Tom Wilkie examines the scientific explanations for why the planet is getting warmer
The hole in the Antarctic ozone layer might have gone undetected - were it not for the Falklands war.

The British Antarctic Survey (BAS) had been short of money and considered stopping ozone-measuring. After the Falklands war, Margaret Thatcher channelled more money to the BAS for reasons of political prestige in the South Atlantic. Measuring ozone was reprieved. It was measurements by this programme in 1985 that helped to confirm earlier suggestions by a Japanese researcher (which had been roundly dismissed) that the ozone layer was being depleted.

The materials responsible for ozone depletion - the chlorofluorocarbons (CFCs) used in refrigerators and air-conditioning units, and as propellants in aerosols - are potent greenhouse chemicals. Molecule for molecule, CFCs are far more efficient as a warming blanket than carbon dioxide. However, there is far less of the stuff in the atmosphere. Carbon dioxide is the main greenhouse gas.

There is a further link. Global warming could increase the rate at which the remaining CFCs deplete ozone, according to Professor Sherwood Rowland from the University of California.

Professor Rowland and Dr Mario Molina shared the Nobel Prize in Chemistry last year for predicting, more than a decade before the hole in the Antarctic ozone layer was discovered, that CFCs could damage the ozone layer.

Professor Rowland believes that over the next 50 years, the ozone layer will recover as a result of the decreasing concentrations of CFCs. But it could be a close-run thing.

Professor Rowland said: "The worry about low ozone in the next couple of decades is the possibility of volcanic eruptions putting surfaces in there for chlorine to take out ozone more efficiently."

Volcanoes can increase ozone depletion by throwing huge quantities of sulphur dioxide gas into the stratosphere, where it forms sulphuric acid aerosol droplets. These provide reactive surfaces to increase the efficiency with which chlorine from CFCs reacts with and removes the ozone.

The effect of sunlight on the chemistry of aerosols on the surfaces of ice-crystals in stratospheric clouds may appear recondite; it is relatively straightforward compared with modelling the effects of carbon dioxide on climate.

One complication: pollution from burning fossil fuels is cooling large areas of the planet's surface as well as warming it. Emissions of carbon dioxide trap heat in the atmosphere, but sulphate aerosols, also produced by burning coal and gas, act as a heat shield.

Powerful computers are the chief scientific weapon against global warming. Scientists at the Hadley Centre for Climate Prediction at Bracknell, Berkshire, are using a Cray C90 computer to calculate how the climate evolved from 1860 to the present day. Once they can recreate the past, the modellers will have more confidence in predicting a future in which concentrations of greenhouse gases continue to rise as the world burns more coal, oil and gas.

The behaviour of the oceans and the atmosphere are critical. There are complex flows of energy and water going on between the two, with the oceans slowing warming or cooling trends in the air.

Then there is the cooling effect of aerosols. These are microscopic particles of sulphate, dust and water produced by the burning of fossil fuels as well as nature - volcanoes, sea spray and forest fires. They appear as a haze in the lower atmosphere.

Sulphate aerosol cools by reflecting incoming sunlight back into space. They are also "seed" clouds and boost the number of water droplets they contain. The clouds are then whiter and brighter and so reflect more incoming sunshine back to space.

Clouds, which could have a critical influence on the progress of any man-made global warming, are just as confusing. A warmer atmosphere would probably contain more water vapour - itself a greenhouse gas - causing further warming. But more vapour could mean more clouds. They trap some of the earth's outgoing heat radiation, but they also reflect incoming sunlight, preventing solar radiation from heating the lower atmosphere and ground. In a warming world, our scientists will need to have their heads in the clouds.

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