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

===== 6.6.2.3.1 Aviation =====

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Aviation is associated with a range of SLCFs, in particular emissions of NO <sub>x</sub> and aerosol particles, alongside emissions of water vapour and CO <sub>2.</sub> The largest SLCF effects are those from the formation of persistent condensation trails (contrails) and NO <sub>x</sub> emissions. Persistent contrails are ice-crystal clouds, formed around aircraft soot particles (and water vapour from the engine), injected in ambient cold and ice-supersaturated atmosphere, which can spread and form contrail cirrus clouds. The ‘net NO <sub>x</sub> ’ effect arises from the formation of tropospheric ozone, counterbalanced by the destruction of ambient methane and associated cooling effects of reductions in stratospheric water vapour and background ozone. The AR5 assessed the radiative forcing from persistent linear contrails to be +0.01 [+0.005 to +0.03] W m <sup>–2</sup> for the year 2011, with ''medium confidence'' ( [[#Boucher--2013|Boucher et al., 2013]] ). The combined linear contrail and their subsequent evolution to contrail cirrus radiative forcing from aviation was assessed to be +0.05 [+0.02 to +0.15] W m <sup>–2</sup> , with ''low confidence'' . An additional forcing of +0.003 W m <sup>–2</sup> due to emissions of water vapour in the stratosphere by aviation was also reported ( [[#Boucher--2013|Boucher et al., 2013]] ). The aviation sector was also estimated to lead to a net surface warming at 20- and 100-year horizons following a one-year pulse emission. This net temperature response was determined by similar contributions from contrails, contrail cirrus and CO <sub>2</sub> over a 20-year time horizon, and dominated by CO <sub>2</sub> in a 100-year perspective (Figure 8.34 in AR5, [[#Myhre--2013|Myhre et al., 2013]] ).

Our assessment is built upon [[#Lee--2021|Lee et al. (2021)]] . Their study consists of an updated, comprehensive assessment of aviation climate forcing in terms of RF and ERF based on a large number of studies and the most recent air-traffic and fuel-use datasets available (for 2018), new calculations and the normalization of values from published modelling studies, and combining the resulting best estimates via a Monte-Carlo analysis. [[#Lee--2021|Lee et al. (2021)]] report a net aviation ERF for year-2018 emissions of +0.101 [0.055–0.145] W m <sup>–2</sup> with major contributions from contrail cirrus (0.057 W m <sup>–2</sup> ), CO <sub>2</sub> (0.034 W m <sup>–2</sup> ) and NO <sub>x</sub> (0.017 W m <sup>–2</sup> ). Contrails and aviation-induced cirrus yield the largest individual positive ERF followed by CO <sub>2</sub> and NO <sub>x</sub> emissions (Lee et al. 2021). The ''confidence'' level in ERF due to contrails and aviation-induced cirrus is assessed to be ''low'' in [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] ( [[IPCC:Wg1:Chapter:Chapter-7#7.3.4.2|Section 7.3.4.2]] ) due to potential missing processes. The formation and emission of sulphate aerosols yield a negative (cooling) term. SLCF forcing terms contribute about eight times more than CO <sub>2</sub> to the uncertainty in the aviation net ERF in 2018 ( [[#Lee--2021|Lee et al., 2021]] ). The largest uncertainty in assessing aviation climate effects is on the interactions of BC and sulphate aerosols on cirrus and mixed-phase clouds, for which no best estimates of the ERFs were provided ( [[#Lee--2021|Lee et al., 2021]] ).

One of the most significant changes between AR5 and AR6 in terms of aviation SLCFs is the explicit calculation of a contrail cirrus ERF found to be 35% of the corresponding RF (Bickel et al. , 2020), confirming the studies indicating smaller efficacy of linear contrails ( [[#Ponater--2005|Ponater et al., 2005]] ; [[#Rap--2010|Rap et al., 2010]] ). The net-NO <sub>x</sub> term is generally agreed to be a positive RF in the present day, although attribution in a non-linear chemical system is problematic ( [[#Grewe--2019|Grewe et al., 2019]] ), but [[#Skowron--2021|Skowron et al. (2021)]] point out that the sign of net NO <sub>x</sub> term is dependent on background conditions and could be negative under certain future scenarios.

The best estimate ERFs from aviation ( [[#Lee--2021|Lee et al., 2021]] ) have been used to calculate aviation-specific Absolute Global Temperature change Potential (AGTP) using the method described in [[#Lund--2020|Lund et al. (2020)]] and subsequently compute the effect of a one-year pulse of aviation emissions on global mean surface temperature on a 10- and 100-year time horizon (Section 6.6.2.3.4 and Figure 6.16). The effect of contrail-cirrus is most important for the estimated net-GSAT response after the first decade, followed by similar warming contributions from NO <sub>x</sub> and CO <sub>2</sub> emissions. At a 20-year time horizon, the net contribution from aviation to GSAT has switched from a positive to a small negative effect (see Supplementary Material 6.SM.4). This is due to the combination of rapidly decaying contrail-cirrus warming and the complex time variation of the net temperature response to NO <sub>x</sub> emissions, which changes sign between 10 and 20 years due to the balance between the positive short-lived ozone forcing and negative forcing from changes in methane and methane-induced changes in ozone and stratospheric water vapour. The net GSAT response to aviation emissions has previously been estimated to be positive on a 20-year time horizon (AR5, Chapter 8; [[#Lund--2017|Lund et al., 2017]] ). This difference in net GSAT after 20 years in AR5 compared to AR6, results primarily from a shorter time scale of the climate response in the underlying AGTP calculations in [[#Lund--2020|Lund et al. (2020)]] , which means the initial, strong impacts of the most short-lived SLCFs, including the warming by contrail-cirrus decay faster, in turn giving the net NO <sub>x</sub> effect a relatively higher importance after 20 years. On longer time horizons, the net GSAT response switches back to positive, as CO <sub>2</sub> becomes the dominating warming contribution.

In summary, the net aviation ERF is assessed to be +0.1 W m <sup>–2</sup> (±0.045) for the year 2018 ( ''low confidence)'' . This confidence level is largely a result of the fact that the SLCF-related terms, which account for more than half (66%) of the net aviation ERF, are the most uncertain terms. The climate response to SLCF-related aviation terms exhibits substantial spatio-temporal heterogeneity in characteristics ( ''high confidence'' ). Overall, cirrus and contrail cirrus warming, as well as NO <sub>x</sub> -induced ozone increase, induce strong but short-lived warming contributions to the GSAT response 10 years after a one-year pulse of present-day aviation emissions ( ''medium confidence'' ), while CO <sub>2</sub> both gives a warming effect in the near term and dominates the long-term warming impact ( ''high confidence'' ).

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