Editing IPCC:AR6/WGIII/Chapter-10 (section)

=== 10.5.2 Short-lived Climate Forcers and Aviation ===

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Aviation’s net warming effect results from its historical and current emissions of CO 2 , and non-CO 2 emissions of water vapour, soot, sulphur dioxide (from sulphur in the fuel), and nitrogen oxides (NO x , = NO + NO 2 ) ( [[#IPCC--1999|IPCC 1999]] ; [[#Lee--2021|Lee et al. 2021]] ; [[#Szopa--2021|Szopa et al. 2021]] ). Although the effective radiative forcing (ERF) of CO 2 from historic aviation emissions is not currently the largest forcing term, it is difficult to address because of the sector’s current dependency on fossil-based hydrocarbon fuels and the longevity of CO 2 . A residual of emissions of CO 2 today will still have a warming effect in many thousands of years ( [[#Archer--2009|Archer et al. 2009]] ; [[#Canadell--2021|Canadell et al. 2021]] ) whereas water vapour, soot, and NO x emissions will have long ceased to contribute to warming after some decades. As a result, CO 2 mitigation of aviation to net zero levels, as required in 1.5°C scenarios, requires fundamental shifts in technology, fuel types, or changes of behaviour or demand.

The non-CO 2 effects of aviation on climate fall into the category of short-lived climate forcers (SLCFs). Emissions of NO x currently result in net positive warming from the formation of short-term ozone (warming) and the destruction of ambient methane (cooling). If the conditions are suitable, emissions of soot and water vapour can trigger the formation of contrails ( [[#Kärcher--2018|Kärcher 2018]] ), which can spread to form extensive contrail-cirrus cloud coverage. Such cloud coverage is estimated to have a combined ERF that is about 57% of the current net ERF of global aviation ( [[#Lee--2021|Lee et al. 2021]] ), although a comparison of cirrus cloud observations under pre- and post-COVID-19 pandemic conditions suggest that this forcing could be smaller ( [[#Digby--2021|Digby et al. 2021]] ). Additional effects from aviation from aerosol-cloud interactions on high-level ice clouds through soot ( [[#Chen--2013|Chen and Gettelman 2013]] ; [[#Zhou--2014|Zhou and Penner 2014]] ; [[#Penner--2018|Penner et al. 2018]] ), and lower-level warm clouds through sulphur ( [[#Righi--2013|Righi et al. 2013]] ; [[#Kapadia--2016|Kapadia et al. 2016]] ) are highly uncertain, with no best estimates available ( [[#Lee--2021|Lee et al. 2021]] ). In total, the net ERF from aviation’s non-CO 2 SLCFs is estimated to be approximately 66% of aviation’s current total forcing. It is important to note that the fraction of non-CO 2 forcing to total forcing is not a fixed quantity and is dependent on the recent history of growth (or otherwise) of CO 2 emissions ( [[#Klöwer--2021|Klöwer et al. 2021]] ). The non-CO 2 effects from aviation are the subject of discussion for mitigation options ( [[#Arrowsmith--2020|Arrowsmith et al. 2020]] ). However, the issues are complex, potentially involving technological and operational trade-offs with CO 2 .

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