Editing IPCC:AR6/WGI/Chapter-4 (section)

==== 4.6.3.1 Emergence of the Climate Response to Mitigation ====

<div id="h3-41-siblings" class="h3-siblings"></div>

Reducing GHG emissions will eventually slow and limit the degree of climate change relative to high-emissions scenarios such as SSP5-8.5 ( ''very'' ''high confidence'' ). Even when CO <sub>2</sub> emissions are reduced, however, atmospheric CO <sub>2</sub> concentrations continue to increase as long as emissions exceed removal by sinks ( [[#Millar--2017|Millar et al., 2017]] ). Surface warming would likewise initially continue under scenarios of decreasing emissions, resulting in a substantial lag between a peak in CO <sub>2</sub> emissions and peak warming ( ''high confidence'' ) ( [[#Ricke--2014|Ricke and Caldeira, 2014]] ; [[#Zickfeld--2015|Zickfeld and Herrington, 2015]] ). The lag between peak emissions and warming depends on the emissions history prior to the peak and also on the rate of the subsequent emissions reductions ( [[#Matthews--2010|Matthews, 2010]] ; [[#Ricke--2014|Ricke and Caldeira, 2014]] ; [[#Zickfeld--2015|Zickfeld and Herrington, 2015]] ).

In addition to the lag between peak emissions and peak warming,the climate response to reduced emissions would be overlain by internal variability, which can amplify or attenuate the forced response. The resulting masking of differences between scenarios is illustrated in Figure 4.36 for GSAT trends over 2021–2040 ( [[#Maher--2020|Maher et al., 2020]] ). The overall trends conform to expectations in that most simulations show warming almost everywhere, especially under scenario RCP8.5 (Figure 4.36 bottom row). But any individual grid point can in principle show no warming or even cooling, even under RCP 8.5, over the near term (Figure 4.36, middle row). The magnitude of pointwise maximum and minimum temperature trends can be as large as 0.5 <sup>°</sup> C per year (Figure 4.36, top and middle rows), exceeding possible trends in the global mean by one order of magnitude. While it is only a small fraction of the surface that simultaneously can show cooling, cooling at any given location is fully consistent with globally averaged surface warming over the near term ( ''high confidence'' , since the findings of [[#Maher--2020|Maher et al. (2020)]] are consistent across six different large initial-condition ensembles).

<div id="_idContainer090" class="Basic-Text-Frame"></div>

[[File:f83d9eafc0470a6d1c009854fbbaeaee IPCC_AR6_WGI_Figure_4_36.png]]

'''Figure''' '''4.36 |''' '''Masking of climate response to mitigation by internal variability in the near term.''' Near-term (2021–2040) pointwise maximum '''(top row)''' and pointwise minimum '''(middle row)''' surface air temperature trends in the large initial-condition ensemble from MPI '''(left and centre columns)''' , and CESM '''(right-hand column)''' models in the RCP2.6 '''(left-hand column)''' and RCP8.5 scenarios '''(centre and right columns)''' . The percentage of ensemble members with a warming trend in the near term is shown in the bottom panels. Figure modified from [[#Maher--2020|Maher et al. (2020)]] . Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

An important development since AR5 has been the quantification of when the climate response to mitigation can be expected to emerge from the background noise of internal variability (illustrated in Figure 4.36; see [[IPCC:Wg1:Chapter:Chapter-1#1.4.2.2|Section 1.4.2.2]] and Glossary). A basic ambiguity arises because once mitigation measures are in place, it is no longer possible to observe what the climate would have been without these measures, and any statement about emergence of the response to mitigation is contingent upon the assumed strength of mitigation in relation to an assumed (‘counterfactual’) no-mitigation scenario. Still, there is ''high agreement'' on the emergence of the climate response to mitigation across a number of independent studies using different models and different statistical approaches.

Among global quantities, emergence of the response to differing CO <sub>2</sub> emissions – representing differences between low- and high-emissions scenarios – is first expected to arise in global mean CO <sub>2</sub> concentrations, about 10 years after emissions pathways have started diverging ( ''high confidence'' ) ( [[#Tebaldi--2013|Tebaldi and Friedlingstein, 2013]] ; [[#Peters--2017|Peters et al., 2017]] ; [[#Schwartzman--2020|Schwartzman and Keeling, 2020]] ; [[#Spring--2020|Spring et al., 2020]] ). In these studies, emergence is generally defined as the time at which the global mean concentration first differs between mitigation and non-mitigation scenarios by more than two standard deviations of internal variability, although there are some methodological differences.

Emergence in GSAT would be delayed further, owing to the inertia in the climate system. Although not investigating emergence as defined here in AR6, [[#Tebaldi--2021|Tebaldi et al. (2021)]] used a 20-year running-mean GSAT and compared pairwise either model-by-model or between CM IP6 ensemble means from the core set of five scenarios assessed in this chapter. Differences by more than 0.1°C showed up in most cases in the near term, with only some of the individual models and the comparisons of the closest scenarios showing a delay until the mid-term. Taking internal variability explicitly into account, [[#Tebaldi--2013|Tebaldi and Friedlingstein (2013)]] and [[#Samset--2020|Samset et al. (2020)]] found emergence of mitigation benefits in GSAT changes about 25–30 years after RCP2.6 emissions diverge from the higher-emissions trajectories in RCP4.5 and RCP8.5. Consistently, [[#Marotzke--2019|Marotzke (2019)]] found about one-third likelihood that a trend reduction in GSAT, over the period 2021–2035 relative to 2005–2020, would be attributable to the emissions reductions implied by the difference between RCP2.6 and RCP4.5. Emergence of the GSAT response to mitigation of individual short-lived climate forcers (SLCFs) would likewise not occur until several decades after emissions trajectories diverge, owing to the relatively small influence of individual SLCFs on the total ERF ( [[#Samset--2020|Samset et al., 2020]] ), see also [[#4.4.4|Section 4.4.4]] and Figure 4.18.

In contrast to the earlier studies, emergence in GSAT within the near-term has recently been found by [[#McKenna--2021|McKenna et al. (2021)]] who investigated the likelihood that under the SSP scenarios GSAT trends will exceed the largest historical observed 20-year trends. They found that under scenario SSP1-1.9, the 20-year GSAT trends would ''likely'' be lower than in SSP3-7.0 and SSP5-8.5 within the near term. This earlier diagnosed time of emergence compared to [[#Marotzke--2019|Marotzke (2019)]] , while using a similar statistical approach, presumably arose because of the longer-period trends (20 rather than 15 years) and the larger difference between emissions trajectories considered ( ''medium confidence'' ). Using 20-year temperature anomalies relative to 1995–2014 instead of 20-year trends yielded a low probability of emergence ( [[#McKenna--2021|McKenna et al., 2021]] ), consistent with the AR5 (Collins et al., 2013; [[#Kirtman--2013|Kirtman et al., 2013]] ; [[#Tebaldi--2013|Tebaldi and Friedlingstein, 2013]] ; [[#Samset--2020|Samset et al., 2020]] ). It is not yet understood why GSAT trends appear to show faster emergence of mitigation benefits, compared to GSAT anomalies.

Emergence of mitigation benefits has been studied much less for quantities other than globally and annually averaged CO <sub>2</sub> concentration and surface temperature. Boreal-winter temperatures are more challenging for emergence, due to larger variability in boreal winter and adding a decade to the time of emergence, whereas emergence times for boreal-summer averages are similar to the annual temperature averages ( [[#Tebaldi--2013|Tebaldi and Friedlingstein, 2013]] ). Emergence happens later at the regional scale, with a median time of emergence of 30–45 years after emissions paths separate in RCP2.6 relative to RCP4.5 and RCP8.5; a stricter requirement of 95% confidence level instead of median induces a delay of several decades, bringing time of emergence toward the end of the 21st century at regional scales ( [[#Tebaldi--2013|Tebaldi and Friedlingstein, 2013]] ).

Attribution to emissions reductions, for the case of RCP2.6 relative to RCP4.5, is not substantially more likely for 2021–2035 trends in upper-2000 m OHC than for GSAT ( [[#Marotzke--2019|Marotzke, 2019]] ), although OHC change is thought to be less susceptible to internal variability. Furthermore, [[#Marotzke--2019|Marotzke (2019)]] found only around 10% likelihood of mitigation-benefit emergence during 2021–2035 for change in AMOC and September Arctic sea ice area. [[#Tebaldi--2018|Tebaldi and Wehner (2018)]] showed that the differences in temperature extremes between RCP4.5 and RCP8.5 over all land areas become statistically significant by 2050. The seemingly contrasting result of [[#Ciavarella--2017|Ciavarella et al. (2017)]] that mitigation benefits arise earlier for climate extremes poses no contradiction, because [[#Ciavarella--2017|Ciavarella et al. (2017)]] did not look at emergence as defined here but at the extremes of a distribution, which differ between scenarios already at a time when the distributions are still largely overlapping.

In summary, if strong mitigation is applied from 2020 onward as reflected in SSP1-1.9, its effect on 20-year trends in GSAT would ''likely'' emerge during the near term, measured against an assumed non-mitigation scenario such as SSP3-7.0 and SSP5-8.5. However, the response of many other climate quantities to mitigation would be largely masked by internal variability during the near term, especially on the regional scale ( ''high confidence'' ). The mitigation benefits for these quantities would emerge only later during the 21st century ( ''high confidence'' ). During the near term, a small fraction of the surface can show cooling under all scenarios assessed here, so near-term cooling at any given location is fully consistent with globally averaged surface warming ( ''high confidence'' ).

<div id="4.6.3.2" class="h3-container"></div>

<span id="climate-response-to-mitigation-by-carbon-dioxide-removal"></span>