Climate forcing due to tropospheric and stratospheric ozone

Radiative forcings, preindustrial to present, are presented due to changes in both tropospheric and total ozone. Unlike previous studies, we use a combination of present-day satellite data, ozonsondes, and a chemistry model to produce a hybrid monthly mean ozone data set from 1870 to 1990. We calculate the radiative forcing due to the prescribed ozone distribution using a three-dimensional climate model that employs seasonally evolving fixed dynamical heating in the stratosphere. We find that tropospheric ozone causes a seasonal fixed dynamical radiative forcing (SEFDH) of 0.30 W m^-2 over the time period 1870-1990. The spatial pattern of forcing is correlated strongly with the change in tropospheric ozone. When we allow for observed changes in stratospheric ozone we calculate that the forcing due to total ozone from 1870 to 1990 is 0.29 W m^-2, with 0.18 from shortwave and 0.11 W m^-2 from longwave processes. Thus reduction of stratospheric ozone since 1970 offsets the forcing by only -0.01 W m^-2. We also consider to what extent increased tropospheric ozone offsets the direct forcing by sulfate aerosols. We find that for June-July-August the sulfate forcing is -0.76 W m^-2, while the addition of ozone forcing reduces this to -0.40 W m^-2. Finally, we carry out sensitivity studies with regard to the uncertainty in preindustrial ozone levels.