Emissions of CO_2 and other greenhouse gases, as CH_4, N_2Oand several chlorofluorocarbons (CFC's) have the potential to impact the climate of our planet (see e.g. IPCC, 1990; 1995; 1996). These gases, which cause a warming of the troposphere, have lifetimes of several years in the atmosphere, and they are therefore well mixed in the troposphere. Emissions of SO_2 lead to formation of sulphate aerosols that cool the troposphere. The particles have a shorter lifetime, and their concentrations and the radiative forcing have a regional structure, reflecting largely the emission areas. Human activity has also caused ozone changes, due to emissions of substances that deplete ozone in the stratosphere and precursors that generate ozone in the troposphere. The ozone changes, in particular in the troposphere, vary on regional scales. As discussed in WMO (1995; 1999) and IPCC (1996) the radiative forcing due to ozone has a longwave as well as a shortwave component and there is a critical dependence on the vertical distribution of ozone changes. Several papers have investigated the sensitivity of radiative forcing to the altitude of an ozone change. Forster and Shine (1997) used radiative transfer models and observed climatologies of temperatures and clouds to study the relative impact of ozone changes in separate altitude regions. Like Wang et al. (1980) and Lacis et al. (1990) they found that the region of largest influence is the tropopause region. However, they also pointed to the fact that when relative rather than absolute changes in ozone are considered, the importance of ozone changes in the middle and upper troposphere, as well as the middle stratosphere, are strengthened relative to those near the tropopause. In a GCM study including all feedbacks (e.g. water vapour, clouds and surface albedo) Hansen et al. (1997a) found that the impact of ozone changes in the mid-to-lower troposphere was strengthened due to the impact of the ozone change on cloudiness. In this paper estimates of radiative forcing due to changes in stratospheric as well as tropospheric ozone given in the literature are reviewed. The review is largely based on the WMO (1999) assessment.
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