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AR4 did not explicitly discuss measured stratospheric ozone trends. For the current assessment report such trends are relevant because they are the basis for revising the RF from -0.05 ± 0.10 W/m2 in 1750 to -0.10 ± 0.15 W/m2 in 2005 (Section 8.3.3.2). These values strongly depend on the vertical distribution of the stratospheric ozone changes.

Total ozone is a good proxy for stratospheric ozone because tropospheric ozone accounts for only about 10% of the total ozone column. Long-term total ozone changes over various latitudinal belts, derived from Weber et al. (2012), are illustrated in Figure 2.6 (a–d). Annually averaged total column ozone declined during the 1980s and early 1990s and has remained constant for the past decade, about 3.5 and 2.5% below the 1964–1980 average for the entire globe (not shown) and 60°S to 60°N, respectively, with changes occurring mostly outside the tropics, particularly the SH, where the current extratropical (30ºS to 60ºS) mean values are 6% below the 1964–1980 average, compared to 3.5% for the NH extratropics (Douglass et al., 2011). In the NH, the 1993 minimum of about -6% was caused primarily by ozone loss through heterogeneous reactions on volcanic aerosols from Mt. Pinatubo.

Two altitude regions are mainly responsible for long-term changes in total column ozone (Douglass et al., 2011). In the upper stratosphere (35 to 45 km), there was a strong and statistically significant decline (about 10%) up to the mid-1990s and little change or a slight increase since. The lower stratosphere, between 20 and 25 km over mid-latitudes, also experienced a statistically significant decline (7 to 8%) between 1979 and the mid-1990s, followed by stabilization or a slight (2 to 3%) ozone increase.

Springtime averages of total ozone poleward of 60° latitude in the Arctic and Antarctic are shown in Figure 2.6e. By far the strongest ozone loss in the stratosphere occurs in austral spring over Antarctica (ozone hole) and its impact on SH climate is discussed in Chapters 11, 12 and 14. Interannual variability in polar stratospheric ozone abundance and chemistry is driven by variability in temperature and transport due to year-to-year differences in dynamics. This variability is particularly large in the Arctic, where the most recent large depletion occurred in 2011, when chemical ozone destruction was, for the first time in the observational record, comparable to that in the Antarctic (Manney et al., 2011).

In summary, it is certain that global stratospheric ozone has declined from pre-1980 values. Most of the decline occurred prior to the mid- 1990s; since then there has been little net change and ozone has remained nearly constant at about 3.5% below the 1964–1980 level.

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