Attribution of historical ozone forcing to anthropogenic emissions.

Anthropogenic ozone radiative forcing is traditionally separately attributed to tropospheric and stratospheric changes assuming that these have distinct causes. Using the interactive composition-climate model GISS-E2-R we find that this assumption is not justified. Our simulations show that changes in emissions of tropospheric ozone precursors have substantial effects on ozone in both regions, as do anthropogenic halocarbon emissions. On the basis of our results, further simulations with the NCAR-CAM3.5 model, and published studies, we estimate industrial era (1850-2005) whole-atmosphere ozone forcing of ∼0.5 W m⁻² due to anthropogenic tropospheric precursors and about −0.2 W m⁻² due to halocarbons. The net troposphere plus stratosphere forcing is similar to the net halocarbon plus precursor ozone forcing, but the latter provides a more useful perspective. The halocarbon-induced ozone forcing is roughly two-thirds the magnitude of the halocarbon direct forcing but opposite in sign, yielding a net forcing of only ∼0.1 W m⁻². Thus, the net effect of halocarbons has been smaller, and the effect of tropospheric ozone precursors has been greater, than generally recognized. [ABSTRACT FROM AUTHOR]