Editing IPCC:AR6/WGI/TS (section)

==== TS.3.3.3 Relating Different Forcing Agents ====

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'''When including other GHGs, the choice of emissions metric affects the quantification of net zero GHG emissions and their resulting temperature outcome ( ''high confidence'' ). Reaching and sustaining net zero GHG emissions typically leads to a peak and decline in temperatures when quantified with the global warming potential over a 100-year period (GWP-100). Carbon-cycle responses are more robustly accounted for in emissions metrics compared to AR5 ( ''high confidence'' ). New emissions metric approaches can be used to generate equivalent cumulative emissions of CO <sub>2</sub> for short-lived greenhouse gases based on their rate of emissions. Links to chapters 7.6.2''' '''Over 10- to 20-year time scales, the temperature response to a single year’s worth of current emissions of short-lived climate forcers (SLCFs) is at least as large as that of CO <sub>2</sub> , but because the effects of SLCFs decay rapidly over the first few decades after emission, the net long-term temperature response to a single year’s worth of emissions is predominantly determined by cumulative CO <sub>2</sub> emissions.''' '''Emissions reductions in 2020 associated with COVID-19 containment led to small and positive global ERF; however, global and regional climate responses to the forcing are undetectable above internal variability due to the temporary nature of emissions reductions. Links to chapters 6.6, Cross-Chapter Box 6.1'''
The relative climate effects of different forcing agents are typically quantified using emissions metrics that compare the effects of an idealised pulse of 1 kg of some climate forcing agent against a reference climate forcing agent, almost always CO <sub>2</sub> . The two most prominent pulse emissions metrics are the global warming potential (GWP) and global temperature change potential (GTP) (see Glossary). The climate responses to CO <sub>2</sub> emissions by convention include the effects of warming on the carbon cycle, so for consistency these also need to be determined for non-CO <sub>2</sub> emissions. The methodology for doing this has been placed on a more robust scientific footing compared to AR5 ( ''high confidence'' ). Methane from fossil fuel sources has slightly higher emissions metric values than those from biogenic sources since it leads to additional fossil CO <sub>2</sub> in the atmosphere ( ''high confidence'' ). Updates to the chemical adjustments for CH <sub>4</sub> and N <sub>2</sub> O emissions (Section TS.3.1) and revisions in their lifetimes result in emissions metrics for GWP and GTP that are slightly lower than in AR5 ( ''medium confidence'' ). Emissions metrics for the entire suite of GHGs assessed in the AR6 have been calculated for various time horizons. Links to chapters 7.6.1, Table 7.15, Table 7.SM.7

New emissions metric approaches, such as GWP* and Combined-GTP (CGTP), relate changes in the emissions rate of short-lived greenhouse gases to equivalent cumulative emissions of CO <sub>2</sub> (CO <sub>2</sub> -e). Global surface temperature response from aggregated emissions of short-lived greenhouse gases over time is determined by multiplying these cumulative CO <sub>2</sub> -e by TCRE (see Section TS.3.2.1). When GHGs are aggregated using standard metrics such as GWP or GTP, cumulative CO <sub>2</sub> -e emissions are not necessarily proportional to future global surface temperature outcomes ( ''high confidence'' ) Links to chapters 7.6.1, Box 7.3

Emissions metrics are needed to aggregate baskets of gases to determine net zero GHG emissions. Generally, achieving net zero CO <sub>2</sub> emissions and declining non-CO <sub>2</sub> radiative forcing would halt human-induced warming. Reaching net zero GHG emissions quantified by GWP-100 typically leads to declining temperatures after net zero GHGs emissions are achieved if the basket includes short-lived gases, such as CH <sub>4</sub> . Net zero GHG emissions defined by CGTP or GWP* imply net zero CO <sub>2</sub> and other long-lived GHG emissions and constant (CGTP) or gradually declining (GWP*) emissions of short-lived gases. The warming evolution resulting from net zero GHG emissions defined in this way corresponds approximately to reaching net zero CO <sub>2</sub> emissions, and would thus not lead to declining temperatures after net zero GHG emissions are achieved but to an approximate temperature stabilization ( ''high confidence'' ). The choice of emissions metric hence affects the quantification of net zero GHG emissions, and therefore the resulting temperature outcome of reaching and sustaining net zero GHG emissions levels ( ''high confidence'' ). Links to chapters 7.6.1.4, 7.6.2, 7.6.3

As pointed out in AR5, ultimately, it is a matter for policymakers to decide which emissions metric is most applicable to their needs. This Report does not recommend the use of any specific emissions metric, as the most appropriate metric depends on the policy goal and context (see Chapter 7, [[IPCC:Wg1:Chapter:Chapter-7#7.6|Section 7.6]] ). A detailed assessment of GHG metrics to support climate change mitigation and associated policy contexts is provided in the WGIII contribution to the AR6.

The global surface temperature response following a climate change mitigation measure that affects emissions of both short- and long-lived climate forcers depends on their lifetimes, their ERFs, how fast and for how long the emissions are reduced, and the thermal inertia in the climate system. Mitigation, relying on emissions reductions and implemented through new legislation or technology standards, implies that emissions reductions occur year after year. Global temperature response to a year’s worth of current emissions from different sectors informs about the mitigation potential (Figure TS.20). Over 10- to 20-year time scales, the influence of SLCFs is at least as large as that of CO <sub>2</sub> , with sectors producing the largest warming being fossil fuel production and distribution, agriculture, and waste management. Because the effects of the SLCFs decay rapidly over the first few decades after emission, the net long-term temperature effect from a single year’s worth of current emissions is predominantly determined by CO <sub>2</sub> . Fossil fuel combustion for energy, industry and land transportation are the largest contributing sectors on a 100-year time scale ( ''high confidence'' ). Current emissions of CO <sub>2</sub> , N <sub>2</sub> O and SLCFs from East Asia and North America are the largest regional contributors to additional net future warming on both short ( ''medium confidence'' ) and long time scales (10 and 100 years, respectively) ( ''high confidence'' ). Links to chapters 6.6.1, 6.6.2, Figure 6.16

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'''Figure TS.20 |''' '''Global surface temperature change 10 and 100 years after a one-year pulse of present-day emissions.''' ''The intent of this figure is to show the sectoral contribution to present-day climate change by specific climate forcers, including carbon dioxide (CO'' 2 '') as well as short-lived climate forcers (SLCFs).'' The temperature response is broken down by individual species and shown for total anthropogenic emissions (top) , and sectoral emissions on 10-year (left) and 100-year time scales (right) . Sectors are sorted by (high-to-low) net temperature effect on the 10-year time scale. Error bars in the top panel show the 5–95% range in net temperature effect due to uncertainty in radiative forcing only (calculated using a Monte Carlo approach and best estimate uncertainties from the literature). Emissions for 2014 are from the Coupled Model Intercomparison Project Phase 6 (CMIP6) emissions dataset, except for hydrofluorocarbons (HFCs) and aviation H 2 O, which rely on other datasets (see Section 6.6.2 for more details). CO <sub>2</sub> emissions are excluded from open biomass burning and residential biofuel use. Links to chapters 6.6.2, Figure 6.16

COVID-19 restrictions led to detectable reductions in global anthropogenic emissions of nitrogen oxides (NO x ) (about 35% in April 2020) and fossil CO <sub>2</sub> (7%, with estimates ranging from 5.8% to 13.0%), driven largely by reduced emissions from the transportation sector ( ''medium confidence'' ). There is ''high confidence'' that, with the exception of surface ozone, reductions in pollutant precursors contributed to temporarily improved air quality in most regions of the world. However, these reductions were lower than what would be expected from sustained implementation of policies addressing air quality and climate change ( ''medium confidence'' ). Overall, the net global ERF from COVID-19 containment was likely small and positive for 2020 (with a temporary peak value less than 0.2 W m <sup>–2</sup> ), thus temporarily adding to the total anthropogenic climate influence, with positive forcing (warming influence) from aerosol changes dominating over negative forcings (cooling influence) from CO <sub>2</sub> , NO ''x'' and contrail cirrus changes. Consistent with this small net radiative forcing, and against a large component of internal variability, Earth system models show no detectable effect on global or regional surface temperature or precipitation ( ''high confidence'' ). Links to chapters Cross Chapter Box 6.1

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'''Box TS.7 | Climate and Air Quality Responses to Short-lived Climate Forcers in Shared''' '''Socio-economic''' '''Pathways'''

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'''Future changes in emissions of short-lived climate forcers (SLCFs) are expected to cause an additional global mean warming, with a large diversity in the end-of-century response across the WGI core set of Shared Socio-economic Pathways (SSPs), depending upon the level of climate change and air pollution mitigation (Box TS.7, Figure 1). This additional warming is either due to reductions in cooling aerosols for air pollution regulation or due to increases in methane (CH <sub>4</sub> ), ozone and hydrofluorocarbons (HFCs). This additional warming is stable after 2040 in SSPs associated with lower global air pollution as long as CH <sub>4</sub> emissions are also mitigated, but the overall warming induced by SLCF changes is higher in scenarios in which air quality continues to deteriorate (induced by growing fossil fuel use and limited air pollution control) ( ''high confidence'' ).''' '''Sustained CH <sub>4</sub> mitigation reduces global surface ozone, contributing to air quality improvements, and also reduces surface temperature in the longer term, but only sustained CO <sub>2</sub> emissions reductions allow long-term climate stabilization ( ''high confidence'' ). Future changes in air quality (near-surface ozone and particulate matter, or PM) at global and local scales are predominantly driven by changes in ozone and aerosol precursor emissions rather than climate ( ''high confidence'' ). Air quality improvements driven by rapid decarbonization strategies, as in SSP1-1.9 and SSP1-2.6, are not sufficient in the near term to achieve air quality guidelines set by the World Health Organization in some highly polluted regions ( ''high confidence'' ). Additional policies (e.g., access to clean energy, waste management) envisaged to attain United Nations Sustainable Development Goals bring complementary SLCF reduction. Links to chapters 4.4.4, 6.6.3, 6.7.3, Box 6.2'''
The net effect of SLCF emissions changes on temperature will depend on how emissions of warming and cooling SLCFs will evolve in the future. The magnitude of the cooling effect of aerosols remains the largest uncertainty in the effect of SLCFs in future climate projections. Since the SLCFs have undergone large changes over the past two decades, the temperature and air pollution responses are estimated relative to the year 2019 instead of 1995–2014.

'''Temperature Response'''
In the next two decades, it is ''very likely'' that SLCF emissions changes will cause a warming relative to 2019, across the WGI core set of SSPs (see Section TS.1.3.1), in addition to the warming from long-lived GHGs. The net effect of SLCF and HFC changes in global surface temperature across the SSPs is a ''likely'' warming of 0.06°C–0.35°C in 2040 relative to 2019. This near-term global mean warming linked to SLCFs is quite similar in magnitude across the SSPs due to competing effects of warming (CH <sub>4</sub> , ozone) and cooling (aerosols) forcers (Box TS.7, Figure 1). There is greater diversity in the end-of-century response among the scenarios. SLCF changes in scenarios with no climate change mitigation (SSP3-7.0 and SSP5-8.5) will cause a warming in the ''likely'' range of 0.4°C–0.9°C in 2100 relative to 2019 due to increases in CH <sub>4</sub> , tropospheric ozone and HFC levels. For the stringent climate change and pollution mitigation scenarios (SSP1-1.9 and SSP1-2.6), the cooling from reductions in CH <sub>4</sub> , ozone and HFCs partially balances the warming from reduced aerosols, primarily sulphate, and the overall SLCF effect is a ''likely'' increase in global surface temperature of 0.0°C–0.3°C in 2100, relative to 2019. With intermediate climate change and air pollution mitigations, SLCFs in SSP2-4.5 add a ''likely'' warming of 0.2°C–0.5°C to global surface temperature change in 2100, with the largest warming resulting from reductions in aerosols. Links to chapters 4.4.4, 6.7.3

Assuming implementation and efficient enforcement of both the Kigali Amendment to the Montreal Protocol on Substances that Deplete the Ozone Layer and current national plans result in limiting emissions (as in SSP1-2.6), the effects of HFCs on global surface temperature, relative to 2019, would remain below +0. 02°C from 2050 onwards versus about +0.04°C–0.08°C in 2050 and +0.1°C–0.3°C in 2100 considering only national HFC regulations decided prior to the Kigali Amendment (as in SSP5-8.5) ( ''medium confidence'' ). Links to chapters 6.6.3, 6.7.3

'''Air Quality Responses'''
Air pollution projections range from strong reductions in global surface ozone and PM (e.g., SSP1-2.6, with stringent mitigation of both air pollution and climate change) to no improvement and even degradation (e.g., SSP3-7.0 without climate change mitigation and with only weak air pollution control) ( ''high confidence'' ). Under the SSP3-7.0 scenario, PM levels are projected to increase until 2050 over large parts of Asia, and surface ozone pollution is projected to worsen over all continental areas through 2100 ( ''high confidence'' ). In SSP5-8.5, a scenario without climate change mitigation but with stringent air pollution control, PM levels decline through 2100, but high CH <sub>4</sub> levels hamper the decline in global surface ozone at least until 2080 ( ''high confidence'' ). Links to chapters 6.7.1

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'''Box TS.7, Figure 1 |'''

'''Effects of short-lived climate forcers (SLCFs) on global surface temperature and air pollution across the WGI core set of Shared Socio-economic Pathways (SSPs).''' ''The intent of this figure is to show the climate and air quality (surface ozone and particulate matter smaller than 2.5 microns in diameter, or PM'' ''2.5'' '') response to SLCFs in the SSP scenarios for the near and long-term.'' Effects of net aerosols, tropospheric ozone, hydrofluorocarbons (HFCs; with lifetimes less than 50 years), and methane (CH <sub>4</sub> ) are compared with those of total anthropogenic forcing for 2040 and 2100 relative to year 2019. The global surface temperature changes are based on historical and future evolution of effective radiative forcing (ERF) as assessed in [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] of this Report. The temperature responses to the ERFs are calculated with a common impulse response function (RT) for the climate response, consistent with the metric calculations in [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] (Box 7.1). The RT has an equilibrium climate sensitivity of 3.0°C for a doubling of atmospheric CO <sub>2</sub> concentration (feedback parameter of –1.31 W m <sup>–2</sup> °C <sup>–1</sup> ). The scenario total (grey bar) includes all anthropogenic forcings (long- and short-lived climate forcers, and land-use changes). Uncertainties are 5–95% ranges. The global changes in air pollutant concentrations (ozone and PM 2.5 ) are based on multimodel Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations and represent changes in five-year mean surface continental concentrations for 2040 and 2098 relative to 2019. Uncertainty bars represent inter-model ±1 standard deviation. Links to chapters 6.7.2, 6.7.3, Figure 6.24

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'''Box TS.8 | Earth System Response to Solar Radiation Modification'''

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'''Since AR5, further modelling work has been conducted on aerosol-based solar radiation modification (SRM) options such as stratospheric aerosol injection, marine cloud brightening, and cirrus cloud thinning <sup>[[#footnote-000|21]]</sup> and their climate and biogeochemical effects. These investigations have consistently shown that SRM could offset some of the effects of increasing greenhouse gases on global and regional climate, including the carbon and water cycles ( ''high confidence'' ). However, there would be substantial residual or overcompensating climate change at the regional scales and seasonal time scales ( ''high confidence'' ), and large uncertainties associated with aerosol–cloud–radiation interactions persist. The cooling caused by SRM would increase the global land and ocean CO <sub>2</sub> sinks ( ''medium confidence'' ), but this would not stop CO <sub>2</sub> from increasing in the atmosphere or affect the resulting ocean acidification under continued anthropogenic emissions ( ''high confidence'' ). It is ''likely'' that abrupt water cycle changes will occur if SRM techniques are implemented rapidly. A sudden and sustained termination of SRM in a high CO <sub>2</sub> emissions scenario would cause rapid climate change ( ''high confidence'' ). However, a gradual phase-out of SRM combined with emissions reduction and carbon dioxide removal (CDR) would avoid these termination effects ( ''medium confidence'' ). Links to chapters 4.6.3, 5.6.3. 6.4.6, 8.6.3 .'''
Solar radiation modification (SRM) refers to deliberate, large-scale climate intervention options that are studied as potential supplements to deep mitigation, for example, in scenarios that overshoot climate stabilization goals. SRM options aim to offset some of the warming effects of GHG emissions by modification of Earth’s shortwave radiation budget. Following SR1.5, the SRM assessed in this Report also includes some options, such as cirrus cloud thinning, that alter the longwave radiation budget.

SRM contrasts with climate change mitigation activities, such as emissions reductions and CDR, as it introduces a ‘mask’ to the climate change problem by altering Earth’s radiation budget, rather than attempting to address the root cause of the problem, which is the increase in GHGs in the atmosphere. By masking only the climate effects of GHG emissions, SRM does not address other issues related to atmospheric CO <sub>2</sub> increase, such as ocean acidification. This Report assesses physical understanding of the Earth system response to proposed SRM, and the assessment is based primarily on idealized climate model simulations. There are other important considerations, such as risk to human and natural systems, perceptions, ethics, cost, governance, and trans-boundary issues and their relationship to the United Nations Sustainable Development Goals – issues that the WGII (Chapter 16) and WGIII (Chapter 14) Reports address. Links to chapters 4.6.3

SRM options include those that increase surface albedo, brighten marine clouds by increasing the amount of cloud condensation nuclei, or reduce the optical depth of cirrus clouds by seeding them with ice nucleating particles. However, the most commonly studied approaches attempt to mimic the cooling effects of major volcanic eruptions by injecting reflective aerosols (e.g., sulphate aerosols) or their precursors (e.g., sulphur dioxide) into the stratosphere. Links to chapters 4.6.3, 5.6.3, 6.4.6

SRM could offset some effects of greenhouse gas-induced warming on global and regional climate, but there would be substantial residual and overcompensating climate change at the regional scale and seasonal time scales ( ''high confidence'' ). Since AR5, more modelling work has been conducted with more sophisticated treatment of aerosol-based SRM approaches, but the uncertainties in cloud–aerosol–radiation interactions are still large ( ''high confidence'' ). Modelling studies suggest that it is possible to stabilize multiple large-scale temperature indicators simultaneously by tailoring the deployment strategy of SRM options ( ''medium confidence'' ) but with large residual or overcompensating regional and seasonal climate changes. Links to chapters 4.6.3

SRM approaches targeting shortwave radiation are ''likely'' to reduce global mean precipitation, relative to future CO <sub>2</sub> emissions scenarios, if all global mean warming is offset. In contrast, cirrus cloud thinning, targeting longwave radiation, is expected to cause an increase in global mean precipitation ( ''medium confidence'' ). If shortwave approaches are used to offset global mean warming, the magnitude of reduction in regional precipitation minus evapotranspiration (P–E) (Box TS.5), which is more relevant to freshwater availability, is smaller than precipitation decrease because of simultaneous reductions in both precipitation and evapotranspiration ( ''medium confidence'' ). Links to chapters 4.6.3, 8.2.1, 8.6.3 .

If SRM is used to cool the planet, it would cause a reduction in plant and soil respiration and slow the reduction of ocean carbon uptake due to warming ( ''medium confidence'' ). The result would be an enhancement of the global land and ocean CO <sub>2</sub> sinks ( ''medium confidence'' ) and a slight reduction in atmospheric CO <sub>2</sub> concentration relative to unmitigated climate change. However, SRM would not stop CO <sub>2</sub> from increasing in the atmosphere or affect the resulting ocean acidification under continued anthropogenic emissions ( ''high confidence'' ). Links to chapters 5.6.3

The effect of stratospheric aerosol injection on global temperature and precipitation is projected by models to be detectable after one to two decades, which is similar to the time scale for the emergence of the benefits of emissions reductions. A sudden and sustained termination of SRM in a high GHG emissions scenario would cause rapid climate change and a reversal of the SRM effects on the carbon sinks ( ''high confidence'' ). It is also ''likely'' that a termination of strong SRM would drive abrupt changes in the water cycle globally and regionally, especially in the tropical regions by shifting the Inter-tropical Convergence Zone and Hadley cells. At the regional scale, non-linear responses cannot be excluded, due to changes in evapotranspiration. However, a gradual phase-out of SRM combined with emissions reductions and CDR would avoid larger rates of changes ( ''medium confidence'' ). Links to chapters 4.6.3, 5.6.3, 8.6.3 .

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'''Box TS.9 | Irreversibility, Tipping Points and Abrupt Changes'''

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'''The present rates of response of many aspects of the climate system are proportionate to the rate of recent temperature change, but some aspects may respond disproportionately. Some climate system components are slow to respond, such as the deep ocean overturning circulation and the ice sheets (Box TS.4). It is ''virtually certain'' that irreversible, committed change is already underway for the slow-to-respond processes as they come into adjustment for past and present emissions.''' '''The paleoclimate record indicates that tipping elements exist in the climate system where processes undergo sudden shifts toward a different sensitivity to forcing, such as during a major deglaciation, where 1°C degree of temperature change might correspond to a large or small ice-sheet mass loss during different stages (Box TS.2). For global climate indicators, evidence for abrupt change is limited, but deep ocean warming, acidification and sea level rise are committed to ongoing change for millennia after global surface temperatures initially stabilize and are irreversible on human time scales ( ''very high confidence'' ). At the regional scale, abrupt responses, tipping points and even reversals in the direction of change cannot be excluded ( ''high confidence'' ). Some regional abrupt changes and tipping points could have severe local impacts, such as unprecedented weather, extreme temperatures and increased frequency of droughts and forest fires.''' '''Models that exhibit such tipping points are characterized by abrupt changes once the threshold is crossed, and even a return to pre-threshold surface temperatures or to atmospheric carbon dioxide concentrations does not guarantee that the tipping elements return to their pre-threshold state. Monitoring and early warning systems are being put into place to observe tipping elements in the climate system. Links to chapters 1.3, 1.4.4, 1.5, 4.3.2, Table 4.10, 5.3.4, 5.4.9, 7.5.3, 9.2.2, 9.2.4, 9.4.1, 9.4.2, 9.6.3, Cross-chapter Box 12.1'''
Understanding of multi-decadal reversibility (i.e., the system returns to the previous climate state within multiple decades after the radiative forcing is removed) has improved since AR5 for many atmospheric, land surface and sea ice climate metrics following sea surface temperature recovery. Some processes suspected of having tipping points, such as the Atlantic Meridional Overturning Circulation (AMOC), have been found to often undergo recovery after temperature stabilization with a time delay ( ''low confidence'' ). However, substantial irreversibility is further substantiated for some cryosphere changes, ocean warming, sea level rise, and ocean acidification. Links to chapters 4.7.2, 5.3.3, 5.4.9, 9.2.2, 9.2.4, 9.4.1, 9.4.2, 9.6.3

Some climate system components are slow to respond, such as the deep ocean overturning circulation and the ice sheets. It is ''likely'' that under stabilization of global warming at 1.5°C, 2.0°C or 3.0°C relative to 1850–1900, the AMOC will continue to weaken for several decades by about 15%, 20% and 30% of its strength and then recover to pre-decline values over several centuries ( ''medium confidence'' ). At sustained warming levels between 2°C and 3°C, there is ''limited evidence'' that the Greenland and West Antarctic ice sheets will be lost almost completely and irreversibly over multiple millennia; both the probability of their complete loss and the rate of mass loss increases with higher surface temperatures ( ''high confidence'' ). At sustained warming levels between 3°C and 5°C, near-complete loss of the Greenland Ice Sheet and complete loss of the West Antarctic Ice Sheet is projected to occur irreversibly over multiple millennia ( ''medium confidence'' ); with substantial parts or all of Wilkes Subglacial Basin in East Antarctica lost over multiple millennia ( ''low confidence'' ). Early-warning signals of accelerated sea level rise from Antarctica could possibly be observed within the next few decades. For other hazards (e.g., ice-sheet behaviour, glacier mass loss and global mean sea level change, coastal floods, coastal erosion, air pollution, and ocean acidification) the time and/or scenario dimensions remain critical, and a simple and robust relationship with global warming level cannot be established ( ''high confidence'' ). Links to chapters 4.3.2, 4.7.2, 5.4.3, 5.4.5, 5.4.8, 8.6, 9.2, 9.4, Box 9.3, Cross-Chapter Box 12.1

For global climate indicators, evidence for abrupt change is limited. For global warming up to 2°C above 1850–1900 levels, paleoclimate records do not indicate abrupt changes in the carbon cycle ( ''low confidence'' ). Despite the wide range of model responses, uncertainty in atmospheric CO <sub>2</sub> by 2100 is dominated by future anthropogenic emissions rather than uncertainties related to carbon–climate feedbacks ( ''high confidence'' ). There is no evidence of abrupt change in climate projections of global temperature for the next century: there is a near-linear relationship between cumulative CO <sub>2</sub> emissions and maximum global mean surface air temperature increase caused by CO <sub>2</sub> over the course of this century for global warming levels up to at least 2°C relative to 1850–1900. The increase in global ocean heat content (Section TS.2.4) will likely continue until at least 2300 even for low emissions scenarios, and global mean sea level will continue to rise for centuries to millennia following cessation of emissions (Box TS.4) due to continuing deep ocean heat uptake and mass loss of the Greenland and Antarctic ice sheets ( ''high confidence'' ). Links to chapters 2.2.3; Cross-Chapter Box 2.1; 5.1.1; 5.4; Cross-Chapter Box 5.1; Figures 5.3, 5.4, 5.25, and 5.26; 9.2.2; 9.2.4

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 climate change ( ''high'' '''''Box TS.9'''''

''confidence'' ). It is ''very unlikely'' that gas clathrates (mostly methane) in deeper terrestrial permafrost and subsea clathrates will lead to a detectable departure from the emissions trajectory during this century. Possible abrupt changes and tipping points in biogeochemical cycles lead to additional uncertainty in 21st century atmospheric GHG concentrations, but future anthropogenic emissions remain the dominant uncertainty ( ''high confidence'' ). There is potential for abrupt water cycle changes in some high emissions scenarios, but there is no overall consistency regarding the magnitude and timing of such changes. Positive land surface feedbacks, including vegetation, dust, and snow, can contribute to abrupt changes in aridity, but there is only ''low confidence'' that such changes will occur during the 21st century. Continued Amazon deforestation, combined with a warming climate, raises the probability that this ecosystem will cross a tipping point into a dry state during the 21st century ( ''low confidence'' ). (Section TS.3.2.2) Links to chapters 5.4.3, 5.4.5, 5.4.8, 5.4.9, 8.6.2, 8.6.3, Cross-Chapter Box 12.1

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