Aircraft emit nitrogen oxides (NOx) that indirectly warm and cool the climate by increasing tropospheric ozone and decreasing methane, two major greenhouse gases. These opposing effects on climate (quantified as radiative forcing, RF) mostly cancel. Through a survey of prior modeling studies we find that the net steady-state RF from aviation NOx is +4.5 ± 4.5 mW/m2. The 100-year global warming potential (GWP100y) from aviation NOx is thus 52 ± 52. Despite the large relative uncertainty in NOx effects, the climate warming due to aviation CO2 emissions is about 14 times greater, over a 100 year timespan. This work diagnoses the sources of uncertainty in RF with two complementary methods. We find strong correlations among the O3 and CH4 responses to aviation NOx across widely differing models, which is likely caused by differing NOx abundances in each model. Measurements of NOx and other reactive species in the free troposphere could reduce the uncertainty in the climate impact from aviation NOx and other emission sources.
Steady-state radiative forcing (RF) from O3 and CH4 caused by aviation NOx emissions. RF components are strongly correlated across models, indicating an underlying factor that determines the net climate RF. The UCI CTM reproduces this variability through changes to background NOx.