Emissions may affect climate indirectly through chemical interactions in the atmosphere, but quantifications of such effects are difficult and uncertain due to incomplete knowledge and inadequate methods. A preliminary assessment of the climatic impact of changes in tropospheric 03 and CH4 in response to various emissions is given. For a 10% increase in the CH4 emissions the relative increase in concentration has been estimated to be 37% larger. The radiative forcing from enhanced levels of tropospheric O3 is estimated to 37% of the forcing from changes in CH4. Inclusion of indirect effects approximately doubles the climatic impact of CH4 emissions. Emissions of NOx increase tropospheric 03, while the levels of CH4 are reduced. For emissions of NOx from aircraft, the positive effects via 03 changes are significantly larger than the negative through changes in CH4. For NOx emitted from surface sources, the effects through changes in 03 and CH4 are estimated to be of similar magnitude and large uncertainty is connected to the sign of the net effect. Emissions of CO have positive indirect effects on climate through enhanced levels of tropospheric 03 and increased lifetime of CH4. These results form the basis for estimates of global warming potentials for sustained step increases in emissions.
[1] Perturbations in H 2 O caused by aircraft in the year 2015 are calculated with a chemical transport model (CTM) and used as input for radiative forcing calculations. The focus is on a hypothetical fleet of cryoplanes, i.e., liquid-hydrogen-powered aircraft. For comparison, the effects of subsonic and supersonic kerosene aircraft are assessed. The CTM applies the accurate second-order moment scheme for advective transport and is run in T42 resolution (2.8°Â 2.8°) with 40 layers between the surface and 10 hPa. Aircraft emissions are taken from NASA inventories for the year 2015. In the cryoplane experiments the projected subsonic kerosene fleet is replaced completely by cryoplanes, which emit 2.55 times as much H 2 O. Longwave and shortwave components of radiative forcing due to the modeled H 2 O increases are calculated for different seasons. In northern midlatitudes near the tropopause, the fleet of cryoplanes is calculated to increase zonal-mean H 2 O by more than 250 ppbv on an annual average. The resulting radiative forcing at the tropopause strongly depends on season, ranging from 0.0027 W/m 2 in October to 0.0135 W/m 2 in April on a global average. Subsonic kerosene aircraft are found to have a rather small impact on H 2 O levels and lead to an annually averaged global-mean radiative forcing of 0.0026 W/m 2 . Supersonic kerosene-powered aircraft have a more pronounced impact on H 2 O concentrations than subsonic cryoplanes and cause a radiative forcing of nearly 0.05 W/m 2 . Several sensitivity studies are performed for cryoplanes, dealing with cruising altitude, tropopause height, tropospheric lifetime, and stratospheric sinks of aircraft-emitted water vapor.
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