carbon sources in 2050, such as renewables or fossil fuels with CO 2 capture and storage, combined with increased electrification of energy demand. Such pathways meet energy service demand with relatively low energy use, through e.g., enhanced energy efficiency and behavioural changes and increased electrification of energy end use. Modelled global pathways limiting global warming to 1.5°C (&gt;50%) with no or limited overshoot generally implement such changes faster than pathways limiting global warming to 2°C (&gt;67%). ( ''high confidence'' ) { ''WGIII SPM C.3, WGIII SPM C.3.2, WGIII SPM C.4, WGIII TS.4.2; SR1.5 SPM C.2.2'' }

'''AFOLU mitigation options, when sustainably implemented, can deliver large-scale GHG emission reductions and enhanced CO''' '''2''' '''removal; however, barriers to implementation and trade-offs may result from the impacts of climate change, competing demands on land, conflicts with food security and livelihoods, the complexity of land ownership and management systems, and cultural aspects (see 3.4.1).''' All assessed modelled pathways that limit warming to 2°C (&gt;67%) or lower by 2100 include land-based mitigation and land-use change, with most including different combinations of reforestation, afforestation, reduced deforestation, and bioenergy. However, accumulated carbon in vegetation and soils is at risk from future loss (or sink reversal) triggered by climate change and disturbances such as flood, drought, fire, or pest outbreaks, or future poor management.. ( ''high confidence'' ). { ''WGI SPM B.4.3; WGII SPM B.2.3, WGII SPM B.5.4; WGIII SPM C.9, WGIII SPM C.11.3, WGIII SPM D.2.3, WGIII TS.4.2, 3.4; SR1.5 SPM C.2.5; SRCCL SPM B.1.4, SRCCL SPM B.3, SRCCL SPM B.7'' }

'''In addition to deep, rapid, and sustained emission reductions, CDR can fulfil three complementary roles: lowering net CO''' '''2''' '''or net GHG emissions in the near term; counterbalancing ‘ hard-to-abate’ residual emissions (e.g., some emissions from agriculture , aviation, shipping, industrial processes) to help reach net zero CO''' '''2''' '''or GHG emissions, and achieving net negative CO''' '''2''' '''or GHG emissions if deployed at levels exceeding annual residual emissions (''' '''''high confidence)''''' '''.''' CDR methods vary in terms of their maturity, removal process, time scale of carbon storage, storage medium, mitigation potential, cost, co-benefits, impacts and risks, and governance requirements. ( ''high confidence'' ). Specifically, maturity ranges from lower maturity (e.g., ocean alkalinisation) to higher maturity (e.g., reforestation); removal and storage potential ranges from lower potential (&lt;1 Gt CO 2 yr ''-1'' , e.g., blue carbon management) to higher potential (&gt;3 Gt CO 2 yr ''-1'' , e.g., agroforestry); costs range from lower cost (e.g., –45 to 100 USD tCO 2 ''-1'' for soil carbon sequestration) to higher cost (e.g., 100 to 300 USD tCO 2 ''-1'' for direct air carbon dioxide capture and storage) ( ''medium confidence'' ). Estimated storage timescales vary from decades to centuries for methods that store carbon in vegetation and through soil carbon management, to ten thousand years or more for methods that store carbon in geological formations. ( ''high confidence'' ). Afforestation, reforestation, improved forest management, agroforestry and soil carbon sequestration are currently the only widely practiced CDR methods ( ''high confidence'' ). Methods and levels of CDR deployment in global modelled mitigation pathways vary depending on assumptions about costs, availability and constraints ( ''high confidence'' ). { ''WGIII SPM C.3.5, WGIII SPM C.11.1, WGIII SPM C.11.4'' }

<div id="3.3.4" class="h3-container"></div>

<span id="overshoot-pathways-increased-risks-and-other-implications"></span>
==== 3.3.4 Overshoot Pathways: Increased Risks and Other Implications ====

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'''Exceeding a specific remaining carbon budget results in higher global warming. Achieving and sustaining net negative global CO''' '''2''' '''emissions could reverse the resulting temperature exceedance (''' '''''high confidence)''''' '''''.''''' Continued reductions in emissions of short-lived climate forcers, particularly methane, after peak temperature has been reached, would also further reduce warming ( ''high confidence'' ). Only a small number of the most ambitious global modelled pathways limit global warming to 1.5°C (&gt;50%) without overshoot. { ''WGI SPM D.1.1, WGI SPM D.1.6, WGI SPM D.1.7; WGIII TS.4.2'' }

Overshoot of a warming level results in more adverse impacts, some irreversible, and additional risks for human and natural systems compared to staying below that warming level, with risks growing with the magnitude and duration of overshoot ( ''high confidence'' ). Compared to pathways without overshoot, societies and ecosystems would be exposed to greater and more widespread changes in climatic impact-drivers, such as extreme heat and extreme precipitation, with increasing risks to infrastructure, low-lying coastal settlements, and associated livelihoods ( ''high confidence'' ). Overshooting 1.5°C will result in irreversible adverse impacts on certain ecosystems with low resilience, such as polar, mountain, and coastal ecosystems, impacted by ice-sheet melt, glacier melt, or by accelerating and higher committed sea level rise ( ''high confidence'' ). Overshoot increases the risks of severe impacts, such as increased wildfires, mass mortality of trees, drying of peatlands, thawing of permafrost and weakening natural land carbon sinks; such impacts could increase releases of GHGs making temperature reversal more challenging ( ''medium confidence'' ). { ''WGI SPM C.2, WGI SPM C.2.1, WGI SPM C.2.3; WGII SPM B.6, WGII SPM B.6.1, WGII SPM B.6.2; SR1.5 3.6'' }

The larger the overshoot, the more net negative CO 2 emissions needed to return to a given warming level ( ''high confidence'' ). Reducing global temperature by removing CO 2 would require net negative emissions of 220 GtCO 2 (best estimate, with a ''likely'' range of 160 to 370 GtCO 2 ) for every tenth of a degree ( ''medium confidence'' ). Modelled pathways that limit warming to 1.5°C (&gt;50%) with no or limited overshoot reach median values of cumulative net negative emissions of 220 GtCO 2 by 2100, pathways that return warming to 1.5°C (&gt;50%) after high overshoot reach median values of 360 GtCO 2 ( ''high confidence'' ). '''[[#footnote-020|137]]''' More rapid reduction in CO 2 and non-CO 2 emissions, particularly methane, limits peak warming levels and reduces the requirement for net negative CO 2 emissions and CDR, thereby reducing feasibility and sustainability concerns, and social and environmental risks ( ''high confidence'' ). { ''WGI SPM D.1.1; WGIII SPM B.6.4, WGIII SPM C.2, WGIII SPM C.2.2, WGIII Table SPM.2'' }

<div id="3.4" class="h2-container"></div>

<span id="long-term-interactions-between-adaptation-mitigation-and-sustainable-development"></span>
=== 3.4 Long-Term Interactions Between Adaptation, Mitigation and Sustainable Development ===

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'''Mitigation and adaptation can lead to synergies and trade-offs with sustainable development ( '''''high confidence''''' ) '''''.''''' Accelerated and equitable mitigation and adaptation bring benefits from avoiding damages from climate change and are critical to achieving sustainable development ( '''''high confidence).''''' Climate resilient development '''[[#footnote-019|138]]''' pathways are progressively constrained by every increment of further warming ( '''''very high confidence''''' ) '''''.''''' There is a rapidly closing window of opportunity to secure a liveable and sustainable future for all ( '''''very high confidence''''' ).'''
<div id="3.4.1" class="h3-container fshow"></div>

<span id="synergies-and-trade-offs-costs-and-benefits"></span>
==== 3.4.1 Synergies and trade-offs, costs and benefits ====

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'''Mitigation and adaptation options can lead to synergies and trade-offs with other aspects of sustainable development (see also Section 4.6, Figure 4.4).''' Synergies and trade-offs depend on the pace and magnitude of changes and the development context including inequalities, with consideration of climate justice. The potential or effectiveness of some adaptation and mitigation options decreases as climate change intensifies (see also Sections 3.2, 3.3.3, 4.5). ( ''high confidence'' ) { ''WGII SPM C.2, WGIIFigure SPM.4b; WGIII SPM D.1, WGIII SPM D.1.2, WGIII TS.5.1, WGIII Figure SPM.8; SR1.5 SPM D.3, SR1.5 SPM D.4; SRCCL SPM B.2, SRCCL SPM B.3, SRCCL SPM D.3.2, SRCCL Figure SPM.3'' }

In the energy sector, transitions to low-emission systems will have multiple co-benefits, including improvements in air quality and health. There are potential synergies between sustainable development and, for instance, energy efficiency and renewable energy. ( ''high confidence'' ). { ''WGIII SPM C.4.2, WGIII SPM D.1.3'' }

For agriculture, land, and food systems, many land management options and demand-side response options (e.g., dietary choices, reduced post-harvest losses, reduced food waste) can contribute to eradicating poverty and eliminating hunger while promoting good health and well-being, clean water and sanitation, and life on land ( ''medium confidence)'' . In contrast, certain adaptation options that promote intensification of production, such as irrigation, may have negative effects on sustainability (e.g., for biodiversity, ecosystem services, groundwater depletion, and water quality) ( ''high confidence'' ). { ''WGII TS.D.5.5; WGIII SPM D.10; SRCCL SPM B.2.3'' }

Reforestation, improved forest management, soil carbon sequestration, peatland restoration and coastal blue carbon management are examples of CDR methods that can enhance biodiversity and ecosystem functions, employment and local livelihoods, depending on context '''[[#footnote-018|139]]''' . However, afforestation or production of biomass crops for bioenergy with carbon dioxide capture and storage or biochar can have adverse socio-economic and environmental impacts, including on biodiversity, food and water security, local livelihoods and the rights of Indigenous Peoples, especially if implemented at large scales and where land tenure is insecure. ( ''high confidence'' ) { ''WGII SPM B.5.4, WGII SPM C.2.4; WGIII SPM C.11.2; SR1.5 SPM C.3.4, SR1.5 SPM C.3.5; SRCCL SPM B.3, SRCCL SPM B.7.3, SRCCL Figure SPM.3'' }

Modelled pathways that assume using resources more efficiently or shift global development towards sustainability include fewer challenges, such as dependence on CDR and pressure on land and biodiversity, and have the most pronounced synergies with respect to sustainable development ( ''high confidence'' ). { ''WGIII SPM C.3.6; SR1.5 SPM D.4.2'' }

'''Strengthening climate change mitigation action entails more rapid transitions and higher up-front investments, but brings benefits from avoiding damages from climate change and reduced adaptation costs.''' The aggregate effects of climate change mitigation on global GDP (excluding damages from climate change and adaptation costs) are small compared to global projected GDP growth. Projected estimates of global aggregate net economic damages and the costs of adaptation generally increase with global warming level. ( ''high confidence'' ) { ''WGII SPM B. 4.6, WGII TS.C.10; WGIII SPM C.12.2, WGIII SPM C.12.3'' } .

Cost-benefit analysis remains limited in its ability to represent all damages from climate change, including non-monetary damages, or to capture the heterogeneous nature of damages and the risk of catastrophic damages ( ''high confidence'' ). Even without accounting for these factors or for the co-benefits of mitigation, the global benefits of limiting warming to 2°C exceed the cost of mitigation ( ''medium confidence'' ). This finding is robust against a wide range of assumptions about social preferences on inequalities and discounting over time ( ''medium confidence'' ). Limiting global warming to 1.5°C instead of 2°C would increase the costs of mitigation, but also increase the benefits in terms of reduced impacts and related risks (see 3.1.1, 3.1.2) and reduced adaptation needs ( ''high confidence'' ) '''[[#footnote-017|140]]''' . { ''WGII SPM B.4, WGII SPM B.6; WGIII SPM C.12, WGIII SPM C.12.2, WGIII SPM C.12.3 WGIII Box TS.7; SR1.5 SPM B.3, SR1.5 SPM B.5, SR1.5 SPM B.6'' }

Considering other sustainable development dimensions, such as the potentially strong economic benefits on human health from air quality improvement, may enhance the estimated benefits of mitigation ( ''medium confidence'' ). The economic effects of strengthened mitigation action vary across regions and countries, depending notably on economic structure, regional emissions reductions, policy design and level of international cooperation ( ''high confidence'' ). Ambitious mitigation pathways imply large and sometimes disruptive changes in economic structure, with implications for near-term actions (Section 4.2), equity ( [[#4.4|Section 4.4]] ), sustainability ( [[#4.6|Section 4.6]] ), and finance (Section 4.8) (. ''igh confidence'' ). { ''WGIII SPM C.12.2, WGIII SPM D.3.2, WGIII TS.4.2'' }

<div id="3.4.2" class="h3-container"></div>

<span id="advancing-integrated-climate-action-for-sustainable-development"></span>
==== 3.4.2 Advancing Integrated Climate Action for Sustainable Development ====

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'''An inclusive, equitable approach to integrating adaptation, mitigation and development can advance sustainable development in the long term (''' '''''high confidence).''''' Integrated responses can harness synergies for sustainable development and reduce trade-offs ( ''high confidence'' ). Shifting development pathways towards sustainability and advancing climate resilient development is enabled when governments, civil society and the private sector make development choices that prioritise risk reduction, equity and justice, and when decision-making processes, finance and actions are integrated across governance levels, sectors and timeframes ( ''very high confidence'' ) (see also Figure 4.2). Inclusive processes involving local knowledge and Indigenous Knowledge increase these prospects ( ''high confidence'' ). However, opportunities for action differ substantially among and within regions, driven by historical and ongoing patterns of development ( ''very high confidence'' ). Accelerated financial support for developing countries is critical to enhance mitigation and adaptation action ( ''high confidence'' ). { . ''GII SPM C.5.4, WGII SPM D.1, WGII SPM D.1.1, WGII SPM D.1.2, WGII SPM D.2, WGII SPM D.3, WGII SPM D.5, WGII SPM D.5.1, WGII SPM D.5.2; WGIII SPM D.1, WGIII SPM D.2, WGIII SPM D.2.4, WGIII SPM E.2.2, WGIII SPM E.2.3, WGIII SPM E.5.3, WGIII Cross-Chapter Box 5'' } .

'''Policies that shift development pathways towards sustainability can broaden the portfolio of available mitigation and adaptation responses (''' '''''medium confidence)''''' '''.''' Combining mitigation with action to shift development pathways, such as broader sectoral policies, approaches that induce lifestyle or behaviour changes, financial regulation, or macroeconomic policies can overcome barriers and open up a broader range of mitigation options ( ''high confidence'' ). Integrated, inclusive planning and investment in everyday decision-making about urban infrastructure can significantly increase the adaptive capacity of urban and rural settlements. Coastal cities and settlements play an important role in advancing climate resilient development due to the high number of people living in the Low Elevation Coastal Zone, the escalating and climate compounded risk that they face, and their vital role in national economies and beyond ( ''high confidence'' ). { ''WGII SPM.D.3, WGII SPM D.3.3; WGIII SPM E.2, WGIII SPM E.2.2; SR1.5 SPM D.6'' }

'''Observed adverse impacts and related losses and damages, projected risks, trends in vulnerability, and adaptation limits demonstrate that transformation for sustainability and climate resilient development action is more urgent than previously assessed (''' '''''very high confidence). Climate resilient development integrates adaptation and GHG mitigation to advance sustainable development for all.''''' Climate resilient development pathways have been constrained by past development, emissions and climate change and are progressively constrained by every increment of warming, in particular beyond 1.5°C ( ''very high confidence'' ). Climate resilient development will not be possible in some regions and sub-regions if global warming exceeds 2°C ( ''medium confidence'' ). Safeguarding biodiversity and ecosystems is fundamental to climate resilient development, but biodiversity and ecosystem services have limited capacity to adapt to increasing global warming levels, making climate resilient development progressively harder to achieve beyond 1.5°C warming. ( ''very high confidence'' ). { ''WGII SPM D.1, WGII SPM D.1.1, WGII SPM D.4, WGII SPM D.4.3, WGII SPM D.5.1; WGIII SPM D.1.1'' } .

'''The cumulative scientific evidence is unequivocal: climate change is a threat to human well-being and planetary health (''' '''''very high confidence). Any further delay in concerted anticipatory global action on adaptation and mitigation will miss a brief and rapidly closing window of opportunity to secure a liveable and sustainable future for all (''''' '''''very high confidence).''''' Opportunities for near-term action are assessed in the following section. { ''WGII SPM D.5.3; WGIII SPM D.1.1'' }

<div id="Section 4: Near-Term Responses in a Changing Climate" class="h1-container"></div>

<span id="section-4-near-term-responses-in-a-changing-climate"></span>