Life on Earth depends on the ozone layer to screen harmful ultraviolet solar radiation (UV). This stratospheric ozone forms a layer extending from 20 to 50km above the earth's surface and removes some 99 per cent of the UV (1). Depletion of the ozone layer has occurred due to human activities which have introduced artificially high quantities of chlorine, bromine and other ozone depleting substances into the stratosphere, causing higher quantities of ultraviolet radiation to reach the Earth's surface. A decrease of one per cent in the ozone layer causes a 1.5 - 2 per cent increase in UV-B transmission (2).

At this point in time, the ozone layer is more depleted than ever before. Decades of assault from man-made chemicals, notably chlorofluorocarbons (CFCs), halons and methyl bromide, have caused substantial thinning of the ozone layer, leading to an increase in the amount of UV reaching the Earth's surface. The ozone hole over Antarctica grows bigger every year, and in September 2000 extended over a city for the first time. The portents are for a similar hole to appear over the Northern Hemisphere during the next 20 years.

While the Montreal Protocol has dramatically reduced the use of ozone-depleting substances, the original projections for full recovery of the ozone layer by 2050 appear increasingly optimistic, due to interactions with climate change, non compliance with the Protocol and the illegal trade in ODS.

Antarctica

Although systematic measurements of stratospheric ozone began in the 1950s (3), it was only with the discovery of the ozone hole measured over Antarctica in 1985 by the British Antarctic Survey (4), (5) that the effects of ozone depletion were generally recognised. Following the discovery of this hole, satellite measurements confirmed that the ozone loss has reappeared in the austral spring for all successive years since 1985, and generally the Antarctic ozone hole grows bigger and lasts for longer each year.

By mid-September 2001 this hole covered an area of 24 million km2 and significant depletion extended round the Southern Ocean (6). The largest and most severe Antarctic ozone depletion event to date occurred in the austral spring of 2000. The US National Aeronautical and Space Administration (NASA) measured its maximum size at 28.4 million km2 on the 10th September 2000 (7), (8). In early October 2000 minimum ozone values were amongst the lowest on record; some 40 per cent of normal values (8). In mid- to late October 2000, the ozone hole drifted over areas of Argentina, Chile and the Falkland Islands, including the cities of Punta Arenas and Ushaia, where ozone levels were reduced by up to 70 per cent (9). For the first time the reality of living under an ozone hole became clear as the public were advised to stay indoors during the peak hours of 11 am and 3 pm to avoid exposure to the harmful UV rays.

Northern Hemisphere

Ozone losses over the Arctic were smaller compared with the Antarctic in the 1980s and early 1990s. Yet in recent years significant ozone depletion events have occurred over the Arctic region. Alarming reductions in ozone levels over mid-latitudes as well as polar latitudes of the Northern Hemisphere have been observed in the winter/spring seasons of the majority of years over the last decade. These reductions in ozone generally reached 20-30 per cent of pre-1976 averages (10). Recently significant depletion of the ozone layer over the Arctic, Europe and North America has occurred, reaching 60 per cent in some areas (11).

Considerable losses of stratospheric ozone were observed in the Arctic in 1999, and the highest ever local ozone loss observed at any altitude was observed in this region, with losses recorded at greater than 70 per cent (12). In November 1999, in addition to depleted ozone levels over the entire Arctic, there was evidence of significant ozone layer thinning over the United Kingdom, northern Germany, the Netherlands, southern Sweden, Denmark and the Baltic States (13). Although stratospheric ozone depletion in the last few years in the Northern Hemisphere has not been as high as levels reached in some years in the 1990s (14), (15), this weaker than usual ozone depletion reflects natural variations that affect seasonal losses. This is due to atmospheric conditions in the stratosphere, principally warmer temperatures, rather than a decrease in the concentration of ozone-depleting substances in the atmosphere.