Radiative forcing of climate by changes in the vertical distribution of ozone

Abstract

We describe a simple method for evaluating the radiative forcing of surface temperature caused by changes in the vertical distribution of ozone. The method employs a parameterization based on one‐dimensional radiative‐convective equilibrium calculations; these calculations predict that the surface temperature should warm in response to both decreases in ozone above 30 km and increases in ozone below 30 km. The parameterization is used to investigate the response of surface temperature to observed changes in the vertical distribution of ozone at northern mid‐latitudes. We show that the observed ozone trends, taken at face value, suggest a cooling of the surface temperature at northern mid‐latitudes during the 1970s equal in magnitude to about half the warming predicted for CO2 for the same time period. However, the measurement uncertainty of the observed trends is large, with the best estimates for mid‐latitude cooling being −0.05±0.05°C. The surface cooling is caused by ozone decreases in the lower stratosphere, which outweigh the warming effects of ozone increases in the troposphere. The results obtained differ from predictions based on one‐dimensional photochemical model simulations of ozone trends for the 1970s, which suggest a warming of the surface temperature equal to ∼20% of the warming contributed by CO2. Also, the ozone decreases observed in the lower stratosphere during the 1970s produce atmospheric cooling by several tenths of a degree in the 12‐ to 20‐km altitude region over the northern mid‐latitudes. This temperature decrease is larger than the cooling due to CO2 and thus may obscure the expected stratospheric CO2 greenhouse signature.