A two-dimensional photochemical model is used to examine changes to the ozone layer caused by emissions of CFCl3, CF2Cl2, CH3CCl3 and CCl4. The influence of a possible secular increase in tropospheric methane up to 2% per year was found to be small, although it acts to mask decreases in total ozone caused by the chlorocarbons. Increasing NOx emissions caused by industrialization also tend to mask decreases in total ozone and may have caused total ozone to increase by ~1%. The model calculated ozone decreases are estimated to be about 3% by 1980. This estimate is higher than estimates by similar models, although we note that CCl4 and CH3CCl3 emissions are included in the model in addition to CFCl3 and CF2Cl2. This is significant because the model indicates that CCl4 has dominated the ozone depletions so far, and knowledge of the historical emission rate of CCl4 to the atmosphere is incomplete. There remain sufficient significant disagreements between theoretical and observed concentrations and variabilities, particularly for odd nitrogen and ClO, to caution against assigning too much confidence in the calculated ozone depletion.