The first evidence that millions of tonnes of a greenhouse gas 20 times more potent than carbon dioxide is being released into the atmosphere from beneath the Arctic seabed has been discovered by scientists.
Details of preliminary findings suggest that massive deposits of sub-sea methane are bubbling to the surface as the Arctic region becomes warmer and its ice retreats.
Underground stores of methane are important because scientists believe their sudden release has in the past been responsible for rapid increases in global temperatures, dramatic changes to the climate, and even the mass extinction of species.
Scientists aboard a research ship that has sailed the entire length of Russia's northern coast have discovered intense concentrations of methane - sometimes at up to 100 times background levels - over several areas covering thousands of square kilometres of the Siberian continental shelf.
In the past few days, the researchers have seen areas of sea foaming with gas bubbling up through "methane chimneys" rising from the sea floor.
They believe the sub-sea layer of permafrost, which has acted like a "lid" to prevent the gas from escaping, has melted away to allow methane to rise from underground deposits formed before the last ice age.
They calculate that the amount of methane being released from this area of the Arctic could easily match the entire emissions from the rest of the world's oceans and have warned the rise is likely to be linked with the rapid warming the region has experienced in recent years.
Methane is about 20 times more powerful as a greenhouse gas than carbon dioxide and many scientists fear its release could accelerate global warming in a giant positive feedback where more atmospheric methane causes higher temperatures, leading to further permafrost melting and the release of yet more methane.
The amount of methane stored beneath the Arctic is calculated to be greater than the total amount of carbon locked up in global coal reserves so there is intense interest in the stability of these deposits as the region warms at a faster rate than other places on earth.
Orjan Gustafsson of Stockholm University in Sweden, one of the leaders of the expedition, described the scale of the methane emissions in an email exchange sent from the Russian research ship Jacob Smirnitskyi.
"An extensive area of intense methane release was found," said Dr Gustafsson. "At earlier sites we had found elevated levels of dissolved methane.
"Yesterday, for the first time, we documented a field where the release was so intense that the methane did not have time to dissolve into the seawater but was rising as methane bubbles to the sea surface. These 'methane chimneys' were documented on echo sounder and with seismic [instruments]."
At some locations, methane concentrations reached 100 times background levels. These anomalies have been seen in the East Siberian Sea and the Laptev Sea, covering several tens of thousands of square kilometres, amounting to millions of tonnes of methane, said Dr Gustafsson.
"The conventional thought has been that the permafrost 'lid' on the sub-sea sediments on the Siberian shelf should cap and hold the massive reservoirs of shallow methane deposits in place.
"The growing evidence for release of methane in this inaccessible region may suggest that the permafrost lid is starting to get perforated and thus leak methane ... The permafrost now has small holes. We have found elevated levels of methane above the water surface and even more in the water just below. It is obvious that the source is the seabed."
Preliminary findings of the International Siberian Shelf Study 2008, being prepared for publication by the American Geophysical Union, are being overseen by Igor Semiletov of the Russian Academy of Sciences.
Since 1994, he has led about 10 expeditions in the Laptev Sea but during the 1990s he did not detect any elevated levels of methane. However, since 2003 he reported a rising number of methane "hotspots".
Dr Semiletov has suggested several possible reasons why methane is now being released from the Arctic, including the rising volume of relatively warmer water being discharged from Siberia's rivers due to the melting of the permafrost on the land.
The Arctic region as a whole has seen a 4C rise in average temperatures over recent decades.
THE ULTIMATE GAS LEAK
There are two significant facts about methane in terms of global warming.
It is about 20 times more potent as a greenhouse gas than carbon dioxide, and there are massive stores of it locked away under the permafrost of the Northern Hemisphere.
Methane is produced naturally by the decay of water-logged vegetation.
Over thousands of years it has accumulated under the ground at northern latitudes and has effectively been taken out of circulation by the permafrost acting as an impermeable lid.
What makes methane so potentially dangerous is that its release from under the now-leaking permafrost could accelerate global warming, which in turn would speed the melting of the permafrost and release even more methane.
Scientists believe this has happened in the geological past with devastating consequences for the global climate and life.
Like carbon dioxide, average methane concentrations in the atmosphere have risen significantly since the Industrial Revolution, increasing from about 700 parts per billion (ppb) in 1800 to about 1790ppb today. Much of this increase is down to human activities, notably oil and gas exploration, and agriculture.
For the past 10 years, average global methane concentrations have levelled out, probably because of improvements in Russian oil and gas exploration.
However, for the first time in more than a decade, scientists recorded an increase in global methane in 2007 and are set to measure a further increase this year.
The good news about methane is that it is quickly degraded in the environment, with an average lifetime of about 12 years, compared to the 100 years of carbon dioxide.
The bad news is that we do not understand how the methane stores in the north will behave as the region experiences more extensive thaws.
The fear is that the amounts released will make global warming far worse than expected.