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

==== 6.6.2.4 GSAT Response to Emissions Pulse of Current Emissions ====

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Figure 6.16 presents the GSAT response to an idealized pulse of year-2014 emissions of individual SLCF and LLGHG. The GSAT response is calculated for 11 sectors and 10 regions accounting for best available knowledge and geographical dependence of the forcing efficacy of different SLCFs ( [[#Lund--2020|Lund et al., 2020]] ). Two time horizons are shown (of 10 and 100 years) to represent near- and long-term effects (and a 20-year horizon is presented in Supplementary Material Figure 6.SM.3). Other time-horizon choices may affect the relative importance, and even sign in the case of the NO <sub>x</sub> effect, of the temperature response from some of the SLCFs, or be more relevant for certain applications. GSAT response is calculated using the concept of AGTP ( [[IPCC:Wg1:Chapter:Chapter-7#7.6.2.2|Section 7.6.2.2]] ). Further details of the AGTP emulator applied in Figure 6.16 are provided in [[#Lund--2020|Lund et al. (2020)]] and Supplementary Material 6.SM.4 ( [[IPCC:Wg1:Chapter:Chapter-7#7.6.1.2|Section 7.6.1.2]] , Cross-Chapter Box 7.1 and Supplementary Material 7.SM.7.2). As discussed by [[#Lund--2020|Lund et al. (2020)]] , the AGTP framework is primarily designed to study the relative importance of individual emissions and sources, but the absolute magnitude of temperature responses should be interpreted with care due to the linearity of the AGTP, which does not necessarily capture all the non-linear effects of SLCFs emissions on temperature.

Differences in the mix of emissions result in net effects on GSAT that vary substantially, in both magnitude and sign, between sectors and regions. SLCFs contribute substantially to the GSAT effects of sectors on short time horizons (10–20 years) but CO <sub>2</sub> dominates on longer time horizons (Figure 6.16). As the effect of the SLCFs decays rapidly over the first few decades after emission, the net long-term temperature effect is predominantly determined by CO <sub>2</sub> . N <sub>2</sub> O adds a small contribution to the long-term effect of agriculture. CO <sub>2</sub> emissions cause an important contribution to near-term warming that is not always fully acknowledged in discussions of LLGHGs and SLCFs ( [[#Lund--2020|Lund et al., 2020]] ).

The global sectoral ranking for near- and long-term global temperature effects is similar to the AR5 assessment despite regional reductions in aerosol precursor emissions between 2008 and 2014. This report has applied updated climate policy metrics for SLCFs and treatment of aerosol–cloud interactions for SO <sub>2</sub> , BC and OC ( [[#Lund--2020|Lund et al., 2020]] ). By far the largest 10-year GSAT effects are from the energy production (fossil fuel mining and distribution), agriculture and waste management sectors ( ''high confidence'' ). Methane is the dominant contributor in the energy production, agriculture and waste management sectors. On the 10-year time horizon, other net warming sectors are residential fossil fuel and energy combustion (dominated by CO <sub>2</sub> ) and aviation and residential biofuel (dominated by SLCFs and cloud) ( ''medium confidence'' ). The total residential and commercial sector, including biofuel and fossil fuels, is the fourth largest contributor to warming globally on short time horizons of 10–20 years. The energy combustion sector has considerable cooling from high emissions of SO <sub>2</sub> that result in a relatively small net GSAT temperature effect on short time horizons, despite the high CO <sub>2</sub> emissions from this activity. On the 10-year time horizon, global emissions from industry and shipping cause a net cooling effect despite a considerable warming from CO <sub>2</sub> emissions. On the 100-year time horizon, the net effects of agriculture and waste management are small, while energy combustion is the largest individual contributor to warming due to its high CO <sub>2</sub> emissions. The second largest driver of long-term temperature change is industry, demonstrating the importance of non-CO <sub>2</sub> emissions for shaping relative weight over different time frames. Transport contributes a small net warming on the 10-year time horizon that increases by a factor of three on the 100-year time horizon. In contrast, the aviation sector contribution to warming shrinks by about a factor of three between the 10- and 100-year time horizons. Results for the 20-year time horizon are provided in the Supplementary Material 6.SM.4. Compared to the 10-year time horizon, there are some changes in ranking, especially of sectors and regions with a strong SO <sub>2</sub> contribution, which decays substantially between 10 and 20 years. Aviation is the sector with the most distinct difference between 10- and 20-year time horizons, such that the net GSAT effect after 20 years becomes small but negative. This is due to a switch in sign for the NO <sub>x</sub> AGTP for this sector and the stronger effect of short-lived ozone response over these two short-term horizons in the case of aviation compared with other sectors.

In terms of source regions, the largest contributions to net short-term warming are caused by emissions in Eastern Asia, Latin America and North America, followed by Africa, Eastern Europe, West-Central Asia and South East Asia ( ''medium confidence'' ). However, the relative contributions from individual species vary. In Eastern Asia, North America, Europe and Southern Asia, the effect of current emissions of cooling and warming SLCFs approximately balance in the near term and these regions cause comparable net warming effects on 10- and 100-year time horizons (Figure 6.16). In Latin America, Africa, and South East Asia and Developing Pacific, methane and BC emissions are currently high while emissions of CO <sub>2</sub> and cooling aerosols are low compared to other regions, resulting in a net warming effect after 10 years that is substantially higher than that of CO <sub>2</sub> alone.

Overall, the global sectors that contribute the largest warming on short time scales are the methane-dominated sources, that is energy production (fossil fuel mining and distribution), and agriculture and waste management ( ''high confidence'' ). On short time scales, other net warming sectors are residential fossil fuel and energy combustion (dominated by CO <sub>2</sub> ), and aviation and residential biofuel (dominated by SLCFs) ( ''medium confidence'' ). On short time scales, global emissions from industry and shipping cause a net cooling effect despite a considerable warming from CO <sub>2</sub> emissions ( ''high confidence'' ). On longer time horizons, the sectors that contribute the largest warming are energy combustion and industry due to the large CO <sub>2</sub> emissions ( ''high confidence'' ).

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