Time scales in atmospheric chemistry: Theory, GWPs for CH4 and CO, and runaway growth



Atmospheric CH4 perturbations, caused directly by CH4 emissions or indirectly by those of CO are enhanced by chemical feedbacks. They can be diagnosed in terms of the natural modes of atmospheric chemistry that are general solutions of the continuity equations. Each mode is a pattern in the global distribution of all chemical species, and each has a single time-constant that accurately describes its exponential decay about a given atmospheric state. This mathematical theory extends earlier work and is general for 2-D and 3-D chemistry-transport models. A formal proof relates the steady-state distribution and its lifetime to the integral of the true time-dependent response (properly included in the recent IPCC assessment). Changes in CO are also known to perturb CH4; however, the impact of CO emissions on climate has not been formally assessed in part because the short lifetime of CO (months) relative to that of CH4 (decade) was believed to limit the integrated impact. Using the IPCC model studies, this theory predicts that adding 5 CO molecules to today's atmosphere is equivalent to adding 1 CH4 molecule with the same decadal duration as direct CH4 addition. Extrapolating these results, CH4 sources would have to triple before runaway growth, wherein CH4 emissions exceed the oxidizing capacity of the troposphere.