Estimates of indirect global warming potentials for CH4, CO and NOx

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 [O.sub.3] and C[H.sub.4] in response to various emissions is given. For a 10% increase in the C[H.sub.4] emissions the relative increase in concentration has been estimated to be 37% larger. The radiative forcing from enhanced levels of tropospheric [O.sub.3] is estimated to 37% of the forcing from changes in C[H.sub.4]. Inclusion of indirect effects approximately doubles the climatic impact of C[H.sub.4] emissions. Emissions of N[O.sub.x] increase tropospheric [O.sub.3], while the levels of C[H.sub.4] are reduced. For emissions of N[O.sub.x] from aircraft, the positive effects via [O.sub.3] changes are significantly larger than the negative through changes in C[H.sub.4]. For N[O.sub.x] emitted from surface sources, the effects through changes in [O.sub.3] and C[H.sub.4] 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 [O.sub.3] and increased lifetime of C[H.sub.4]. These results form the basis for estimates of global warming potentials for sustained step increases in emissions.