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Our current understanding of the global and polar stratospheric O3 has been described and discussed in detail in Chapters 3 and 4 of the recent WMO Scientific Assessment of Ozone Depletion: 2006 (WMO, 2007) and the references therein. The key findings are: The global values of stratospheric O3 concentrations are ~3.5% less for the average 2002-2005 period as compared with the 1964-1980 average. Northern and southern hemispheric mid-latitude total column O3 is respectively 3% and 5.5% lower. The 2002-2005 values are similar to 1998-2001 values, indicating that the decrease in O3 appears to have been halted. Empirical and model studies have shown that changes in dynamics account for some of the observed northern hemisphere mid-latitude O3 decline. In particular an unexplained variability in BDC appears to be modulating the amount of O3 transported from the tropics (e.g., Dhomse et al., 2006). However significant and unexplained decreases of ~3% have also been observed between the tropopause and 25 km in the tropics (Randel and Wu, 2006). In the Arctic column O3 loss has been in recent winters among the largest ever observed. The volume of air able to support the formation of polar stratospheric clouds (PSCs) has increased since the 1960s (Rex et al., 2006). In the Arctic in contrast to the Antarctic the halogen activation is not saturated and year to year variations related to temperature variability make any trend assessment more difficult. In the Antarctic the O3 depletion appears to have stabilised. However in September 2002, the first ever major warming was observed in the Antarctic. It remains unclear whether this was an exceptional event or an early warning of changing dynamics.

After a steady ozone decrease until the middle 1990s, a turnaround in ozone concentration has been observed globally that in parts reflects the starting slow decline of stratospheric chlorine. However global climate change impacts on the rate of recovery from the change in ozone depleting substances (ODSs) in a manner which is not yet understood (WMO, 2007). CCMs consistently indicate that the recovery of total O3 will be accelerated in some regions if stratospheric temperatures will further decrease. However, the situation is much more complicated in polar regions where a cooler stratosphere will lead to an increase in the amount of PSCs. Therefore, the future evolution in polar O3 may differ from that in the tropics and middle latitudes. Moreover, the possibility of future strengthening of the BDC would also affect O3 via changes in transport.

In recent years it has been realised that in addition to the long-lived O3 depleting substances (such as CFCs and halons) very short-lived substances (VSLS), mostly of natural origin, can also contribute significantly to the stratospheric halogen loading, and thus stratospheric O3 loss (e.g., Salawitch et al., 2005; WMO, 2007). Very little is currently known how the impact of VSLS on the stratospheric O3 layer will evolve in the next decades under the influence of a changing climate (e.g., Chapter 2 in WMO, 2007).