In this study we examine the rate of change in the total ozone decline observed during past years over both the northern and the southern hemisphere. This is studied from the analysis of the longest available reevaluated series of total ozone observations (1964--1993), with the aid of a two-dimensional chemical/transport model of the middle atmosphere, which takes into account microphysical formation of polar stratospheric clouds (PSCs) and heterogeneous mechanisms under different stratospheric chlorine-loading conditions. In our analysis the zonal mean total ozone decreases during the last two decades were compared to the corresponding mean ozone levels during the undisturbed period 1964--1973. The effects of quasi-cyclical and transient phenomena such as the quasi-biennial oscillation, the El Nino/Southern Oscillation and the solar cycle, which are known to significantly influence the ozone variability, have been filtered out from the observations, allowing a more straightforward comparison between model results and observations. These model results are in good agreement with the observed change in the rate of total ozone decline over the middle to high latitudes of both the northern and the southern hemisphere. These model calculations indicate that the ozone depletion at high latitudes in both hemispheres is mainly caused by heterogeneous reactions on the surface of PSCs. At northern midlatitudes, though, the ozone losses are caused by heterogeneous reactions on the surface of sulfate aerosols, especially after large volcanic eruptions. From a sensitivity study performed with the model, a 2¿ decrease in temperature was found to affect significantly the ozone field, especially in the northern hemisphere, by facilitating the heterogeneous conversion processes on PSC surfaces through enhancement of their concentration, and by accelerating the reactions on the surface of sulfate aerosols. ¿ 1997 American Geophysical Union