The SUNY-SPB 2-D model is used to model ozone behavior during the period 1970--2000. The roles of different natural and human-induced impacts (solar variability, volcanoes, changes in dynamics, increasing chlorine and bromine concentrations) on the column ozone are studied separately as well as in combination. We found that variations in solar UV radiation produce global total ozone fluctuations that are comparable to those due to volcanic eruptions and increasing concentrations of chlorine and bromine during this period. We also suggest that the physical reason for the different observed time behavior of total ozone in the Northern and Southern Hemisphere middle latitudes during the 1990s may be found in the greater importance of mixing of polar processed air into middle latitudes in the Southern hemisphere.