Editing IPCC:AR6/WGI/Chapter-7 (section)

=== Climate Response ===

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'''The total human-forced GSAT change from 1750 to 2019 is calculated to be 1.29 [0.99 to 1.65] °C. This calculation is an emulator-based estimate, constrained by the historic GSAT and ocean heat content changes from ( [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] and the ERF, ECS and TCR from this chapter.''' The calculated GSAT change is composed of a well-mixed greenhouse gas warming of 1.58 [1.17 to 2.17] °C ( ''high confidence'' ), a warming from ozone changes of 0.23 [0.11 to 0.39] °C ( ''high confidence'' ), a cooling of –0.50 [–0.22 to –0.96] °C from aerosol effects ( ''medium confidence'' ), and a –0.06 [–0.15 to +0.01] °C contribution from surface reflectance changes from land-use change and light-absorbing particles on ice and snow ( ''medium confidence'' ). Changes in solar and volcanic activity are assessed to have together contributed a small change of –0.02 [–0.06 to +0.02] °C since 1750 ( ''medium confidence'' ). {7.3.5}

'''Uncertainties regarding the true value of ECS and TCR are the dominant source of uncertainty in global temperature projections over the 21st century under moderate to high greenhouse gas emissions scenarios. For scenarios that reach net zero carbon dioxide emissions, the uncertainty in the ERF values of aerosol and other short-lived climate forcers contribute substantial uncertainty in projected temperature.''' Global ocean heat uptake is a smaller source of uncertainty in centennial-time scale surface warming ( ''high confidence'' ). {7.5.7}

'''The assessed historical and future ranges of GSAT change in this Report are shown to be internally consistent with the Report’s assessment of key physical-climate indicators: greenhouse gas ERFs, ECS and TCR.''' When calibrated to match the assessed ranges within the assessment, physically based emulators can reproduce the best estimate of GSAT change over 1850–1900 to 1995–2014 to within 5% and the ''very likely'' range of this GSAT change to within 10%. Two physically based emulators match at least two-thirds of the Chapter 4-assessed projected GSAT changes to within these levels of precision. When used for multi-scenario experiments, calibrated physically based emulators can adequately reflect assessments regarding future GSAT from Earth system models and/or other lines of evidence ( ''high confidence'' ). {Cross-Chapter Box 7.1}

'''It is now well understood that the Arctic warms more quickly than the Antarctic due to differences in radiative feedbacks and ocean heat uptake between the poles, but that surface warming will eventually be amplified in both the Arctic and Antarctic''' ( ''high confidence'' ''').''' The causes of this polar amplification are well understood, and the evidence is stronger than at the time of AR5, supported by better agreement between modelled and observed polar amplification during warm paleo time periods ( ''high confidence'' ) ''.'' The Antarctic warms more slowly than the Arctic owing primarily to upwelling in the Southern Ocean, and even at equilibrium is expected to warm less than the Arctic. The rate of Arctic surface warming will continue to exceed the global average over this century ( ''high confidence'' ). There is also ''high confidence'' that Antarctic amplification will emerge as the Southern Ocean surface warms on centennial time scales, although only ''low confidence'' regarding whether this feature will emerge during the 21st century. {7.4.4}

'''The assessed global warming potentials (GWP) and global temperature-change potentials (GTP) for methane and nitrous oxide are slightly lower than in AR5 due to revised estimates of their lifetimes and updated estimates of their indirect chemical effects''' ( ''medium confidence'' ''').''' The assessed metrics now also include the carbon cycle response for non-CO <sub>2</sub> gases. The carbon cycle estimate is lower than in AR5, but there is ''high confidence'' in the need for its inclusion and in the quantification methodology. Metrics for methane from fossil fuel sources account for the extra fossil CO <sub>2</sub> that these emissions contribute to the atmosphere and so have slightly higher emissions metric values than those from biogenic sources ( ''high confidence'' ). {7.6.1}

'''New emissions metric approaches such as GWP* and the combined-GTP (CGTP) are designed to relate emissions rates of short-lived gases to cumulative emissions of CO''' <sub>2</sub> '''. These metric approaches are well suited to estimate the GSAT response from aggregated emissions of a range of gases over time, which can be done by scaling the cumulative CO''' <sub>2</sub> '''equivalent emissions calculated with these metrics by the transient climate response to cumulative emissions of CO''' <sub>2</sub> '''.''' For a given multi-gas emissions pathway, the estimated contribution of emissions to surface warming is improved by using either these new metric approaches or by treating short- and long-lived GHG emissions pathways separately, as compared to approaches that aggregate emissions of GHGs using standard GWP or GTP emissions metrics. By contrast, if emissions are weighted by their 100-year GWP or GTP values, different multi-gas emissions pathways with the same aggregated CO <sub>2</sub> equivalent emissions rarely lead to the same estimated temperature outcome ( ''high confidence'' ). {7.6.1, Box 7.3}

'''The choice of emissions metric affects the quantification of net zero GHG emissions and therefore the resulting temperature outcome after net zero emissions are achieved.''' In general, achieving net zero CO <sub>2</sub> emissions and declining non-CO <sub>2</sub> radiative forcing would be sufficient to prevent additional human-caused warming. Reaching net zero GHG emissions as quantified by GWP-100 typically results in global temperatures that peak and then decline after net zero GHGs emissions are achieved, though this outcome depends on the relative sequencing of mitigation of short-lived and long-lived species. In contrast, reaching net zero GHG emissions when quantified using new emissions metrics such as CGTP or GWP* would lead to approximate temperature stabilization ( ''high confidence'' ). {7.6.2}

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