Nitrogen oxides emitted from aircraft engines alter the chemistry of the atmosphere, perturbing the greenhouse gases methane (CH4) and ozone (O-3). We quantify uncertainties in radiative forcing (RF) due to short-lived increases in O-3, long-lived decreases in CH4 and O-3, and their net effect, using the ensemble of published models and a factor decomposition of each forcing. The decomposition captures major features of the ensemble, and also shows which processes drive the total uncertainty in several climate metrics. Aviation-specific factors drive most of the uncertainty for the short-lived O-3 and long-lived CH4 RFs, but a nonaviation factor dominates for long-lived O-3. The model ensemble shows strong anticorrelation between the short-lived and long-lived RF perturbations (R-2 = 0.87). Uncertainty in the net RF is highly sensitive to this correlation. We reproduce the correlation and ensemble spread in one model, showing that processes controlling the background tropospheric abundance of nitrogen oxides are likely responsible for the modeling uncertainty in climate impacts from aviation.