The earth's ability to soak up the gases causing global warming is beginning to fail because of rising temperatures, in a long-feared sign of "positive feedback," new research reveals today. Climate change itself is weakening one of the principal "sinks" absorbing carbon dioxide - the Southern Ocean around Antarctica - a new study has found. As a result, atmospheric CO2 levels may rise faster and bring about rising temperatures more quickly than previously anticipated. Stabilising the CO2 level, which must be done to bring the warming under control, is likely to become much more difficult, even if the world community agrees to do it.

The news may give added urgency to the meeting in three weeks' time between the G8 group of rich nations and the leading developing countries led by China, at Heiligendamm in Germany, when an attempt will be made to put together the framework of a new world climate treaty to succeed the current Kyoto protocol. "This is a timely warning in advance of Heiligendamm and the G8 that the climate clock is beginning to tick faster," said the leading environmentalist Tom Burke, visiting professor at Imperial College London.

"The shift that has been detected in a four-year study by researchers from the University of East Anglia, the British Antarctic Survey and the Max-Planck Institute for Biogeochemistry, published in the journal Science, is one of the most ominous in the development of climate change. It implies a breach in the planet's own defences against global warming. Human society has hugely benefited from the earth's natural carbon absorption facility, which means oceans and forests take up roughly half of the CO2 pumped into the atmosphere, in the so-called carbon cycle. What is left in the atmosphere is known as the "airborne fraction".

If sinks weakened, the airborne fraction would be likely to get bigger. Although supercomputer models of the climate have for some time predicted the weakening of the ocean and terrestrial sinks, no example of it happening has actually been detected - until now. Now the research team has found the vast Southern Ocean, which is the earth's biggest carbon sink, accounting for about 15 per cent of the total absorption potential, has become effectively CO2-saturated.

The level of the gas it is absorbing has remained static since 1981 - but in that time the amount emitted has grown by 40 per cent, so it has stopped keeping pace and much more CO2 is left over to trap the sun's heat. The effect - revealed by scrutinising observations of atmospheric CO2 from 40 stations around the world, is thought to have been caused by an increase in ocean wind speeds. Stormier weather and stronger waves are churning up the sea and bringing natural CO2 stored there closer to the surface - which reduces the ability of the surface to absorb the gas from the air.

The increased winds are believed to be caused by altered atmospheric temperature regimes produced by two separate processes - the depletion of the ozone layer over Antarctica by chlorofluorocarbon gases from aerosol spray cans (now phased out), and global warming. It is thus a positive feedback - an effect of climate change which itself makes climate change worse. Some researchers fear that feedbacks may make global warming happen much faster, and harder to control, than generally appreciated. The pessimism of scientists such as James Lovelock is largely based on the fact that most feedbacks in the earth's system are likely to work against us.

"This is the first unequivocal detection of a carbon sink weakening because of recent climate change," said the lead author of the study, Corinne Le Quéré, of the University of East Anglia. "This is serious. Whenever the world has greatly warmed in the past, the weakening of CO2 sinks has contributed to it." Professor Chris Rapley, director of the British Antarctic Survey, said: "Since the beginning of the Industrial Revolution the world's oceans have absorbed about a quarter of the 500 gigatons [millions of tonnes] of carbon emitted by humans. The possibility that in a warmer world the Southern Ocean is weakening is a cause for concern."

The Government's chief scientific adviser, Sir David King, said: "We have quite a large number of positive feedbacks to worry about, and this appears to be another one. But the seriousness of it would depend on if it was affecting the whole ocean, or merely the Southern Ocean." In recent years it has become clear that the rate at which CO2 was accumulating is itself increasing. The level currently stands at about 382 parts per million by volume (ppm), up from 315 ppm in 1958.

In the past decade the rate has jumped from about 1.6ppm annually to well above 2ppm - a fact which, as The Independent reported in October 2004, may well signal that the earth's absorption ability is shrinking.Asked if this rate increase could now be linked to weakening sinks, Dr Le Quéré said: "I think we are just at the border of detecting that." She added: "All the carbon cycle experts have their eyes on it."Saturations of the Southern Ocean CO2 sink due to recent climate change, Le Quéré et al, published this week in Science

(By Michael McCarthy, The Independent, 18/05/2007)