Editing IPCC:AR6/WGI/SPM (section)

== D. Limiting Future Climate Change ==

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''Since AR5, estimates of remaining carbon budgets have been improved by a new methodology first presented in SR1.5, updated evidence, and the integration of results from multiple lines of evidence. A comprehensive range of possible future air pollution controls in scenarios is used to consistently assess the effects of various assumptions on projections of climate and air pollution. A novel development is the ability to ascertain when climate responses to emissions reductions would become discernible above natural climate variability, including internal variability and responses to natural drivers.'' '''D.1 From a physical science perspective, limiting human-induced global warming to a specific level requires limiting cumulative CO <sub>2</sub> emissions, reaching at least net zero CO <sub>2</sub> emissions, along with strong reductions in other greenhouse gas emissions. Strong, rapid and sustained reductions in CH <sub>4</sub> emissions would also limit the warming effect resulting from declining aerosol pollution and would improve air quality.   Expand [[#figure-spm-10|Figure SPM.10]] [[#table-spm-2|Table SPM.2]] Links to chapters 3.3, 4.6, 5.1, 5.2, 5.4, 5.5, 5.6, Box 5.2, Cross-Chapter Box 5.1, 6.7, 7.6, 9.6'''
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D.1.1 This Report reaffirms with ''high confidence'' the AR5 finding that there is a near-linear relationship between cumulative anthropogenic CO <sub>2</sub> emissions and the global warming they cause. Each 1000 GtCO <sub>2</sub> of cumulative CO <sub>2</sub> emissions is assessed to ''likely'' cause a 0.27°C to 0.63°C increase in global surface temperature with a best estimate of 0.45°C. <sup>[[#footnote-008|41]]</sup> This is a narrower range compared to AR5 and SR1.5. This quantity is referred to as the transient climate response to cumulative CO <sub>2</sub> emissions (TCRE). This relationship implies that reaching net zero anthropogenic CO <sub>2</sub> emissions '''[[#footnote-007|42]]''' is a requirement to stabilize human-induced global temperature increase at any level, but that limiting global temperature increase to a specific level would imply limiting cumulative CO <sub>2</sub> emissions to within a carbon budget. <sup>[[#footnote-006|43]]</sup>   [[#figure-spm-10|Figure SPM.10]] Links to chapters 5.4, 5.5, TS.1.3, TS.3.3, Box TS.5

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[[File:6440bca4ee1d4c6bb18f687899b3439b IPCC_AR6_WGI_SPM_Figure_10.png]]

Figure SPM.10 | Near-linear relationship between cumulative CO <sub>2</sub> emissions and the increase in global surface temperature

'''Top panel:''' Historical data (thin black line) shows observed global surface temperature increase in °C since 1850–1900 as a function of historical cumulative carbon dioxide (CO <sub>2</sub> ) emissions in GtCO <sub>2</sub> from 1850 to 2019. The grey range with its central line shows a corresponding estimate of the historical human-caused surface warming (see Figure SPM.2). 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 <sub>2</sub> emissions from 2020 until year 2050 for the set of illustrative scenarios (SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5; see Figure SPM.4). Projections use the cumulative CO <sub>2</sub> emissions of each respective scenario, and the projected global warming includes the contribution from all anthropogenic forcers. The relationship is illustrated over the domain of cumulative CO <sub>2</sub> emissions for which there is ''high confidence'' that the transient climate response to cumulative CO <sub>2</sub> emissions (TCRE) remains constant, and for the time period from 1850 to 2050 over which global CO <sub>2</sub> emissions remain net positive under all illustrative scenarios, as there is ''limited evidence'' supporting the quantitative application of TCRE to estimate temperature evolution under net negative CO <sub>2</sub> emissions.

'''Bottom panel:''' Historical and projected cumulative CO <sub>2</sub> emissions in GtCO <sub>2</sub> for the respective scenarios.   Links to chapters Section 5.5, Figure 5.31, Figure TS.18

D.1.2 Over the period 1850–2019, a total of 2390 ± 240 ( ''likely'' range) GtCO <sub>2</sub> of anthropogenic CO <sub>2</sub> was emitted. Remaining carbon budgets have been estimated for several global temperature limits and various levels of probability, based on the estimated value of TCRE and its uncertainty, estimates of historical warming, variations in projected warming from non-CO <sub>2</sub> emissions, climate system feedbacks such as emissions from thawing permafrost, and the global surface temperature change after global anthropogenic CO <sub>2</sub> emissions reach net zero.   [[#table-spm-2|Table SPM.2]] Links to chapters 5.1, 5.5, Box 5.2, TS.3.3

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'''Table SPM.2 | Estimates of historical carbon dioxide (CO''' ''2'' ''') emissions and remaining carbon budgets.''' Estimated remaining carbon budgets are calculated from the beginning of 2020 and extend until global net zero CO <sub>2</sub> emissions are reached. They refer to CO <sub>2</sub> emissions, while accounting for the global warming effect of non-CO <sub>2</sub> emissions. Global warming in this table refers to human-induced global surface temperature increase, which excludes the impact of natural variability on global temperatures in individual years.

Links to chapters Table 3.1, 5.5.1, 5.5.2, Box 5.2, Table 5.1, Table 5.7, Table 5.8, Table TS.3

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<sup>a</sup> Values at each 0.1°C increment of warming are available in Tables TS.3 and 5.8.

<sup>b</sup> This likelihood is based on the uncertainty in transient climate response to cumulative CO <sub>2</sub> emissions (TCRE) and additional Earth system feedbacks and provides the probability that global warming will not exceed the temperature levels provided in the two left columns. Uncertainties related to historical warming (±550 GtCO <sub>2</sub> ) and non-CO <sub>2</sub> forcing and response (±220 GtCO <sub>2</sub> ) are partially addressed by the assessed uncertainty in TCRE, but uncertainties in recent emissions since 2015 (±20 GtCO <sub>2</sub> ) and the climate response after net zero CO <sub>2</sub> emissions are reached (±420 GtCO <sub>2</sub> ) are separate.

<sup>c</sup> Remaining carbon budget estimates consider the warming from non-CO <sub>2</sub> drivers as implied by the scenarios assessed in SR1.5. The Working Group III Contribution to AR6 will assess mitigation of non-CO <sub>2</sub> emissions.

D.1.3 Several factors that determine estimates of the remaining carbon budget have been re-assessed, and updates to these factors since SR1.5 are small. When adjusted for emissions since previous reports, estimates of remaining carbon budgets are therefore of similar magnitude compared to SR1.5 but larger compared to AR5 due to methodological improvements. <sup>[[#footnote-005|44]]</sup>   [[#table-spm-2|Table SPM.2]] Links to chapters 5.5, Box 5.2, TS.3.3

D.1.4 Anthropogenic CO <sub>2</sub> removal (CDR) has the potential to remove CO <sub>2</sub> from the atmosphere and durably store it in reservoirs ( ''high confidence'' ). CDR aims to compensate for residual emissions to reach net zero CO <sub>2</sub> or net zero GHG emissions or, if implemented at a scale where anthropogenic removals exceed anthropogenic emissions, to lower surface temperature. CDR methods can have potentially wide-ranging effects on biogeochemical cycles and climate, which can either weaken or strengthen the potential of these methods to remove CO <sub>2</sub> and reduce warming, and can also influence water availability and quality, food production and biodiversity <sup>[[#footnote-004|45]]</sup> ( ''high confidence'' ).   Links to chapters 5.6, Cross-Chapter Box 5.1, TS.3.3

D.1.5 Anthropogenic CO <sub>2</sub> removal (CDR) leading to global net negative emissions would lower the atmospheric CO <sub>2</sub> concentration and reverse surface ocean acidification ( ''high confidence'' ). Anthropogenic CO <sub>2</sub> removals and emissions are partially compensated by CO <sub>2</sub> release and uptake respectively, from or to land and ocean carbon pools ( ''very high confidence'' ). CDR would lower atmospheric CO <sub>2</sub> by an amount approximately equal to the increase from an anthropogenic emission of the same magnitude ( ''high confidence'' ). The atmospheric CO <sub>2</sub> decrease from anthropogenic CO <sub>2</sub> removals could be up to 10% less than the atmospheric CO <sub>2</sub> increase from an equal amount of CO <sub>2</sub> emissions, depending on the total amount of CDR ( ''medium confidence'' ).   Links to chapters 5.3, 5.6, TS.3.3

D.1.6 If global net negative CO <sub>2</sub> emissions were to be achieved and be sustained, the global CO <sub>2</sub> -induced surface temperature increase would be gradually reversed but other climate changes would continue in their current direction for decades to millennia ( ''high confidence'' ). For instance, it would take several centuries to millennia for global mean sea level to reverse course even under large net negative CO <sub>2</sub> emissions ( ''high confidence'' ).   Links to chapters 4.6, 9.6, TS.3.3

D.1.7 In the five illustrative scenarios, simultaneous changes in CH <sub>4</sub> , aerosol and ozone precursor emissions, which also contribute to air pollution, lead to a net global surface warming in the near and long term ( ''high confidence'' ). In the long term, this net warming is lower in scenarios assuming air pollution controls combined with strong and sustained CH <sub>4</sub> emissions reductions ( ''high confidence'' ). In the low and very low GHG emissions scenarios, assumed reductions in anthropogenic aerosol emissions lead to a net warming, while reductions in CH <sub>4</sub> and other ozone precursor emissions lead to a net cooling. Because of the short lifetime of both CH <sub>4</sub> and aerosols, these climate effects partially counterbalance each other, and reductions in CH <sub>4</sub> emissions also contribute to improved air quality by reducing global surface ozone ( ''high confidence'' ).   [[#figure-spm-2|Figure SPM.2]] [[#box-spm-1|Box SPM.1]] Links to chapters 6.7, Box TS.7

D.1.8 Achieving global net zero CO <sub>2</sub> emissions, with anthropogenic CO <sub>2</sub> emissions balanced by anthropogenic removals of CO <sub>2</sub> , is a requirement for stabilizing CO <sub>2</sub> -induced global surface temperature increase. This is different from achieving net zero GHG emissions, where metric-weighted anthropogenic GHG emissions equal metric-weighted anthropogenic GHG removals. For a given GHG emissions pathway, the pathways of individual GHGs determine the resulting climate response, <sup>[[#footnote-003|46]]</sup> whereas the choice of emissions metric <sup>[[#footnote-002|47]]</sup> used to calculate aggregated emissions and removals of different GHGs affects what point in time the aggregated GHGs are calculated to be net zero. Emissions pathways that reach and sustain net zero GHG emissions defined by the 100-year global warming potential are projected to result in a decline in surface temperature after an earlier peak ( ''high confidence'' ).   Links to chapters 4.6, 7.6, Box 7.3, TS.3.3

'''D.2 Scenarios with very low or low GHG emissions (SSP1-1.9 and SSP1-2.6) lead within years to discernible effects on greenhouse gas and aerosol concentrations and air quality, relative to high and very high GHG emissions scenarios (SSP3-7.0 or SSP5-8.5). Under these contrasting scenarios, discernible differences in trends of global surface temperature would begin to emerge from natural variability within around 20 years, and over longer time periods for many other climatic impact-drivers ( high confidence ).   Expand [[#figure-spm-8|Figures SPM.8]] , [[#figure-spm-10|SPM.10]] Links to chapters 4.6, 6.6, 6.7, Cross-Chapter Box 6.1, 9.6, 11.2, 11.4, 11.5, 11.6, Cross-Chapter Box 11.1, 12.4, 12.5'''
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D.2.1 Emissions reductions in 2020 associated with measures to reduce the spread of COVID-19 led to temporary but detectable effects on air pollution ( ''high confidence'' ) and an associated small, temporary increase in total radiative forcing, primarily due to reductions in cooling caused by aerosols arising from human activities ( ''medium confidence'' ). Global and regional climate responses to this temporary forcing are, however, undetectable above natural variability ( ''high confidence'' ). Atmospheric CO <sub>2</sub> concentrations continued to rise in 2020, with no detectable decrease in the observed CO <sub>2</sub> growth rate ( ''medium confidence'' ). <sup>[[#footnote-001|48]]</sup>   Links to chapters Cross-Chapter Box 6.1, TS.3.3

D.2.2 Reductions in GHG emissions also lead to air quality improvements. However, in the near term, <sup>[[#footnote-000|49]]</sup> even in scenarios with strong reduction of GHGs, as in the low and very low GHG emissions scenarios (SSP1-2.6 and SSP1-1.9), these improvements are not sufficient in many polluted regions to achieve air quality guidelines specified by the World Health Organization ( ''high confidence'' ). Scenarios with targeted reductions of air pollutant emissions lead to more rapid improvements in air quality within years compared to reductions in GHG emissions only, but from 2040, further improvements are projected in scenarios that combine efforts to reduce air pollutants as well as GHG emissions, with the magnitude of the benefit varying between regions ( ''high confidence'' ).   Links to chapters 6.6, 6.7, Box TS.7 .

D.2.3 Scenarios with very low or low GHG emissions (SSP1-1.9 and SSP1-2.6) would have rapid and sustained effects to limit human-caused climate change, compared with scenarios with high or very high GHG emissions (SSP3-7.0 or SSP5-8.5), but early responses of the climate system can be masked by natural variability. For global surface temperature, differences in 20-year trends would ''likely'' emerge during the near term under a very low GHG emissions scenario (SSP1-1.9), relative to a high or very high GHG emissions scenario (SSP3-7.0 or SSP5-8.5). The response of many other climate variables would emerge from natural variability at different times later in the 21st century ( ''high confidence'' ).   [[#figure-spm-8|Figure SPM.8]] [[#figure-spm-10|Figure SPM.10]] Links to chapters 4.6, Cross-Section Box TS.1

D.2.4 Scenarios with very low and low GHG emissions (SSP1-1.9 and SSP1-2.6) would lead to substantially smaller changes in a range of CIDs <sup>[[#footnote-013|36]]</sup> beyond 2040 than under high and very high GHG emissions scenarios (SSP3-7.0 and SSP5-8.5). By the end of the century, scenarios with very low and low GHG emissions would strongly limit the change of several CIDs, such as the increases in the frequency of extreme sea level events, heavy precipitation and pluvial flooding, and exceedance of dangerous heat thresholds, while limiting the number of regions where such exceedances occur, relative to higher GHG emissions scenarios ( ''high confidence'' ). Changes would also be smaller in very low compared to low GHG emissions scenarios, as well as for intermediate (SSP2-4.5) compared to high or very high GHG emissions scenarios ( ''high confidence'' ).   Links to chapters 9.6, 11.2, 11.3, 11.4, 11.5, 11.6, 11.9, Cross-Chapter Box 11.1, 12.4, 12.5, TS.4.3

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[[#footnote-048-backlink|1]] Decision IPCC/XLVI-2.

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[[#footnote-047-backlink|2]] The three Special Reports are: Global Warming of 1.5°C: An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty (SR1.5); Climate Change and Land: An IPCC Special Report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems (SRCCL); IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC).

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[[#footnote-046-backlink|3]] The assessment covers scientific literature accepted for publication by 31 January 2021.

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[[#footnote-045-backlink|4]] Each finding is grounded in an evaluation of underlying evidence and agreement. A level of confidence is expressed using five qualifiers: very low, low, medium, high and very high, and typeset in italics, for example, ''medium confidence'' . The following terms have been used to indicate the assessed likelihood of an outcome or result: virtually certain 99–100% probability; very likely 90–100%; likely 66–100%; about as likely as not 33–66%; unlikely 0–33%; very unlikely 0–10%; and exceptionally unlikely 0–1%. Additional terms (extremely likely 95–100%; more likely than not &gt;50–100%; and extremely unlikely 0–5%) are also used when appropriate. Assessed likelihood is typeset in italics, for example, ''very likely'' . This is consistent with AR5. In this Report, unless stated otherwise, square brackets [x to y] are used to provide the assessed ''very likely'' range, or 90% interval.

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[[#footnote-044-backlink|5]] The Interactive Atlas is available at [https://interactive-atlas.ipcc.ch/ https://interactive-atlas.ipcc.ch]

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[[#footnote-043-backlink|6]] Other GHG concentrations in 2019 were: perfluorocarbons (PFCs) – 109 parts per trillion (ppt) CF <sub>4</sub> equivalent; sulphur hexafluoride (SF <sub>6</sub> ) – 10 ppt; nitrogen trifluoride (NF <sub>3</sub> ) <sub></sub> – 2 ppt; hydrofluorocarbons (HFCs) – 237 ppt HFC-134a equivalent; other Montreal Protocol gases (mainly chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs)) – 1032 ppt CFC-12 equivalent). Increases from 2011 are 19 ppm for CO <sub>2</sub> , 63 ppb for CH <sub>4</sub> and 8 ppb for N <sub>2</sub> O.

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[[#footnote-042-backlink|7]] Land and ocean are not substantial sinks for other GHGs.

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[[#footnote-041-backlink|8]] The term ‘global surface temperature’ is used in reference to both global mean surface temperature and global surface air temperature throughout this SPM. Changes in these quantities are assessed with ''high confidence'' to differ by at most 10% from one another, but conflicting lines of evidence lead to ''low confidence'' in the sign (direction) of any difference in long-term trend.   Links to chapters Cross-Section Box TS.1

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[[#footnote-040-backlink|9]] The period 1850–1900 represents the earliest period of sufficiently globally complete observations to estimate global surface temperature and, consistent with AR5 and SR1.5, is used as an approximation for pre-industrial conditions.

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[[#footnote-039-backlink|10]] Since AR5, methodological advances and new datasets have provided a more complete spatial representation of changes in surface temperature, including in the Arctic. These and other improvements have also increased the estimate of global surface temperature change by approximately 0.1°C, but this increase does not represent additional physical warming since AR5.

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[[#footnote-038-backlink|11]] The period distinction with A.1.2 arises because the attribution studies consider this slightly earlier period. The observed warming to 2010–2019 is 1.06 [0.88 to 1.21] °C.

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[[#footnote-037-backlink|12]] Throughout this SPM, ‘main driver’ means responsible for more than 50% of the change.

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[[#footnote-036-backlink|13]] As stated in section B.1, even under the very low emissions scenario SSP1-1.9, temperatures are assessed to remain elevated above those of the most recent decade until at least 2100 and therefore warmer than the century-scale period 6500 years ago.

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[[#footnote-035-backlink|14]] As indicated in footnote 12, throughout this SPM, ‘main driver’ means responsible for more than 50% of the change.

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[[#footnote-034-backlink|15]] Agricultural and ecological drought (depending on the affected biome): a period with abnormal soil moisture deficit, which results from combined shortage of precipitation and excess evapotranspiration, and during the growing season impinges on crop production or ecosystem function in general (see Annex VII: Glossary). Observed changes in meteorological droughts (precipitation deficits) and hydrological droughts (streamflow deficits) are distinct from those in agricultural and ecological droughts and are addressed in the underlying AR6 material (Chapter 11).

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[[#footnote-033-backlink|16]] The combined processes through which water is transferred to the atmosphere from open water and ice surfaces, bare soils and vegetation that make up the Earth’s surface (Glossary).

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[[#footnote-032-backlink|17]] The global monsoon is defined as the area in which the annual range (local summer minus local winter) of precipitation is greater than 2.5 mm day <sup>–1</sup> (Glossary). Global land monsoon precipitation refers to the mean precipitation over land areas within the global monsoon.

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[[#footnote-031-backlink|18]] Compound extreme events are the combination of multiple drivers and/or hazards that contribute to societal or environmental risk (Glossary). Examples are concurrent heatwaves and droughts, compound flooding (e.g., a storm surge in combination with extreme rainfall and/or river flow), compound fire weather conditions (i.e., a combination of hot, dry and windy conditions), or concurrent extremes at different locations.

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[[#footnote-030-backlink|19]] Cumulative energy increase of 282 [177 to 387] ZJ over 1971–2006 (1 ZJ = 10 <sup>21</sup> joules).

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[[#footnote-029-backlink|20]] Cumulative energy increase of 152 [100 to 205] ZJ over 2006–2018.

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[[#footnote-028-backlink|21]] Understanding of climate processes, the instrumental record, paleoclimates and model-based emergent constraints (Glossary).

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[[#footnote-027-backlink|22]] Throughout this Report, the five illustrative scenarios are referred to as SSPx-y, where ‘SSPx’ refers to the Shared Socio-economic Pathway or ‘SSP’ describing the socio-economic trends underlying the scenario, and ‘y’ refers to the approximate level of radiative forcing (in watts per square metre, or W m <sup>–2</sup> ) resulting from the scenario in the year 2100. A detailed comparison to scenarios used in earlier IPCC reports is provided in Section TS.1.3, and Sections 1.6 and 4.6. The SSPs that underlie the specific forcing scenarios used to drive climate models are not assessed by WGI. Rather, the SSPx-y labelling ensures traceability to the underlying literature in which specific forcing pathways are used as input to the climate models. IPCC is neutral with regard to the assumptions underlying the SSPs, which do not cover all possible scenarios. Alternative scenarios may be considered or developed.

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[[#footnote-026-backlink|23]] Net negative CO <sub>2</sub> emissions are reached when anthropogenic removals of CO <sub>2</sub> exceed anthropogenic emissions (Glossary).

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[[#footnote-025-backlink|24]] Changes in global surface temperature are reported as running 20-year averages, unless stated otherwise.

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[[#footnote-024-backlink|25]] SSP1-1.9 and SSP1-2.6 are scenarios that start in 2015 and have very low and low GHG emissions, respectively, and CO <sub>2</sub> emissions declining to net zero around or after 2050, followed by varying levels of net negative CO <sub>2</sub> emissions.

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[[#footnote-023-backlink|26]] Crossing is defined here as having the assessed global surface temperature change, averaged over a 20-year period, exceed a particular global warming level.

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[[#footnote-022-backlink|27]] The AR6 assessment of when a given global warming level is first exceeded benefits from the consideration of the illustrative scenarios, the multiple lines of evidence entering the assessment of future global surface temperature response to radiative forcing, and the improved estimate of historical warming. The AR6 assessment is thus not directly comparable to the SR1.5 SPM, which reported '''likely''' reaching 1.5°C global warming between 2030 and 2052, from a simple linear extrapolation of warming rates of the recent past. When considering scenarios similar to SSP1-1.9 instead of linear extrapolation, the SR1.5 estimate of when 1.5°C global warming is first exceeded is close to the best estimate reported here.

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[[#footnote-021-backlink|28]] Natural variability refers to climatic fluctuations that occur without any human influence, that is, internal variability combined with the response to external natural factors such as volcanic eruptions, changes in solar activity and, on longer time scales, orbital effects and plate tectonics (Glossary).

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[[#footnote-020-backlink|29]] The internal variability in any single year is estimated to be about ±0.25°C (5–95% range, ''high confidence'' ).

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[[#footnote-019-backlink|30]] Projected changes in agricultural and ecological droughts are primarily assessed based on total column soil moisture. See footnote 15 for definition and relation to precipitation and evapotranspiration.

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[[#footnote-018-backlink|31]] Monthly average sea ice area of less than 1 million km <sup>2</sup> , which is about 15% of the average September sea ice area observed in 1979–1988.

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[[#footnote-017-backlink|32]] These projected adjustments of carbon sinks to stabilization or decline of atmospheric CO <sub>2</sub> are accounted for in calculations of remaining carbon budgets.

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[[#footnote-016-backlink|33]] The other sectoral emissions are calculated as the residual of the net land and ocean CO <sub>2</sub> uptake and the prescribed atmospheric CO <sub>2</sub> concentration changes in the CMIP6 simulations. These calculated emissions are net emissions and do not separate gross anthropogenic emissions from removals, which are included implicitly.

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[[#footnote-015-backlink|34]] Low-likelihood, high-impact outcomes are those whose probability of occurrence is low or not well known (as in the context of deep uncertainty) but whose potential impacts on society and ecosystems could be high. A tipping point is a critical threshold beyond which a system reorganizes, often abruptly and/or irreversibly. (Glossary)   Links to chapters 1.4, Cross-Chapter Box 1.3, 4.7

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[[#footnote-014-backlink|35]] To compare to the 1986–2005 baseline period used in AR5 and SROCC, add 0.03 m to the global mean sea level rise estimates. To compare to the 1900 baseline period used in Figure SPM.8, add 0.16 m.

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[[#footnote-013-backlink|36]] Climatic impact-drivers (CIDs) are physical climate system conditions (e.g., means, events, extremes) that affect an element of society or ecosystems. Depending on system tolerance, CIDs and their changes can be detrimental, beneficial, neutral, or a mixture of each across interacting system elements and regions (Glossary). CID types include heat and cold, wet and dry, wind, snow and ice, coastal and open ocean.

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[[#footnote-012-backlink|37]] The main internal variability phenomena include El Niño–Southern Oscillation, Pacific Decadal Variability and Atlantic Multi-decadal Variability through their regional influence.

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[[#footnote-011-backlink|38]] Based on 2500 year reconstructions, eruptions more negative than –1 W m <sup>–2</sup> occur on average twice per century.

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[[#footnote-010-backlink|39]] Regions here refer to the AR6 WGI reference regions used in this Report to summarize information in sub-continental and oceanic regions. Changes are compared to averages over the last 20–40 years unless otherwise specified.   Links to chapters 1.4, 12.4, Atlas.1 .

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[[#footnote-009-backlink|40]] The specific level of confidence or likelihood depends on the region considered. Details can be found in the Technical Summary and the underlying Report.

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[[#footnote-008-backlink|41]] In the literature, units of °C per 1000 PgC (petagrams of carbon) are used, and the AR6 reports the TCRE ''likely'' range as 1.0°C to 2.3°C per 1000 PgC in the underlying report, with a best estimate of 1.65°C.

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[[#footnote-007-backlink|42]] The condition in which anthropogenic carbon dioxide (CO <sub>2</sub> ) emissions are balanced by anthropogenic CO <sub>2</sub> removals over a specified period (Glossary).

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[[#footnote-006-backlink|43]] The term ‘carbon budget’ refers to the maximum amount of cumulative net global anthropogenic CO <sub>2</sub> emissions that would result in limiting global warming to a given level with a given probability, taking into account the effect of other anthropogenic climate forcers. This is referred to as the total carbon budget when expressed starting from the pre-industrial period, and as the remaining carbon budget when expressed from a recent specified date (Glossary). Historical cumulative CO <sub>2</sub> emissions determine to a large degree warming to date, while future emissions cause future additional warming. The remaining carbon budget indicates how much CO <sub>2</sub> could still be emitted while keeping warming below a specific temperature level.

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[[#footnote-005-backlink|44]] Compared to AR5, and when taking into account emissions since AR5, estimates in AR6 are about 300–350 GtCO <sub>2</sub> larger for the remaining carbon budget consistent with limiting warming to 1.5°C; for 2°C, the difference is about 400–500 GtCO <sub>2</sub> .

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[[#footnote-004-backlink|45]] Potential negative and positive effects of CDR for biodiversity, water and food production are methods-specific and are often highly dependent on local context, management, prior land use, and scale. IPCC Working Groups II and III assess the CDR potential and ecological and socio-economic effects of CDR methods in their AR6 contributions.

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[[#footnote-003-backlink|46]] A general term for how the climate system responds to a radiative forcing (Glossary).

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[[#footnote-002-backlink|47]] The choice of emissions metric depends on the purposes for which gases or forcing agents are being compared. This Report contains updated emissions metric values and assesses new approaches to aggregating gases.

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[[#footnote-001-backlink|48]] For other GHGs, there was insufficient literature available at the time of the assessment to assess detectable changes in their atmospheric growth rate during 2020.

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[[#footnote-000-backlink|49]] Near term: 2021–2040.