28. Stratospheric Ozone Depletion

CHAPTER 28: STRATOSPHERIC OZONE DEPLETION - THE PROBLEM

The so-called ozone layer is located between 10 and 50 km above the Earth's surface and contains approximately 90 per cent of all atmospheric ozone. Under undisturbed conditions stratospheric ozone is formed as the result of a photochemical equilibrium involving oxygen molecules, oxygen atoms and solar radiation. The ozone layer protects life on the Earth's surface since ozone is the only efficient absorbent of the ultraviolet-B radiation (wavelengths 280 to 310 nm) from the sun. UV-B radiation is harmful to organisms in many ways (see Chapter 16).

The abundance of ozone is often measured through its total amount in an atmospheric column going from the ground to the top of the atmosphere. Ground-based and satellite observations have shown a decrease of total column ozone in winter in the northern hemisphere (an average depletion of the global ozone layer of 3 per cent in the period 1979­91, but with large latitudinal and seasonal differences). At mid-latitudes over Europe, the decline of the ozone column is about 6 to 7 per cent during the last decade. The decline is strongest in winter and early spring, when there is relatively little UV-B (Table 28.1). The 1991 Scientific Assessment (WMO, 1991) showed, for the first time, evidence of significant decreases in spring and summer in both the northern and southern hemispheres at middle and high latitudes, as well as in the southern winter. These downward trends were larger during the 1980s than the 1970s (see Figure 28.1). Recent evaluations show that seasonal averages of total ozone over Europe were 10 per cent lower than long-term averages in winter 1991/92, and 13 per cent lower in winter 1992/93 (Bojkov et al, 1993).

The 'ozone hole' observed above Antarctica since the end of the 1970s is an extreme case of ozone layer depletion, with column reduction of about 55 per cent in October (ie, southern hemisphere spring) 1987 and 1989­93. At the 13 to 21 km level (lower stratosphere), the depletion was nearly complete, with loss of 95 per cent. There is great concern that substantial ozone decline could be produced in the northern high latitude regions, where the densely populated regions of Eurasia and North America could be under direct risk.

Continuous stratospheric ozone depletion trends, and the possible occurrence of an Arctic ozone hole, should be of great concern for Europe, with potential effects on human health, plants, animals and the food supply. Europe's responsibility is emphasised by the fact that Europe contributes approximately one third of the global annual emissions of ozone-depleting substances (see Chapters 4 and 14). The issue of stratospheric ozone depletion has stimulated interest and dialogue among the scientific community, policy makers and the public. Scientific concern was already expressed in 1974, but effects stronger than predicted were observed in 1985, presenting the public with the alarming image of a hole in the sky. The first concrete international measures aimed at limiting ozone depletion were agreed in 1985 and 1987. Two important questions now are whether the agreed measures are sufficient for the atmosphere to reach its 'pre-hole' conditions, and what is the extent of any irreversible damage that has occurred to humans and ecosystems on Earth.

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