Radiative forcing due to increased tropospheric ozone concentrations

Increasing tropospheric ozone concentrations have been observed in the past decades in industrialized and remote areas of the Northern Hemisphere. Since ozone absorbs both solar and infrared radiation, several studies concerning the tropospheric ozone-climate problem have been recently conducted mainly with one- and two-dimensional models. In this study, pre-industrial and present-day tropospheric ozone concentrations simulated by a three-dimensional Chemistry Transport Model (3-D CTM) IMAGES (Intermediate Model for the Annual and Global Evolution of Species) are used in conjunction with the Laboratoire de Meteorologie Dynamique General Circulation Model (LMD GCM) to determine the ozone radiative forcing since the pre-industrial era. We find that the ozone forcing is regionally heterogeneous with a marked interhemispheric difference and a strong seasonal variation, peaking over the Northern Hemisphere continents during summer and reaching locally more than 1 W m(-2). Sensitivity simulations confirm that the major contributions to the tropopause forcing arise from ozone changes occurring in the high troposphere. Changes of ozone concentration in the planetary boundary layer are about 10 times less efficient than in the high troposphere in terms of radiative perturbation. These 3D results also confirm the quasi-linear relationship between the radiative forcing and the tropospheric ozone increase for both hemispheres. Some previsions of future forcing change considering a critical constant global rate of ozone increase equal to 10% per decade and the IS92a IPCC emission scenario are realized for the next century.