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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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