Global methane levels have jumped by almost 28 million tonnes in the past two years, and could accelerate the rate of climate change, according to new research.
The methane spike, which began in mid-2006, appears to be generated by the rapid melting of ice across the Arctic circle, driven by hotter than average summer temperatures.
''It's not good news for future global warming. It's a big jump and we don't know if it will continue,'' CSIRO senior climate scientist Paul Fraser said.
A study published online today by the American Geophysical Union says after almost a decade of near-zero growth in atmospheric methane concentrations, levels have now risen to 5.6 billion tonnes. Dr Fraser, who co-authored the study, said global data collected by CSIRO and other research institutions suggested the increase was chiefly due to methane releases in the high latitudes of the northern hemisphere.
Warmer temperatures and flooding caused by the ice melt were causing methane to bubble up from bacteria feeding on organic material on the bottom of lakes and wetlands.
Northern latitudes were ''renowned for their temperature fluctuations'', and a cooler summer could see a drop in methane levels, he said. ''We think this recent increase is coming from wetlands, with rapid Arctic ice melting creating more high latitude wetland areas. At this stage we don't think it's caused by thawing permafrost.
''If it was linked to permafrost, that's a very different situation with serious consequences, because an increase in methane levels from thawing permafrost would continue, even if the next Arctic summer was cooler.''
Methane is the second most important greenhouse gas after carbon dioxide, but methane emissions have 25 times the impact on temperature as carbon dioxide.
Methane concentrations in the Earth's atmosphere have increased by 150 per cent since 1750, and are estimated to account for 20 per cent of global warming.
Dr Fraser said the methane increase wasn't linked to emissions from forest fires or cattle exhaling methane as a product of digestion.
''We've ruled out forest fires as a source because there weren't any big biomass burning events during this period .''
As for cows, they tended to be concentrated in mid-latitudes and the world's cattle population had remained fairly consistent in recent years.
''There aren't too many cows inside the Arctic circle, and there'd have to be a sudden increase in cow numbers to account for the rise in global methane levels we've measured.''
Dr Fraser said an additional cause of the increase could be a weakening of atmospheric oxidation, possibly linked to ozone depletion in the Earth's atmosphere. ''The reality is that scientists have only a very basic understanding of these methane variations,'' he said.
A recent ocean survey by scientists from the University of Sweden reported ''whole fields of methane bubbles'' in the east Siberian Sea, with methane levels in the water up to 10,000 times higher than normal.