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

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We have developed a global model for the study of chemistry‐climate interactions by incorporating a detailed simulation of tropospheric ozone‐NOx‐hydrocarbon chemistry within a general circulation model (GCM). We present a first application of the model to the calculation of radiative forcing from tropospheric ozone since preindustrial times. Longwave and shortwave radiation fluxes are computed every 5 hours in the GCM using the locally simulated ozone fields. In this manner, the model resolves synoptic‐scale correlations between ozone and meteorological variables. A simulation for present‐day conditions is compared to a preindustrial atmosphere (∼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 well the observed distributions of ozone and its precursors in the present‐day atmosphere. Increases in ozone since preindustrial times are 20–200% depending on region and season. The global mean, instantaneous radiative forcing from anthropogenic ozone is 0.44 W m2 (0.35 longwave, 0.09 shortwave). The model reveals large shortwave forcings (0.3–0.7 W m2) over polar regions in summer. The total forcing is greater than 1.0 W m2 over large areas, including the Arctic, during Northern Hemisphere summer. The normalized radiative forcing per unit of added ozone column varies globally from −0.01 to 0.05 W m2. This variance can be explained in large part by the temperature difference between the surface and the tropopause; clouds are an additional factor, particularly at low latitudes. An off‐line radiative calculation using the same ozone fields but averaged monthly shows nearly identical forcings, with differences less than ±2% over most of the Earth. The similarity between the off‐line and on‐line simulations suggests that the common use of off‐line ozone fields is acceptable in radiative forcing calculations. Addition of the direct forcings from anthropogenic sulfate aerosol and tropospheric ozone computed with the same GCM shows compensating effects, with sulfate dominating at northern midlatitudes and ozone usually dominating elsewhere.