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

=== Recent Evolution in Short-lived Climate Forcer (SLCF) Emissions and Abundances ===

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'''Over the last decade (2010–2019), strong shifts in the geographical distribution of emissions have led to changes in atmospheric abundances of highly variable SLCFs''' ( ''high confidence'' '''). Evidence from satellite and surface observations shows strong regional variations in trends of ozone (O''' <sub>3</sub> '''), aerosols and their precursors''' ( ''high confidence'' ''').''' In particular, tropospheric columns of nitrogen dioxide (NO <sub>2</sub> ) and sulphur dioxide (SO <sub>2</sub> ) continued to decline over North America and Europe ( ''high confidence'' ), and to increase over Southern Asia ( ''medium confidence'' ), but have declined over Eastern Asia ( ''high confidence'' ). Global carbon monoxide (CO) abundance has continued to decline ( ''high confidence'' ). The concentrations of hydrofluorocarbons (HFCs) are increasing ( ''high confidence'' ). Global carbonaceous aerosol budgets and trends remain poorly characterized due to limited observations, but sites representative of background conditions have reported multi-year declines in black carbon (BC) over several regions of the Northern Hemisphere. {6.2, 6.3, 2.2.4, 2.2.5, 2.2.6}

'''There is no significant trend in the global mean tropospheric concentration of hydroxyl (OH) radical – the main sink for many SLCFs, including methane (CH''' <sub>4</sub> ''') – from 1850 up to around 1980''' ( ''low confidence'' ''') but OH has remained stable or exhibited a positive trend since the 1980s''' ( ''medium confidence'' ''').''' Global OH cannot be measured directly and is inferred from Earth system and chemistry–climate models (ESMs, CCMs) constrained by emissions and from observationally constrained inversion methods. There is conflicting information from these methods for the 1980–2014 period. ESMs and CCMs concur on a positive trend since 1980 (about a 9% increase over 1980–2014) and there is ''medium confidence'' that this trend is mainly driven by increases in global anthropogenic (human-caused) nitrogen oxide (NO <sub>x</sub> ) emissions and decreases in anthropogenic CO emissions. The observation-constrained methods suggest either positive trends or the absence of trends based on ''limited evidence'' and ''medium agreement.'' Future changes in global OH, in response to SLCF emissions and climate change, will depend on the interplay between multiple offsetting drivers of OH. {6.3.6 and Cross-Chapter Box 5.1}

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