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

=== Effect of SLCFs on Climate and Biogeochemical Cycles ===

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'''Over the historical period, changes in aerosols and their effective radiative forcing (ERF) have primarily contributed to a surface cooling, partly masking the greenhouse gas-driven warming''' ( ''high confidence'' ''').''' Radiative forcings induced by aerosol changes lead to both local and remote temperature responses ( ''high confidence'' ). The temperature response preserves the south to north gradient of the aerosol ERF – hemispherical asymmetry – but is more uniform with latitude and is strongly amplified towards the Arctic ( ''medium confidence'' ). {6.4.1, 6.4.3}

'''Since the mid-1970s, trends in aerosols and their precursor emissions have led to a shift from an increase to a decrease of the magnitude of the negative globally averaged net aerosol ERF''' ( ''high confidence'' ''').''' However, the timing of this shift varies by continental-scale region and has not occured for some finer regional scales. The spatial and temporal distribution of the net aerosol ERF from 1850 to 2014 is highly heterogeneous, with stronger magnitudes in the Northern Hemisphere ( ''high confidence'' ). {6.4.1}

'''For forcers with short lifetimes (e.g., months) and not considering chemical adjustments, the response in surface temperature occurs strongly as soon as a sustained change in emissions is implemented, and that response continues to grow for a few years, primarily due to thermal inertia in the climate system''' ( ''high confidence'' ''')''' . Near its maximum, the response slows down but will then take centuries to reach equilibrium ( ''high confidence'' ). For SLCFs with longer lifetimes (e.g., a decade), a delay equivalent to their lifetimes is appended to the delay due to thermal inertia ( ''high confidence'' ). {6.6.1}

'''Over the 1750–2019 period, changes in SLCF emissions, especially of methane, NO''' <sub>x</sub> '''and SO''' <sub>2</sub> , '''have substantial effects on effective radiative forcing (ERF)''' ( ''high confidence'' ''').''' The net global emissions-based ERF of NO <sub>x</sub> is negative and that of non-methane volatile organic compounds (NMVOCs) is positive, in agreement with the AR5 Assessment ( ''high confidence'' ). For methane, the emissions-based ERF is twice as high as the abundance-based ERF ( ''high confidence'' ) attributed to chemical adjustment mainly via ozone production. SO <sub>2</sub> emissions changes make the dominant contribution to the ERF from aerosol–cloud interactions ( ''high'' ''confidence'' ). Over the 1750–2019 period, the contributions from the emitted compounds to changes in global surface air temperature (GSAT) broadly match their contributions to the ERF ( ''high confidence'' ). Since a peak in emissions-induced SO <sub>2</sub> eRF has already occurred recently and since there is a delay in the full GSAT response, changes in SO <sub>2</sub> emissions have a slightly larger contribution to GSAT change than CO <sub>2</sub> emissions, relative to their respective contributions to ERF. {6.4.2, 6.6.1 and 7.3.5}

'''Reactive nitrogen, ozone and aerosols affect terrestrial vegetation and the carbon cycle through deposition and effects on large-scale radiation''' ( ''high confidence'' ''').''' However, the magnitude of these effects on the land carbon sink, ecosystem productivity and hence their indirect CO <sub>2</sub> forcing remain uncertain due to the difficulty in disentangling the complex interactions between the individual effects. As such, these effects are assessed to be of second order in comparison to the direct CO <sub>2</sub> forcing ( ''high confidence'' ), but effects of ozone on terrestrial vegetation could add a substantial (positive) forcing compared with the direct ozone forcing ( ''low confidence'' ). {6.4.4}

'''Climate feedbacks induced from changes in emissions, abundances or lifetimes of SLCFs mediated by natural processes or atmospheric chemistry are assessed to have an overall cooling effect''' ( ''low confidence'' '''), that is, a total negative feedback parameter of –0.20 [–0.41 to +0.01] W m''' <sup>–2</sup> '''°C''' <sup>−1</sup> '''.''' These non-CO <sub>2</sub> biogeochemical feedbacks are estimated from ESMs, which have advanced since AR5 to include a consistent representation of biogeochemical cycles and atmospheric chemistry. However, process-level understanding of many chemical and biogeochemical feedbacks involving SLCFs, particularly natural emissions, is still emerging, resulting in ''low confidence'' in the magnitude and sign of most SLCF climate feedback parameters. {6.2.2, 6.4.5}

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