Detecting the recovery of total column ozone

International agreements for the limitation of ozone-depleting substances h ave already resulted in decreases in concentrations of some of these chemic als in the troposphere. Full compliance and understanding of all factors co ntributing to ozone depletion are still uncertain; however, reasonable expe ctations are for a gradual recovery of the ozone layer over the next 50 yea rs. Because of the complexity of the processes involved in ozone depletion, it is crucial to detect not just a decrease in ozone-depleting substances but also a recovery in the ozone layer. The recovery is likely to be detect ed in some areas sooner than others because of natural variability in ozone concentrations. On the basis of both the magnitude and autocorrelation of the noise from Nimbus 7 Total Ozone Mapping Spectrometer ozone measurements , estimates of the time required to detect a fixed trend in ozone at variou s locations around the world are presented. Predictions from the Goddard Sp ace Flight Center (GSFC) two-dimensional chemical model are used to estimat e the time required to detect predicted trends in different areas of the wo rld. The analysis is based on our current understanding of ozone chemistry, full compliance with the Montreal Protocol and its amendments, and no inte rvening factors, such as major volcanic eruptions or enhanced stratospheric cooling. The results indicate that recovery of total column ozone is likel y to be detected earliest in the Southern Hemisphere near New Zealand, sout hern Africa, and southern South America and that the range of time expected to detect recovery for most regions of the world is between 15 and 45 year s. Should the recovery be slower than predicted by the GSFC model, owing, f or instance, to the effect of greenhouse gas emissions, or should measureme nt sites be perturbed, even longer times would be needed for detection.