Metrics for ozone and climate: Three-dimensional modeling studies of ozone depletion potentials and indirect global warming potentials

The concepts of ozone depletion potentials (ODPs) and global warming potentials (GWPs) have been extensively used in policy consideration and scientific studies of ozone and climate issues. Most recent candidate- replacement compounds have atmospheric lifetimes shorter than 1 year in order to limit their environmental effects. Especially for chemicals with extremely short lifetimes, on the order of several to tens of days, the stratospheric halogen loading and ozone loss from such gases strongly depend on the location of emissions. Using a state-of-the-art threedimensional global chemistry-transport model (CTM) of the troposphere and the stratosphere, we have calculated the potential effects of very short-lived substances (VSLS) such as n-propyl bromide (nPB), iodotrifluoromethane (CF₃I), and methyl iodine (CH₃I) on atmospheric ozone. The model-derived lifetimes and ODPs of these halogenated compounds for mid-latitude emissions and of CF ₃I for tropical emissions are presented in this chapter. On the other hand, ozone depletion due to emission of bromochlorofluorocarbons, or Halons, leads to cooling of the climate system in the opposite direction to direct warming contribution of the Halons as greenhouse gases. This cooling is a key indirect effect of Halons on radiative forcing or climate. Using atmospheric models, CTMs and a radiative transfer model, we have explicitly calculated the indirect GWPs of Halon-1211 and -1301 for a 100- year time horizon. The calculated indirect effects of Halon-1211 are much smaller than those published in earlier studies. Nevertheless, our new model-based assessment of the indirect GWPs of the two major Halons confirms the importance of indirect effects on climate.