An unexplained jump in greenhouse gases since 2002 might herald a catastrophic acceleration of global warming if it becomes a trend, scientists said yesterday.

But they said the two-year leap might be an anomaly linked, for instance, to forest fires in Siberia or a freak hot summer in Europe last year rather than a portent of runaway climate change linked to human disruption of the climate system.

"There have been two years where the rise of carbon dioxide (CO2) has been faster than average," said Richard Betts, Manager for Ecosystems and Climate Impacts at Britain's Hadley Centre.

"We shouldn't get alarmist about this... If it lasted for more than about five years you'd start to get worried," he said.

Carbon dioxide levels, the main gas blamed for blanketing the planet and pushing up temperatures since the Industrial Revolution, have risen by more than two parts per million (ppm) in the past two years against a recent rate of about 1.5 ppm.

Scientists said the figures were confirmed at sites including Mauna Loa, Hawaii, west Ireland and the Norwegian Arctic island of Svalbard, about 1,300 kilometres from the North Pole. The rise was less in the southern hemisphere.

"CO2 levels are up about two ppm in the past two years - but it would be pushing it to say that it could be the start of runaway global warming," said Kim Holmen, senior scientist at the Norwegian Institute for Air Research (NILU).

The rise in the past two years is quicker than mapped out in UN projections to the year 2100 based on increased human use of fossil fuels like coal, oil or gas. Higher temperatures could trigger everything from desertification to rising sea levels.

On Svalbard, CO2 levels have varied this year from 365-385 ppm, Mr Holmen said. The level is lowest in summer, when plants absorb CO2 as they grow. Organisms from plants to animals emit CO2 when they breathe and the oceans and soil also trap CO2.

A background fear is that extra human emissions, by cars, factories and power plants, may be blunting the planet's ability to absorb CO2. In the worst case, that could lead to a runaway warming.

"These results are deeply worrying, and indicate that the battle against global climate change could be even more pressing than was previously thought," echoed Cathrine Pearce, Friends of the Earth International's climate campaigner.

"It's a worrying sign," said Steve Sawyer, climate policy director at environmental group Greenpeace.