Global aviation operations contribute to anthropogenic climate change via a complex set of processes that lead to a net surface warming. Of importance are aviation emissions of carbon dioxide (CO 2 ), nitrogen oxides (NO x ), water vapor, soot and sulfate aerosols, and increased cloudiness due to contrail formation. Aviation grew strongly over the past decades (1960–2018) in terms of activity, with revenue passenger kilometers increasing from 109 to 8269 billion km yr −1 , and in terms of climate change impacts, with CO 2 emissions increasing by a factor of 6.8–1034 Tg CO 2 yr −1 . Over the period 2013–2018, the growth rates in both terms show a marked increase. Here, we present a new comprehensive and quantitative approach for evaluating aviation climate forcing terms. Both radiative forcing (RF) and effective radiative forcing (ERF) terms and their sums are calculated for the years 2000–2018. Contrail cirrus, consisting of linear contrails and the cirrus cloudiness arising from them, yields the largest positive net (warming) ERF term followed by CO 2 and NO x emissions. The formation and emission of sulfate aerosol yields a negative (cooling) term. The mean contrail cirrus ERF/RF ratio of 0.42 indicates that contrail cirrus is less effective in surface warming than other terms. For 2018 the net aviation ERF is +100.9 mW (mW) m −2 (5–95% likelihood range of (55, 145)) with major contributions from contrail cirrus (57.4 mW m −2 ), CO 2 (34.3 mW m −2 ), and NO x (17.5 mW m −2 ). Non-CO 2 terms sum to yield a net positive (warming) ERF that accounts for more than half (66%) of the aviation net ERF in 2018. Using normalization to aviation fuel use, the contribution of global aviation in 2011 was calculated to be 3.5 (4.0, 3.4) % of the net anthropogenic ERF of 2290 (1130, 3330) mW m −2 . Uncertainty distributions (5%, 95%) show that non-CO 2 forcing terms contribute about 8 times more than CO 2 to the uncertainty in the aviation net ERF in 2018. The best estimates of the ERFs from aviation aerosol-cloud interactions for soot and sulfate remain undetermined. CO 2 -warming-equivalent emissions based on global warming potentials (GWP* method) indicate that aviation emissions are currently warming the climate at approximately three times the rate of that associated with aviation CO 2 emissions alone. CO 2 and NO x aviation emissions and cloud effects remain a continued focus of anthropogenic climate change research and policy discussions.

Aviation emits pollutants that affect the climate, including CO and NO , NO indirectly so, through the formation of tropospheric ozone and reduction of ambient methane. To improve the fuel performance of engines, combustor temperatures and pressures often increase, increasing NO emissions. Conversely, combustor modifications to reduce NO may increase CO. Hence, a technology trade-off exists, which also translates to a trade-off between short-lived climate forcers and a long-lived greenhouse gas, CO. Moreover, the NO -O-CH system responds in a nonlinear manner, according to both aviation emissions and background NO . A simple climate model was modified to incorporate nonlinearities parametrized from a complex chemistry model. Case studies showed that for a scenario of a 20% reduction in NO emissions the consequential CO penalty of 2% actually increased the total radiative forcing (RF). For a 2% fuel penalty, NO emissions needed to be reduced by>43% to realize an overall benefit. Conversely, to ensure that the fuel penalty for a 20% NO emission reduction did not increase overall forcing, a 0.5% increase in CO was found to be the "break even" point. The time scales of the climate effects of NO and CO are quite different, necessitating careful analysis of proposed emissions trade-offs.

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