Trees suck in carbon dioxide (CO2) as part of the natural process of respiration.

So, by such thinking, if Kyoto signatories plant lots and lots of forests, they create wonderful sponges which absorb the dangerous climate-altering gas.

But what about this: What if trees in addition to taking in CO2 also emit a greenhouse gas of their own?

That scenario is sketched in a new study by European scientists, which, if confirmed, would be one of the biggest upheavals in climate science for years.

It would also inflict a serious blow to Kyoto, one of whose key pillars is the faith in "sinks," as forests are called in the treaty's jargon.

Until now, the mainstream belief is that atmospheric methane chiefly comes from bugs: from bacteria working in wet, oxygen-less conditions, such as swamps and rice paddies.

But in a study published on Thursday in Nature, a team led by Frank Keppler of the Max Planck Institute in Heidelberg, Germany, found that living plants, as well as dried leaves and grass, emitted methane in the presence of air.

Nor is this gas just a piffling amount.

The researchers roughly estimate the world's living vegetation emits between 62 and 236 million tonnes of methane per year, and plant litter adds one to seven million tonnes.

This would be equivalent to between 10 and 30 percent of all annual global emissions of methane.

The evidence comes from a series of carefully controlled experiments in the lab and in the field, in which gas chromatography and sensors to monitor carbon-13 isotopes detected and measured methane flows from the vegetation.

The ambient atmosphere was first stripped of background methane before being pumped into enclosed tanks surrounding the plants and leaves in order to get a better chance of spotting the vegetal gas emissions.

Levels of methane were "very temperature sensitive," with concentrations approximately doubling with every 10 C (18F) rise in temperature in a range between 30 and 70 C (86-158 F), a phenomenon that suggests that breakdown by enzymes is not the cause.

In a review of the study, New Zealand atmospheric scientist David Lowe said the findings were a surprise but in fact could explain a nagging puzzle.

Between 1990 and 2000, satellite monitors had detected a slowing of methane flows to the atmosphere by around 20 million tonnes a year.

The cause for this may have been the dramatic rate of deforestation during the same period, Lowe suggested. From 1990-2000, more than 12 percent of the world's tropical forests were hacked down.

Added to this is the anecdotal data from satellite sensors, which have occasionally spotted inexplicably large plumes of methane over old tropical forests, said Lowe.

The study does not seek to explain exactly how the methane is emitted, nor suggest which plant species may emit more than others.

Nor does it challenge scientific opinion on global warming, which has become rock-hard over the past five years and is now questioned only by a small minority.

The consensus is that the global warming is a fact and may already be affecting Earth's climate, and the big culprit is the billions of tonnes of CO2 spewed out by burning oil, gas and coal.

The paper's earliest impact could be political, for it attacks one of Kyoto's conceptual cores.

Under the protocol's notoriously complex rulebook, industrialised signatories that plant forests can offset the supposed benefit against their national quotas of CO2.

The "sink" mechanism hobbled efforts to complete Kyoto in 2001 as Russia, Japan and Canada demanded concessions for their forest industries.

Ironically, "sinks" were initially demanded by the United States under the Clinton adminstration in order to save costs for the oil-dependent US economy.

President George W. Bush then abandoned Kyoto in March 2001, in one of his first acts in office.

Scientists have frequently shaken their heads at the perceived benefits of forests in the global warming equation.

Previous research has already suggested that CO2 storage goes into reverse when a forest matures and its older trees die and rot, surrendering their carbon to the air.

Now doubts over "sinks" have been strengthened, which could mean the extraordinarily bedevilling issue could be opened up again. Negotiations on Kyoto's commitments, after 2012, are due to start by May and are expected to last several years.

"We now have the spectre that new forests might increase greenhouse warming through methane emissions rather than decrease it by being sinks for CO2," Lowe said ruefully.

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