Radiative forcing from tropospheric ozone calculated with a unified chemistry-climate model

We have developed a global model for the study of chemistry-climate interac tions by incorporating a detailed simulation of tropospheric ozone-NO,hydro carbon chemistry within a general circulation model (GCM). We present a fir st application of the model to the calculation of radiative forcing from tr opospheric ozone since preindustrial times. Longwave and shortwave radiatio n fluxes are computed every 5 hours in the GCM using the locally simulated ozone fields. In this manner, the model resolves synoptic-scale correlation s between ozone and meteorological variables. A simulation for present-day conditions is compared to a preindustrial atmosphere (similar to 1800 A.D.) with no fossil fuel combustion, 10% of present-day biomass burning, and 0. 7 ppm methane. The two simulations use the same meteorological fields; the radiative forcing does not feed back into the GCM. The model reproduces wel l the observed distributions of ozone and its precursors in the present-day atmosphere. Increases in ozone since preindustrial times are 20-200% depen ding on region and season. The global mean, instantaneous radiative forcing from anthropogenic ozone is 0.44 W m(2) (0.35 longwave, 0.09 shortwave). T he model reveals large shortwave forcings (0.3-0.7 W m(2)) over polar regio ns in summer. The total forcing is greater than 1.0 W m(2) over large areas , including the Arctic, during Northern Hemisphere summer. The normalized r adiative forcing per unit of added ozone column varies globally from -0.01 to 0.05 W m(2). This variance can be explained in large part by the tempera ture difference between the surface and the tropopause; clouds are an addit ional factor, particularly at low latitudes. An off-line radiative calculat ion using the same ozone fields but averaged monthly shows nearly identical forcings, with differences less than +/-2% over most of the Earth. The sim ilarity between the off-line and on-line simulations suggests that the comm on use of off-line ozone fields is acceptable in radiative forcing calculat ions. Addition of the direct forcings from anthropogenic sulfate aerosol an d tropospheric ozone computed with the same GCM shows compensating effects, with sulfate dominating at northern midlatitudes and ozone usually dominat ing elsewhere.