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

==== 6.7.2.2 Effect of Regional Emissions of SLCFs on GSAT ====

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For SLCFs with lifetimes shorter than typical mixing times in the atmosphere (days to weeks), the effects on secondary forcing agents (e.g., tropospheric ozone, sulphate and nitrate aerosols) depend on where and when the emissions occur due to non-linear chemical and physical processes. Also, the ERF following a change in concentrations depends on the local conditions (Sections 6.2, 6.3 and 6.4).

While the emulators used for GSAT projections shown in Figure 6.22 do not take the regional perspective into account, the set of simulations performed within the Hemispheric Transport of Air Pollutants Phase 2 (HTAP2) project ( [[#Galmarini--2017|Galmarini et al., 2017]] ) allows for this perspective. The results from the chemistry–transport model OsloCTM3 taking part in the HTAP2 have been used by [[#Lund--2020|Lund et al. (2020)]] to derive region-specific absolute global warming potentials (AGTPs; cf. [[#Aamaas--2016|Aamaas et al., 2016]] ) for each emitted SLCF and each HTAP2 region. With this set of AGTPs, [[#Lund--2020|Lund et al. (2020)]] estimate the transient response in GSAT to the regional anthropogenic emissions. There are important differences in the contributions to GSAT in 2040 and 2100 (relative to 2020) between the regions and scenarios, mainly due to the differences in the mixture of emitted SLCFs (Figure 6.23). There is overall good agreement between the total net contribution from all regions to GSAT and the estimate based on global ERF and the two-layer emulator (Figure 6.22).

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'''Figure 6.23 |''' '''Contribution from regional emissions of short-lived climate forcers (SLCFs) to changes in global surface air temperature (GSAT) in 2040 (upper row) and 2100 (lower row), relative to 2020 for four Shared Socio-economic Pathways (SSPs).''' Adapted from [[#Lund--2020|Lund et al. (2020)]] . NO x , CO, and NMVOC account for the impact through changes in ozone and methane, NO x additionally includes the impact through formation of nitrate aerosols. BC, SO <sub>2</sub> and OC accounts for the direct aerosol effect (aerosol–radiation interactions), as well as an estimate of the semi-direct effect for BC due to rapid adjustments and indirect effect (aerosol–cloud interactions) of sulphate aerosols. Regions are the same as shown in the map in Figure 6.7. Further details on data sources and processing are available in the chapter data table (Table 6.SM.3).

In the low- and medium-emissions scenarios (SSP1-2.6 and SSP2-4.5), the warming effects induced by changes in SLCFs on GSAT are dominated by emissions in North America, Europe and Eastern Asia (Figure 6.23). In SSP1-2.6 the emissions of all SLCFs in all regions decrease and the net effect of the changes in SLCFs from all of these three regions is an increase in GSAT of about 0.02°C (per region) in 2040 and about 0.04°C in 2100. For SSP2-4.5, emissions of most SLCFs continue to increase in Southern Asia (Figure 6.19), leading to a net cooling in the near term (–0.03°C in 2040), while in 2100, North America, Europe, Eastern and Southern Asia all contribute to a warming, most pronounced from Eastern Asia (0.05°C). In the SSP3-7.0 scenario, the net effect induced by changes in SLCFs in all regions is an enhanced warming towards the end of the century, driven predominantly by change in methane. Africa is the region contributing the most to predicted global warming due to SLCF changes in 2100 (0.24°C). In SSP5-8.5, methane emissions increase in North America, Europe and Africa, while there is a decrease in the Asian regions. For North America and Europe, the methane increase combined with a reduction in aerosol leads to highest net contribution to GSAT in this scenario (0.06°C and 0.04°C in 2100, respectively). The high growth in methane makes Africa the region with the largest contribution to future warming by SLCFs (0.18°C in 2100 versus 2020) in this scenario.

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