Stratospheric ozone depletion

Stratospheric ozone is thought to have begun forming several billion years ago as a result of the solar-powered destruction and recombination of oxygen. The natural concentration of stratospheric ozone is now maintained through the dynamic equilibrium existing between the production and destruction of ozone. The destruction is catalysed by trace amounts of hydrogen, nitrogen and halogen free radicals (especially chlorine and bromine). These free radicals occur naturally but, in recent decades, their concentration has been increased greatly by industrial activities. This has upset the equilibrium and led to a sustained decline in stratospheric ozone concentrations (2,52).

Stratospheric ozone depletion, as with greenhouse gas accumulation in the lower atmosphere, entails changes in the global climate. That is, although the gaseous emissions arise from diverse localized sources, in all continents, their environmental impact is of a diffuse globalized kind. Local emissions thus contribute to an integrated global change that has potentially serious consequences for human health.

Significant stratospheric ozone has been depleted, mainly at middle and high latitudes. Ozone depletion is more pronounced in winter and spring than in summer (53). These seasonal differences are more marked in the Northern Hemisphere, although, in general, ozone depletion is more pronounced in the Southern Hemisphere. The ozone over Europe has been depleted at approximately 3% per decade (54).

Stratospheric ozone shields the earth's surface from incoming solar ultraviolet radiation, which is harmful to all animals and plants. Increases in ground-level ultraviolet radiation are presumed to have occurred, especially at higher latitudes. However, such trends are difficult to ascertain because ultraviolet radiation has only recently begun to be measured at specific wavelengths. In addition, local factors such as clouds, aerosols and tropospheric ozone pollution can absorb or reflect ultraviolet radiation before it reaches the ground.

The United Nations Environment Programme (54) estimates that, from 1979 to 1992, biologically active radiation (the ultraviolet band that causes erythema) increased by the following:

A phasing out of chlorofluorocarbons and other halocarbons was agreed to internationally in the Montreal Protocol on Substances that Deplete the Ozone Layer (1987), in its London (1990), Vienna (1995) and Montreal (1997) Amendments, and in the Copenhagen adjustment. Nevertheless, the concentration of stratospheric ozone is not expected to return to its normal level until the second half of the 21st century (Fig. 6). There is also evidence that some countries are not observing the global ban on chlorofluorocarbons, and illegal trade in these substances would compromise the recovery of the ozone layer.

Some recent evidence indicates that global warming may alter the atmospheric heat distribution so as to increase the cooling of the stratosphere (3). If such stratospheric cooling continues, the risk of

Fig. 6. Projections of ozone layer recovery under the Montreal Protocol and its Amendments

Fig. 6. Projections of ozone layer recovery under the Montreal Protocol and its Amendments

1900 1920 1940 1960 1980 2000 2020 2040 2060 2080 2100


Note: The curve shows the projected mixing ratio (frequency of occurrence) of equivalent effective chlorine. It is based on the Protocol scenario in the 1998 World Meteorological Organization/United Nations Environment Programme ozone assessment, where one assumes the maximum emissions allowed within the protocols. Thus, a lower concentration in the figure means a higher projected concentration of ozone.

Source: preliminary data from the World Meteorological Organization 1998 ozone assessment (Velders, G., personal communication, 1999).

ozone depletion could continue to increase even after chlorine and bromine loading starts to decline. Shindell et al. (4) have estimated that the projected increase in greenhouse gas concentrations, and subsequent tropospheric heat-trapping, may increase polar ozone losses and thus delay the eventual recovery of the ozone layer.

Was this article helpful?

0 0
Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook

Post a comment