Editing IPCC:AR6/WGI/Chapter-6 (section)

==== 6.7.2.1 Effects of changes in SLCFs on ERF and Climate Response ====

<div id="h3-27-siblings" class="h3-siblings"></div>

This section assesses how the different spatial and temporal evolution of SLCF emissions in the SSPs affects the future global and regional ERFs, and GSAT and precipitation responses. In CMIP6, only a very limited set of simulations (all based on the SSP3-7.0 scenario) have been carried out with coupled ESMs to specifically address the future role of SLCFs (Sections 4.3 and 4.4; [[#Collins--2017|Collins et al., 2017]] ). Note that the ScenarioMIP simulations ( [[IPCC:Wg1:Chapter:Chapter-4#4.3|Section 4.3]] ) include the SLCF emissions (as shown in Figures 6.18 and 6.19), however, they cannot be used to quantify the effect of individual forcers. Coupled ESMs can in principle be used for this through a series of sensitivity simulations (e.g., [[#Allen--2020|Allen et al., 2020]] , 2021), but the amount of computer time required has made this approach prohibitive across the full SSP range. Therefore, to quantify the contribution from emissions of individual forcers spanning the range of the SSP scenarios to GSAT response, the analysis is mainly based on estimates using a two-layer emulator configuration derived from the medians of MAGICC7 and FaIRv1.6.2 ( [[IPCC:Wg1:Chapter:Chapter-1#1.5.3.4|Section 1.5.3.4]] , Cross-Chapter Box 7.1 and Supplementary Material 7.SM5.2). The contribution from SLCFs to changes in GSAT have been calculated based on the global mean ERF for the various components as assessed in [[IPCC:Wg1:Chapter:Chapter-7#7.3.5|Section 7.3.5]] , using the two-layer emulator for the climate response.

The projections of GSAT for a broad group of forcing agents (aerosols, methane, tropospheric ozone and HFCs with lifetimes lower than 50 years) for the SSP scenarios show how much of the future warming or cooling (relative to 2019) can be attributed to the SLCFs (Figure 6.22). Note that during the first two decades, some of these changes in GSAT are due to emissions before 2019, in particular for the longer-lived SLCFs such as methane and HFCs (Figure 6.15). The scenarios SSP3-7.0-lowSLCF-highCH <sub>4</sub> and SSP3-7.0-lowSLCF-lowCH <sub>4</sub> are special cases of the SSP3-7.0 scenario with strong, but realistic, reductions in non-methane SLCFs and all SLCFs, respectively ( [[#Gidden--2019|Gidden et al., 2019]] ).

As discussed in Sections 6.2, 6.3 and 6.4, there are uncertainties relating emissions of SLCFs to changes in abundance (Box 6.2) and further to ERF, in particular for aerosols and tropospheric ozone. Furthermore, there are uncertainties related to climate sensitivity, that is, the relation between ERF and change in GSAT. Uncertainties in the ERF are assessed in [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] and calibrated impulse response function also includes the assessed range (Box 7.1). There are also uncertainties related to the radiative efficacies of the different SLCFs and time scales for the response, in particular for regional emissions ( [[#Schwarber--2019|Schwarber et al., 2019]] ; [[#Yang--2019b|Yang et al., 2019b]] ) that cannot be accounted for with the simple models used here.

Historical emissions have been updated until 2019 (see Supplementary Material 7.SM.1.3.1) and used for ERF for calculating GSAT in Figure 6.22. The year 2019 has been chosen as the base year to be consistent with the attributed temperature changes since 1750 (Figure 7.8). The warming attributed to SLCFs (methane, ozone and aerosols) over the last decade (Figure 7.8) constitutes about 30% of the peak SLCF-driven warming in the most stringent scenarios (SSP1), in good agreement with [[#Shindell--2019|Shindell and Smith (2019)]] , and supported by the recent observed decline in AOD ( [[IPCC:Wg1:Chapter:Chapter-2#2.2.6|Section 2.2.6]] ).

From 2019 and until about 2040, SLCFs and HFCs will contribute to increase GSAT in the WGI core set of SSP scenarios, with a ''very likely'' range of 0.04°C–0.41°C relative to 2019. The warming is most pronounced in the strong mitigation scenarios (i.e., SSP1-1.9 and SSP1-2.6) due to rapid cuts in aerosols. In scenario SSP3-7.0, there is no reduction of aerosols until mid-century and it is the increases in methane and ozone that give a net warming in 2040. The warming is similar in magnitude to that in the SSP1-scenarios, in which the reduction in aerosols is the main driver. Contributions to warming from methane, ozone, aerosols and HFCs make SSP5-8.5 the scenario with the highest warming in 2040 and throughout the century.

After about 2040, it is ''likely'' that, across the scenarios, the net effect of the removal of aerosols is a further increase in GSAT. However, their contribution to the rate of change decreases towards the end of the century (from up to 0.2°C per decade before 2040 to about 0.03°C per decade after 2040). After 2040, the changes in methane, HFCs and tropospheric ozone become equally important as the changes in the aerosols for the GSAT trends. In the low-emissions scenarios (SSP1-1.9 and SSP1-2.6), the contribution to warming from the SLCFs peaks around 2040 with a ''very likely'' range of 0.04°C to 0.34°C. After the peak, the reduced warming from reductions in methane and ozone dominates, giving a best total estimate warming induced by SLCF and HFC changes of 0.12°C and 0.14°C respectively, in 2100, with a ''very likely'' range of –0.07°C to +0.45°C (Figure 6.22). However, in the longer term towards the end of the century there are very significant differences between the scenarios. In SSP3-7.0 there is a near-linear warming due to SLCFs of 0.08°C per decade, while for SSP5-8.5 there is a more rapid early warming. In SSP3-7.0, the limited reductions in aerosols, but a steady increase in methane, HFCs and ozone lead to a nearly linear contribution to GSAT reaching a best estimate of 0.5°C in 2100. Contributions from methane and ozone decrease towards 2100 in SSP5-8.5, however the warming from HFCs still increase and the SSP5-8.5 has the largest SLCF and HFC warming in 2100 with a best estimate of 0.6°C. In the SSP2-4.5 scenario, a reduction in aerosols contributes to about 0.3°C warming in 2100, while contributions from ozone and methane in this scenario are small.

<div id="_idContainer062" class="_idGenObjectStyleOverride-1"></div>

[[File:fa6856a0df781584e847af369e681192 IPCC_AR6_WGI_Figure_6_22.png]]

'''Figure 6.22 |''' '''Time evolution of the effects of changes in short-lived climate forcers (SLCFs) and hydrofluorocarbons (HFCs) on global surface air temperature (GSAT) across the WGI core set of Shared Socio-Economic Pathways (SSPs)''' . Effects of net aerosols, methane, tropospheric ozone and hydrofluorocarbons (HFCs; with lifetimes &lt;50years), and the sum of these, relative to the year 2019 and to the year 1750. The GSAT changes are based on the assessed historic and future evolution of effective radiative forcing (ERF; [[IPCC:Wg1:Chapter:Chapter-7#7.3.5|Section 7.3.5]] ). The temperature responses to the ERFs are calculated with an impulse response function with an equilibrium climate sensitivity of 3.0°C for a doubling of atmospheric CO <sub>2</sub> (feedback parameter of –1.31 W m <sup>–2</sup> °C <sup>–1</sup> , see Cross-Chapter Box 7.1). The vertical bars to the right in each panel show the uncertainties (5–95% ranges) for the GSAT change between 2019 and 2100. Further details on data sources and processing are available in the chapter data table (Table 6.SM.3).

The simplified approach used to estimate the contributions to GSAT in Figure 6.22 has been supplemented with ESM simulations driven by the two versions of the SSP3-7.0-lowSLCF scenario (Section 6.7.1.1). Results from five CMIP6 ESMs with fully interactive atmospheric chemistry and aerosols for the high-methane scenario show ( [[#Allen--2020|Allen et al., 2020]] , 2021) that reductions in emissions of air pollutants would lead to an additional increase in GSAT by 2055 relative to 2015 compared to the standard SSP3-7.0 scenario, with a best estimate of 0.23°C ± 0.05°C, and a corresponding increase in global mean precipitation of 1.3 ± 0.17% (note that uncertainties from the work of Allen et al. here and elsewhere are reported as twice standard deviation). Including methane mitigation (SSP3-7.0-lowSLCF-lowCH <sub>4</sub> ) would lead to a small increase in global precipitation (0.7 ± 0.1%) by mid-century despite a decrease in GSAT (Section 6.7.3), which is related to the higher sensitivity of precipitation to sulphate aerosols than greenhouse gases ( [[IPCC:Wg1:Chapter:Chapter-8#8.2.1|Section 8.2.1]] ; [[#Allen--2021|Allen et al., 2021]] ).

Regionally inhomogeneous ERFs can lead to regionally dependent responses (Section 6.4.3). Mitigation of non-methane SLCFs over the period 2015–2055 (SSP3-7.0-lowSLCF-highCH <sub>4</sub> versus SSP3-7.0) will lead to positive ERF over land regions (Allen et al. , 2020) . There are large regional differences in the ERF from no significant trend over northern Africa to about 0.5 W m <sup>–2</sup> decade <sup>–1</sup> for Southern Asia. The differences are mainly driven by differences in the reductions of sulphate aerosols. There is no strong correspondence between regional warming and the ERF trends. As expected, the sensitivity (temperature change per unit ERF) increases towards higher latitudes due to climate feedbacks and teleconnections. Regionally, the warming rates are higher over continental regions, with the highest increase in temperatures for Central and northern Asia and the Arctic in 2055 relative to 2015. The models agree on an increasing global mean trend in precipitation due to SLCFs, however precipitation trends over land are more uncertain ( [[#Allen--2020|Allen et al., 2020]] ), in agreement with the relationship between aerosol and precipitation trends assessed in Chapter 8.

ESM estimates of future concentrations of various SLCFs vary considerably even when using the same future emissions scenarios, which is related to sources of model structural uncertainty in the several physical, chemical and natural emissions model parametrizations. The general uncertainties in understanding and representing chemical and physical processes governing the life cycle of SLCFs (Box 6.1) necessarily also applies to simulations of future concentrations and ERF. In addition, how the models are able to simulate climate changes (i.e., circulation and precipitation) that affect the dispersion and removal of SLCFs constitute a structural uncertainty in the models. Also SLCF-related climate feedbacks (e.g., NO <sub>x</sub> from lightning or BVOCs from vegetation; Section 6.4.5) add to the uncertainty.

In the near term (2035–2040), it is ''unlikely'' that differences in the socio-economic developments and emissions controls induced by policies (as embedded in the SSPs) can lead to a discernible difference in the net effect of changes of SLCFs on GSAT. This is because the inter-model spread in the estimated net effect of SLCFs on GSAT is as large as the difference between the scenarios due to the compensating effects of change in emissions leading to cooling and warming. However, in the longer term, there is ''high confidence'' that the net warming induced by changes in SLCFs will be lower in the scenario considering strong climate change mitigation (SSP1-1.9 and SSP1-2.6 that include reductions in methane emissions) than in the high CO <sub>2</sub> emissions scenarios (SSP3-7.0 and SSP5-8.5).

<div id="6.7.2.2" class="h3-container"></div>

<span id="effect-of-regional-emissions-of-slcfs-on-gsat"></span>