Climate response to the increase in tropospheric ozone since preindustrial times:

The reliance on global mean radiative forcing as an index of climate change is questionable for highly inhomogeneous forcing agents such as tropospheric ozone or aerosols. Using a general circulation model, we have carried out a pair of equilibrium climate simulations with previously calculated present-day and preindustrial ozone distributions. We show that the radiative forcing of 0.49 W m-2 due to the increase in tropospheric ozone since preindustrial times corresponds to an increase in global mean surface temperature of 0.28 o C. The increase is nearly 0.4 o C in the Northern Hemisphere and about 0.2 o C in the Southern Hemisphere. The largest increases are in northern midlatitudes downwind of Europe and Asia (major ozone source continents). In the lower stratosphere, global mean temperatures decrease by about 0.2 o C due to the diminished upward flux of radiation at 9.6 µm. The largest stratospheric cooling, up to 1.0 o C, occurs over high northern latitudes in winter, with possibly important implications for the formation of polar stratospheric clouds. To identify the characteristics of climate forcing unique to tropospheric ozone, we have conducted two additional climate equilibrium simulations: one in which preindustrial tropospheric ozone concentrations were increased everywhere by 18 ppb, producing the same global radiative forcing as present-day ozone but without the heterogeneity; and one in which CO 2 was decreased by 25 ppm relative to present-day, with ozone at present-day values, to again produce the same global radiative forcing but with the spectral signature of CO 2 rather than ozone. In the first simulation (uniform increase of ozone), the global mean surface temperature increases by 0.25 o C, with an interhemispheric difference of only 0.03 o C, as compared to nearly 0.2 o C for the heterogeneous ozone increase. In the second simulation (equivalent CO 2), the global mean surface temperature increases by 0.36 o C, 30% higher than the increase from tropospheric ozone. The stronger surface warming from CO 2 is in part because CO 2 forcing (obscured by water vapor) is shifted relatively poleward where the positive ice-albedo feedback amplifies the climate response, and in part because the magnitude of the CO 2 forcing in the mid-troposphere is double that of ozone. However, we find that CO 2 is far less effective than tropospheric ozone in driving stratospheric cooling.