Unlayering of the Ozone: An Earth Sans Sunscreen

The formation of the Antarctic ozone hole is a graphic demonstration of how rapidly we can change the atmosphere of our planet. There are many other environmental issues facing us today and we must link them together to understand and debate the underlying causes, rather than treat each issue in isolation. Antarctica is a wonderful continent. Glaciers carve their way to the sea where the waters teem with penguins and whales. Although 70 per cent of the world's fresh water resides in the polar ice cap, the continent is a veritable desert, with liquid water in short supply. The frozen ice takes on many shades, from the brilliant white of freshly fallen snow to the deep indigo at the bottom of a gaping crevasse. This land of contrasts is where the Antarctic ozone hole was discovered.

Ozone is a form of oxygen, similar to the gas that we breathe, but with three atoms instead of two. This makes it highly reactive, and in high concentration it is a toxic gas. When formed by air pollution near the surface it can trigger asthma attacks, but high in the atmosphere it forms a protective sun-shield. This is the ozone layer, a region from about 10 to 35 kilometres in altitude, where the natural concentration of ozone is highest. Ozone forms at this level in the stratosphere through the action of ultraviolet sunlight on oxygen gas, and in the process the most harmful ultraviolet radiation is totally absorbed. Some ultraviolet light does reach the surface, and the intensity is controlled by the amount of ozone -- the more ozone the less ultraviolet, and vice-versa. With a thinning ozone layer more ultraviolet light reaches the surface, exposing us to a greater risk of sunburn, skin cancers or cataracts of the eye.

Ozone observation in the Antarctic began over fifty years ago with the International Geophysical Year of 1957-58. As part of this scientific endeavour, a network of observatories was set up across Antarctica, several of which measured ozone. One of the first to report was the British research station Halley, and the results from the first year of operation showed a surprising difference to those from the equivalent latitude in the Arctic. This was soon recognized as being due to a different stratospheric circulation in the atmosphere above the two poles: in the north the circulation is relatively complex, whilst in the south it is relatively simple with a strong, long lasting winter polar vortex or a large-scale persistent cyclone.

Ozone observations at Halley continued using the same type of instrument, the Dobson ozone spectrophotometer, designed in the 1920s by an Oxford professor of physics, Gordon Dobson; it remains the standard for ozone observations today. The instrument uses ultraviolet light from the sun coming through the ozone layer to measure the amount of ozone. It is very much a manual instrument, and the calculations required to extract the ozone amount from the observations are quite complex, to the extent that in the 1970s a stack of unreduced observations began to build up.

When I joined the British Antarctic Survey, one of my first jobs was to write computer programmes that would process the observations once they were entered into electronic form. Making sure that the entered data was correct was the first part of the process, followed by verifying the software. At about the same time, concern was growing that spray cans and the Concorde supersonic airplane could destroy the ozone layer. When the British Antarctic Survey held its Open Day, it seemed a good opportunity to reassure the public that the ozone layer above Antarctica had not changed. Surprisingly, the data seemed to show that the spring-time ozone values of that year were much lower than they had been a decade earlier, but in the meantime I had yet to process the intervening data. Once this was done, it was obvious that there was a systematic effect, giving rise to the paper that Joe Farman, Brian Gardiner and I wrote, announcing an unexpected effect over Antarctica.

Elsewhere in Antarctica, other observatories had continued to make ozone measurements on a sporadic basis, but they lacked the long-term continuity of the same instrumental technique that was available at Halley. This was a key factor in our discovery, and set a valuable lesson for monitoring the environment. In addition, the centre of the ozone hole is often offset towards the Atlantic, allowing Halley to start making observations several weeks before the sun rose high enough at the South Pole. Once the paper was published in Nature, satellite data was reprocessed to reveal an "ozone hole" over the Southern continent. Whilst satellites give an excellent overview of the changes within the ozone layer, ground-based observations are still needed to provide them with an accurate calibration.

Today we know that this Antarctic ozone hole is caused by chlorine and bromine from ozone-depleting chemicals such as chlorofluorocarbons (CFCs) and halons. The reason for the particularly severe ozone depletion over Antarctica lies with its stable polar vortex, which makes the Antarctic ozone layer roughly ten degrees colder than that in the Arctic. This means that unusual clouds form widely in the Antarctic ozone layer during the winter, and chemistry on the surfaces within these clouds conditions the ozone-depleting chemicals. When sunlight returns, very efficient photocatalytic reactions take place which destroy ozone.

The Montreal Protocol has been a very effective response to the shocking and rapid change in the ozone layer over Antarctica. Now ratified by all but one of the UN Member States, it is having a clear effect in reducing the amount of ozone-destroying substances in the atmosphere. CFCs and allied substances are, however, very stable, so their atmospheric concentration drops very slowly and will not reduce to pre-ozone hole values until at least 2070. It is likely to be several more years before we can be confident that the ozone hole is shrinking and many decades before spring-time ozone levels return to those of the early 1970s. One unintended consequence of the reduction in ozone-destroying substances has been its significant effect on reducing global warming, as the substances are often also powerful greenhouse gases.

Treating the ozone hole was relatively straightforward, with both general acceptance of the need to change and the possibility of alternative products. Another environmental symptom -- that of climate change -- is currently generating much debate, but the amount of greenhouse gases in the atmosphere is rising at the worst-case rate predicted by the Intergovernmental Panel on Climate Change (IPCC). In addition, there are many other global symptoms of environmental stress ranging from water and food shortages and fishery collapses to deforestation habitat destruction, amongst others.

When a doctor treats a patient with an illness, it is essential that all the symptoms are taken into account in making a diagnosis. It must be exactly the same when we are looking after the health of our own planet. My diagnosis is that we must urgently debate and act on reducing our effect on the planet, otherwise evermore symptoms will appear. Such reduction could be achieved through decreasing the consumption of our planet's resources, particularly reducing consumption amongst the developed nations; but we are also likely to need to reduce our own numbers if we are to sustain a healthy planet in the long term. How to do so is the big debate that we must urgently conduct if we are to avoid a fate such as the inhabitants of Easter Island, who used up all their resources. Unfortunately, these warnings, like those of Cassandra, are unlikely to be heeded and it may require a major disaster before action is taken. The United Nations is one forum where the debate should begin.