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Loss of soil carbon "will speed global warming"

Tim Radford

ENGLAND'S SOILS have been losing carbon at the rate of four million tonnes a year for the past 25 years — losses which will accelerate global warming and which have already offset all the cuts in Britain's industrial carbon emissions between 1990 and 2002, scientists have warned. The research dashes hopes that more carbon dioxide emissions might mean more vegetation growth and therefore more carbon removed from the atmosphere.

The unexpected loss of carbon from the soils — consistently, everywhere in England and Wales and therefore probably everywhere in the temperate world — means more carbon dioxide in the atmosphere, which means even more global warming, and yet more carbon lost from the soil.

"All the consequences of global warming will occur more rapidly. That's the scary thing: the amount of time we have got to do something about it is smaller than we thought," Guy Kirk, of Cranfield University, told the British Association Festival of Science, in Dublin. He and colleagues sampled the top 15 centimetres of soil at almost 6,000 fixed points in England and Wales between 1978 and 2003, to measure the changes in living and decaying matter locked in pastures, croplands, forests, bogs, scrubland and heaths.

Their findings, published in Nature on Thursday, show that carbon was being lost from the soil at an average of 0.6 per cent a year: the richer the soils, the higher the rate of loss. When the figures were extrapolated to include all of the U.K., the annual loss was 13 million tonnes. There was no single factor other than global warming that could explain such changes in non-agricultural soils, they said.

In the past 25 years the average temperature has increased by half a degree centigrade and the growing season of the northern hemisphere has been extended by almost 11 days. Warmer soils will have encouraged greater microbial activity, so more rapid decay of organic matter in the soil, leading to greater discharges of gases. For over two decades, scientists have tried to calculate the earth's annual carbon flow. Some of the carbon is absorbed by the oceans, to be trapped as limestone; some is locked in soil as peat or stored in woodland. Latest research implies that in a warmer world much of this "lost" carbon will find its way back into the atmosphere more quickly.

The study confirms the value of long-term research: the national soil inventory was established in 1978 as a network of fixed points at intervals of three miles, and the scientists used went on using the same techniques to measure the changes in soil carbon over more than 20 years.

Professor Kirk said: "It had been reckoned that the Co2 fertilisation effect was somehow offsetting about 25 per cent of the direct human induced carbon dioxide emissions. It was reckoned that the soil temperature emission effect would catch up in maybe 10 to 50 years' time. We are showing that it seems to be happening rather faster than that." —

© Guardian Newspapers Limited 2004

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