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 makes a significant contribution to anthropogenic climate forcing. The impacts arise from emissions of greenhouse gases, aerosols and nitrogen oxides, and from changes in cloudiness in the upper troposphere. An important but poorly understood component of this forcing is caused by 'contrail cirrus'-a type of cloud that consist of young line-shaped contrails and the older irregularly shaped contrails that arise from them. Here we use a global climate model that captures the whole life cycle of these man-made clouds to simulate their global coverage, as well as the changes in natural cloudiness that they induce. We show that the radiative forcing associated with contrail cirrus as a whole is about nine times larger than that from line-shaped contrails alone. We also find that contrail cirrus cause a significant decrease in natural cloudiness, which partly offsets their warming effect. Nevertheless, net radiative forcing due to contrail cirrus remains the largest single radiative-forcing component associated with aviation. Our findings regarding global radiative forcing by contrail cirrus will allow their effects to be included in studies assessing the impacts of aviation on climate and appropriate mitigation options. ARTICLES NATURE CLIMATE CHANGE

Contrail cirrus are a major component of the climate forcing due to air traffic. For a given contrail cirrus cover, ice water content and ice crystal shape, their impact on radiation is dependent on the number and size of ice crystals. Here we use a global climate model to study the impact of a reduction in initially formed ice crystal numbers, as may be caused by reduced soot number emissions. We find that for reduced initial ice crystal numbers the ice water content is decreased and ice crystal sizes increased, leading to a reduction in contrail cirrus optical depth and doubling the fraction of contrail cirrus that cannot be detected by satellite remote sensing. Contrail cirrus lifetimes and coverage are strongly reduced leading to significant reductions in contrail cirrus radiative forcing. The global climate impact of contrail cirrus is nonlinearly dependent on the reduction in initial ice crystal numbers. A reduction in the initial ice crystal number of 80% leads to a decrease in contrail cirrus radiative forcing by 50%, whereas a twofold reduction leads to a decrease in radiative forcing by approximately 20%. Only a few contrail cirrus outbreaks explain a large percentage of the climate impact. The contrail cirrus climate impact can be effectively mitigated by reducing initial ice crystal concentrations in such outbreak situations. Our results are important for assessments dealing with mitigating the climate impact of aviation and discussions about the use of alternative fuels or lean combustion in aviation.

More than one hundred days were simulated over very large domains with fine (0.156 km to 2.5 km) grid spacing for realistic conditions to test the hypothesis that storm (kilometer) and large-eddy (hectometer) resolving simulations would provide an improved representation of clouds and precipitation in atmospheric simulations. At scales that resolve convective storms (storm-resolving for short) scales, the vertical velocity variance becomes resolved and a better physical basis is achieved for representing clouds and precipitation. Similar to past studies we find an improved representation of precipitation at kilometer scales, as compared to models with parameterised convection. The main precipitation features (location, diurnal cycle and spatial propagation) are well captured already at kilometer scales, and refining resolution to hectometer scales does not substantially change the simulations in these respects. It does, however, lead to a reduction in the precipitation on the timescales considered-most notably over the Tropical ocean. Changes in the distribution of precipitation, with less frequent extremes are also found in simulations incorporating hecto-meter scales. Hectometer scales appear more important for the representation of clouds, and make it possible to capture many important aspects of the cloud field, from the vertical distribution of cloud cover, to the distribution of cloud sizes, to the diel (daily) cycle. Qualitative improvements, particularly in the ability to differentiate cumulus from stratiform clouds, are seen when reducing the grid spacing from kilometer to hectometer scales. At the hectometer scale new challenges arise, but the similarity of observed and simulated scales, and the more direct 1 connection between the circulation and the unconstrained degrees of freedom make these challenges less daunting. This quality, combined with an already improved simulation as compared to more parameterised models, underpins our conviction that the use and further development of storm-resolving models offers exciting opportunities for advancing understanding of climate and climate change.

[1] Aviation induces changes in global cirrus cloudiness by producing contrails. In the past, line shaped contrail coverage has been parameterized relying on the scaling of contrail formation frequency to observed values. Coverage due to irregularly shaped contrail cirrus, that develop from line shaped contrails, could not be estimated with this method. We introduce a process-based parameterization of contrail cirrus in a global climate model that does not rely on scaling and that is not restricted to line shaped contrails. A new prognostic cloud class, contrail cirrus, is introduced that is allowed to develop in the parameterized, fractional ice supersaturated area. Initial dimensions of the contrails and a parameter controlling their spreading in a sheared flow are constrained by observational data. In an idealized experiment contrail cirrus coverage is found to be dominated by a major contrail outbreak and scales with supersaturation rather than contrail formation frequency. The global distribution of young contrail coverage is smoothed out due to transport but overall values are similar compared to older estimates. Interannual variability of young contrail coverage can be as large as the mean coverage. The sensitivity of the model simulations to physical model parameters and to parameters concerning the comparison with observational data is studied. The associated uncertainty of global line shaped contrail coverage can be as high as 60% of the reference estimate (0.05%). The simulated coverage due to young contrails agrees reasonably well with most satellite observations of regional line shaped contrail coverage considering the sensitivity to the above parameters and the interannual variability.Citation: Burkhardt, U., and B. Kärcher (2009), Process-based simulation of contrail cirrus in a global climate model, J. Geophys.