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

=== 6.1.2 Treatment of SLCFs in Previous Assessments ===

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Although ozone, aerosols and their precursors have been considered in previous IPCC assessment reports, AR5 considered SLCFs as a specific category of climate-relevant compounds but referred to them as near-term climate forcers (NTCFs; Myhre et al. , 2013) . In AR5, the linkages between air quality and climate change were also considered in a more detailed and quantitative way than in previous reports ( [[#Kirtman--2013|Kirtman et al., 2013]] ; [[#Myhre--2013|Myhre et al., 2013]] ).

The AR5 WGI assessed radiative forcings for short-lived gases, aerosols, aerosol precursors and aerosol–cloud interactions as well as the evolution of confidence levels in the forcing mechanisms from SAR to AR5. Whereas the forcing mechanisms for ozone and aerosol–radiation interactions were estimated to be characterized with ''high confidence'' , the ones induced by aerosols through other processes remained of ''very low'' to ''low confidence'' . The AR5 also reported that forcing agents such as aerosols and ozone are highly heterogeneous spatially and temporally, and these patterns affect global and regional temperature responses as well as other aspects of climate response such as the hydrologic cycle (Myhre et al. , 2013) .

The AR5 WGI also evaluated the air quality-climate interaction through the projected trends of surface ozone and PM <sub>2.5</sub> . [[#Kirtman--2013|Kirtman et al. (2013)]] concluded with ''high confidence'' that the response of air quality to climate-driven changes is more uncertain than the response to emissions-driven changes, and also that locally higher surface temperatures in polluted regions will trigger regional feedbacks in chemistry and local emissions that will increase peak levels of ozone and PM <sub>2.5</sub> ( ''medium confidence'' ).

In the IPCC Special Report on Global Warming of 1.5°C (SR1.5; [[#Allen--2018a|Allen et al., 2018a]] ), [[#Rogelj--2018a|Rogelj et al. (2018a)]] state that the evolution of methane and SO <sub>2</sub> emissions strongly influences the chances of limiting warming to 1.5°C, and that, considering mitigation scenarios to limit warming to 1.5°C or 2°C, a weakening of aerosol cooling would add to future warming in the near term, but can be tempered by reductions in methane emissions ( ''high confidence'' ). In addition, as some SLCFs are co-emitted alongside CO <sub>2</sub> , especially in the energy and transport sectors, low CO <sub>2</sub> scenarios, relying on decline of fossil fuel use, can result in strong abatement of some cooling and warming SLCFs ( [[#Rogelj--2018a|Rogelj et al., 2018a]] ). On the other hand, specific reductions of the warming SLCFs (methane and BC) would, in the short term, contribute significantly to the efforts of limiting warming to 1.5°C. Reductions of BC and methane would have substantial co-benefits, improving air quality and therefore limiting effects on human health and agricultural yields. This would, in turn, enhance the institutional and socio-cultural feasibility of such actions in line with the United Nations’ Sustainable Development Goals (SDGs; [[#Coninck--2018|Coninck et al., 2018]] ).

Following SR1.5, the IPCC Special Report on Climate Change and Land (SRCCL; [[#IPCC--2019a|IPCC, 2019a]] ) took into consideration the emissions on land of three major SLCFs: mineral dust, carbonaceous aerosols (BC and OA) and biogenic volatile compounds (BVOCs) ( [[#Jia--2019|Jia et al., 2019]] ). The SRCCL concluded that: (i) there is no agreement about the direction of future changes in mineral dust emissions; (ii) fossil fuel and biomass burning, and secondary organic aerosols (SOA) from natural BVOC emissions are the main global sources of carbonaceous aerosols whose emissions are expected to increase in the near future due to possible increases in open biomass burning and increase in SOA from oxidation of BVOCs ( ''medium confidence'' ); and (iii) BVOCs are emitted in large amounts by forests and they are rapidly oxidized in the atmosphere to form less volatile compounds that can condense and form SOA, and in a warming planet, BVOC emissions are expected to increase but magnitude is unknown and will depend on future land-use change, in addition to climate ( ''limited evidence'' , ''medium agreement'' ).

Finally, the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC; [[#IPCC--2019b|IPCC, 2019b]] ) discussed the effects of BC deposition on snow and glaciers, concluding that there is ''high confidence'' that darkening of snow through the deposition of BC and other light-absorbing particles enhances snowmelt in the Arctic ( [[#Meredith--2019|Meredith et al., 2019]] ), but that there is ''limited evidence'' and ''low agreement'' that long-term changes in glacier mass of high mountain areas are linked to light-absorbing particles ( [[#Hock--2019|Hock et al., 2019]] ).

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