The NASA Goddard Space Flight Center (GSFC) two-dimensional (2-D) model of stratospheric transport and photochemistry has been used to predict ozone changes that have occurred in the past 20 years from anthropogenic chlorine and bromine emissions, solar cycle ultraviolet flux variations, the changing sulfate aerosol abundance due to several volcanic eruptions including the major eruptions of El Chichón and Mount Pinatubo, solar proton events (SPEs), and galactic cosmic rays (GCRs). The same linear regression technique has been used to derive profile and total ozone trends from both measurements and the GSFC model. Derived 2-D model ozone profile trends are similar in shape to the Solar Backscattered Ultraviolet (SBUV) and SBUV/2 trends with highest percentage decreases in the upper stratosphere at the highest latitudes. The general magnitude of the derived 2-D model upper stratospheric negative ozone trend is larger than the trends derived from the observations, especially in the northern hemisphere. The derived 2-D model negative trend in the lower stratosphere at middle northern latitudes is less than the measured trend. The derived 2-D model total ozone trends are small in the tropics and larger at middle and high latitudes, a pattern that is very similar to the Total Ozone Mapping Spectrometer (TOMS) derived trends. The differences between the derived 2-D model and TOMS trends are generally within 1–2% in the northern hemisphere and the tropics. The derived 2-D model trends are generally more in southern middle and high latitudes by 2–4%. Our 2-D model predictions are also compared with the temporal variations in total ozone averaged between 65°S and 65°N over the TOMS observing period (1979–1993). Inclusion of anthropogenic chlorine and bromine increases, solar cycle ultraviolet flux variations, and the changing sulfate aerosol area abundance into our model captures much of the observed TOMS global total ozone changes. The model simulations predict a decrease in ozone of about 4% from 1979 to 1995 due to the chlorine and bromine increases. The changing sulfate aerosol abundances were computed to significantly affect ozone and result in a maximum decrease of about 2.8% in 1992 in the annually averaged almost global total ozone (AAGTO) computed between 65°S and 65°N. Solar ultraviolet flux variations are calculated to provide a moderate perturbation to the AAGTO over the solar cycle by a maximum of ±0.6% (about 1.2% from solar maximum to minimum). Effects from SPEs are relatively small, with a predicted maximum AAGTO decrease of 0.22% in 1990 after the extremely large events of October 1989. GCRs are computed to cause relatively minuscule variations of a maximum of + 0.02% in AAGTO over a solar cycle.