Key Findings of the Scientific Assessment of Stratospheric Ozone: 1989

Remarkable progress has been made in stratospheric ozone science in the past few years. There have been highly significant advances in the understanding of the impact of human activities on the Earth's protective ozone layer. Since the Montreal Protocol was signed. there have been four major findings, each of which has heightened concern that chlorine- and bromine-containing chemicals can lead to a significant depletion of stratospheric ozone.

• Antarctic ozone hole: The weight of scientific evidence strongly indicates that chlorinated (largely man-made) and brominated chemicals are primarily responsible for the recently discovered substantial decreases in stratospheric ozone over Antarctica in the Southern Hemisphere springtime.

• Perturbed Arctic chemistry: While at present there is no measurable ozone loss over the Arctic comparable to that over the Antarctic, the same potentially ozone-destroying processes have been identified in the Arctic stratosphere. The degree of any future ozone depletion will probably depend on the particular meteorology of each Arctic winter and future atmospheric levels of chlorine and bromine.

• Long-term ozone decreases: The analysis of the total-column ozone data from ground-based Dobson instruments shows measurable downward trends from 1969 to 1988 of 3-5% (i.e. 1.8-2.7% per decade) in the Northern Hemisphere (30deg.N to 64deg.N) in the winter months which cannot be attributed to known natural processes.

• Model limitations: These findings have led to the recognition of major gaps in theoretical models used for assessment studies. Assessment models do not simulate adequately polar stratospheric cloud (PSC) chemistry or polar meteorology. The impact of these shortcomings for the prediction of ozone layer depletion at lower latitudes is uncertain.