PARIS (AFP) - Tropical forest soil warmed in experiments to levels consistent with end-of-century temperature projections released 55 per cent more CO2 than control plots, exposing a previously underestimated source of greenhouse gas emissions, researchers reported on Wednesday (Aug 12).

Before humanity began loading the atmosphere with carbon pollution by burning fossil fuels, the input and outflow of CO2 into soil - one key element in Earth's complex carbon cycle - remained roughly in balance.

Gases emitted by deadwood and decaying leaves, in other words, were cancelled out by microorganisms that feed on such matter.

But climate change has begun to upset that balance, according to a new study, published in Nature.

"Carbon held in tropical soils is more sensitive to warming than previously recognised," lead author Andrew Nottingham, a researcher at the University of Edinburgh's School of Geosciences, told Agence France-Presse.

"Even a small increase in respiration from tropical forest soils could have a large effect on atmospheric CO2 concentrations, with consequences for global climate."

The quantity of carbon cycling each year through soils worldwide is up to 10 times greater than human-generated greenhouse gas emissions.

Just a one-percent imbalance - with more carbon going out than in - "would equal about 10 per cent of global anthropogenic (manmade) carbon emissions," noted Eric Davidson, a researcher at the University of Maryland Center for Environmental Science.

Earth's average surface temperature has risen just over one degree Celsius above pre-industrial levels, enough to boost the severity of droughts, heatwaves and superstorms made more destructive by rising seas.

But the increase in temperatures over land alone - excluding oceans, which cover 70 per cent of the planet - has been nearly 2 deg C, or double the global average.

CARBON 'SINK' TO 'SOURCE'

In the experiments, Nottingham and colleagues placed heating rods in a one-hectare plot of undisturbed primary forest on Barro Colorado Island, Panama.

They warmed the soil to a depth of just over one metre by 4 deg C over a period of two years.

Soil temperature is usually about a degree warmer than air temperature.

While such experiments have been conducted in higher latitude forests, none had been carried out up to now in the tropics.

Climate models seeking to take into account the potential carbon leakage from soil due to rising temperatures have relied on theoretical calculations that underestimate outputs compared to the field tests reported in Nature.

Extrapolating from the new findings, the study estimates that if all the world's tropical soils warmed by 4 deg C for a two-year period some time before 2100, it would release 65 billion tonnes of carbon - equivalent to about 240 billion tonnes of CO2 - into the atmosphere.

"That is more than six times the current annual emissions from human-related sources," Nottingham said.

"This could be an underestimation, because we might see large continued loss beyond the two years in our experiment."

Nor are deeper stores of carbon - below two metres - taken into account, he added.

No sweeping conclusions can be drawn on the basis of a single experiment, the researchers caution.

"But the study adds to recently accumulating evidence that tropical forests are unlikely to continue indefinitely to be carbon sinks as the world warms," said Davidson, who was not among the study's authors.

Up to now, tree cover and the ocean have together consistently absorbed about half of the excess carbon emissions from human activity, but there are signs that some forests may be experiencing CO2 fatigue.

Stored CO2 is also released when trees are cut down.

Last year, a football pitch of primary, old-growth trees was destroyed every six seconds, about 38,000 square kilometres in all, according to Global Forest Watch.