Fig. 4: Net NOx radiative forcing by emission rate, updated CH4 parameterisation.
Aviation net NOx radiative forcing (RF) (the sum of the short-term positive O3 RF perturbation and the negative RF terms caused by a reduction in CH4 lifetime, see Methods), by aviation NOx emission rate according to a range of background emission scenarios, utilising the updated simplified expression for the calculation of CH4 forcing of Etminan et al.23 Aviation net NOx RF systematically decreases with increasing NOx emissions from aviation, showing a variation according to the background surface emissions, with high mitigation (RCP 2.6) having a smaller aviation net NOx RF than, lower mitigation scenarios (RCP 4.5, RCP 8.5) for the same aviation NOx emission. The pattern of behaviour is in contrast to Fig. 1 because the updated CH4 forcing expression accounts for the short-wave forcing of CH4, increasing CH4 RF estimates by approximately 25%, which in the case of aviation net NOx impacts greatly increases the negative terms from the reduction in CH4 lifetime from aviation NOx, tipping the net NOx term from being positive to negative with increasing aviation NOx emissions. Overall uncertainties are indicated by the grey shading, which is one standard deviation (68% confidence interval) from the ensemble of 20 NOx studies presented in Supplementary Fig. 1.
