By Jonathan Amos
BBC News science reporter, Dublin
Higher UK temperatures are causing soils to "exhale" large quantities of carbon dioxide, probably accelerating global warming, scientists report.
It made no difference what the land was used for
They base their assessment on a huge analysis of soil samples gathered from across England and Wales over 25 years.
The team says its findings, if extended to the whole of the UK, suggest some 13 million tonnes of carbon are being lost from British soils each year.
The Cranfield University group reports its work in the journal Nature.
The scientists say computer models used to forecast future climate trends will now have to be revised because the calculations on which they are based will be wide of the mark.
"Our findings suggest the soil part of the equation is scarier than we had thought," Professor Guy Kirk, of Cranfield University, told journalists at the British Association's Festival of Science in Dublin, Ireland.
"The consequence is that there is more urgency about doing something - global warming will accelerate."
Indeed, as an illustration of how big a problem this is, it is likely the carbon lost from British soils since 1990 will have completely wiped out any reductions the country might have made through technological gains over the same period.
In the microbes
The National Soil Inventory of England and Wales is a remarkable project; there is nothing to match it for its scale anywhere else in the world.
At its beginnings in 1978, almost 6,000 soil locations were sampled at various depths down to 15cm. Over the intervening years something like 40% of these sites have been re-sampled and their chemistry analysed in detail.
Professor Kirk and his colleagues have been able to show that the two countries' soils have given up around 0.6% of their carbon content per annum - or just over four million tonnes in the 25 years to 2003.
And although changing how land is used - turning it from crops to woodland, for example - can change its chemistry radically, over England and Wales as a whole this did not seem to be a significant factor in controlling what was going on.
The researchers were left with only one explanation - climate change. Over the 25 years, temperatures in the two nations have risen by about 0.5C.
This, they say, would have increased the rate at which microbes in the soil could have broken down dead vegetation - wood, leaves and roots - and released its carbon content.
The scientists cannot say for sure where all the carbon has gone. Some will have been leached to deeper layers and into waterways but most of it is likely to have gone straight into the atmosphere as carbon dioxide (CO2) - the chief gas thought responsible for driving higher global temperatures.
"If the mechanisms we describe are correct, this will be happening in other temperate countries," said Professor Kirk.
"And this is going to be more important in temperate countries than in the tropics because three-quarters of the world's carbon in soils is in temperate areas."
Nature magazine asked two scientists from Germany's Max Planck Institute for Biogeochemistry to assess the Cranfield work.
Detlef Schulze and Annette Freibauer said: "The scientific and political implications of the new findings are considerable.
"Further research into the carbon cycle and on reducing CO2 emissions must take full account of areas where large pools of organic carbon are stored - or are being released.
"If we intend to stabilise the climate, such areas require much more serious consideration."