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The rate at which global warming gases are accumulating in the atmosphere has taken a sharp leap upwards, leading to fears that the devastating effects of climate change may hit the world even sooner than has been predicted.
Atmospheric levels of carbon dioxide (CO2), the principal greenhouse gas, have made a sudden jump that cannot be explained by any corresponding jump in terrestrial emissions of CO2 from power stations and motor vehicles - because there has been none.
Some scientists think instead that the abrupt speed-up may be evidence of the long-feared climate change "feedback" mechanism, by which global warming causes alterations to the earth's natural systems and then, in turn, causes the warming to increase even more rapidly than before.
Such a development would mean the worldwide droughts, agricultural failure, sea-level rise, increased weather turbulence and flooding all predicted as consequences of climate change would arrive on much shorter time-scales than present scenarios suggest, and the world would have much less time to co-ordinate its response.
Only last month, Tony Blair expressed anxiety that global warming's dire effects would arrive not just in his children's lifetime, but in his own, and would "radically alter human existence".
The feedback phenomenon has already been predicted in the supercomputer models of the global climate on which the current forecasts of warming are based. A key aspect is the weakening, caused by the warming itself, of the earth's ability to remove huge amounts of CO2 from the atmosphere by absorbing it annually in its forests and oceans, in the so-called carbon cycle. (The forests and oceans are referred to as carbon "sinks".)
Hitherto, however, that weakening has been put decades into the future.
The possibility that it may be occurring now is suggested in the long run of atmospheric CO2 measurements that have been made since 1958 at the observatory on the top of Mauna Loa, an 11,000ft volcano in Hawaii, by the American physicist Charles Keeling, from the University of California at San Diego.
When he began, Dr Keeling, who is still in charge of the project and who might be said to be the Grand Old Man of CO2, found the amount of the gas present in the atmosphere to be 315 parts per million by volume (ppm); today, after the remorseless increase in emissions from power stations and motor vehicles over the past four and a half decades, the figure stands at 376ppm.
This growth is what most scientists believe is causing the earth's atmosphere to warm up, as the increasing CO2 retains more and more of the sun's heat in the atmosphere, like the panes of a greenhouse.
But the worry now is not merely the swelling volume of CO2 but the sudden leap in its increase rate. Across all 46 years of Dr Keeling's measurements, the average annual CO2 rise has been 1.3ppm, although in recent decades it has gone up to about 1.6ppm.
There have been several peaks, all associated with El Niņo, the disruption of the atmosphere-ocean system in the tropical Pacific Ocean that causes changes to global weather patterns. In 1988, for example, the annual increase was 2.45ppm; in 1998, 2.74ppm; both were El Niņo years.
Throughout the series those peaks have been followed by troughs, and there has been no annual increase in CO2 above 2ppm that has been sustained for more than a year. Until now.
From 2001 to 2002, the increase was 2.08ppm (from 371.02 to 373.10); and from 2002 to 2003 the increase was 2.54ppm (from 373.10 to 375.64). Neither of these were El Niņo years, and there has been no sudden leap in emissions.
The greater-than-two rise is also visible in two separate sets of CO2 measurements made by America's National Oceanic and Atmospheric Administration, at Mauna Loa and other stations around the world.
At the weekend, Dr Keeling told The Independent the rise was real and worrying as it might indeed represent the beginnings of a feedback.
He said it might be associated with the Southern Oscillation, a pattern of high and low atmospheric pressure previously always associated with El Niņos, or it might be something new.
"The rise in the annual rate of CO2 increase to above two parts per million for two consecutive years is a real phenomenon," Dr Keeling said.
"It is possible this is merely a reflection of the Southern Oscillation, like previous peaks in the rate, but it is possible it is the beginning of a natural process unprecedented in records.
"This could be a decoupling of the Southern Oscillation from El Niņo events, which itself could be caused by increased CO2 in the atmosphere; or it could be a weakening of the earth's carbon sinks. It is a cause for concern."
Leading British scientists and environmentalists agree. "If this is a rate change [in the CO2 rise], of course it will be very significant," said Dr Piers Forster of the meteorology department of the University of Reading. "It will be of enormous concern, because it will imply that all our global warming predictions for the next 100 years or so will have to be redone. If the higher rate of increase continues, things will get very much worse. It will makes our predicament even more catastrophic."
Tom Burke, a former government adviser on green issues who is now an academic and environmental adviser to business, said: "This series of CO2 measurements is the world's climate clock, and it looks as if it may be ticking faster,"
"That means we are running out of time to stabilise the climate. Governments and business will both have to invest dramatically more if we are to avoid the global warming catastrophe that Tony Blair has warned against."
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