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 Hemis phere. Since ozone absorbs both solar and infrared radiation, several studies concerning the tropospheric ozone-climate problem have been re cently conducted mainly with one- and two-dimensional models. In this study, pre-industrial and present-day tropospheric ozone concentration s simulated by a three-dimensional Chemistry Transport Model (3-D CTM) IMAGES (Intermediate Model for the Annual and Global Evolution of Spe cies) are used in conjunction with the Laboratoire de Meteorologie Dyn amique General Circulation Model (LMD GCM) to determine the ozone radi ative forcing since the pre-industrial era. We find that the ozone for cing is regionally heterogeneous with a marked interhemispheric differ ence and a strong seasonal variation, peaking over the Northern Hemisp here 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 trop osphere. Changes of ozone concentration in the planetary boundary laye r are about 10 times less efficient than in the high troposphere in te rms of radiative perturbation. These 3D results also confirm the quasi -linear relationship between the radiative forcing and the tropospheri c ozone increase for both hemispheres. Some previsions of future forci ng change considering a critical constant global rate of ozone increas e equal to 10% per decade and the IS92a IPCC emission scenario are rea lized for the next century.