Study identifies Arctic lakes as a major source of prehistoric methane

Researchers from the University of Alaska-Fairbanks and the University of Southampton have identified Arctic lakes as a likely source of increased levels of atmospheric methane as temperatures rose towards the end of the last ice age.

Previous studies of ice cores from Greenland and Antarctica have shown that when global climates warmed between 14,000 and 11,500 years ago, levels of methane in the atmosphere increased significantly. There was also an unidentified northern source of the gas at that time.

According to new research published in the latest edition of the journal Science (26 October), methane bubbling from Arctic lakes could have been responsible for up to 87 per cent of this northern source. The findings could help climate modellers to assess how warming affects atmospheric levels of methane, a potent greenhouse gas.

Previous hypotheses suggested that the increase came from gas hydrates or wetlands. The new study’s findings indicate that methane bubbling from thermokarst lakes, which are formed when permafrost thaws rapidly, is likely to be a third and major source.

"Our research focused on areas of Siberia and Alaska which, during the last ice age, were dry landscapes underlain by deep permafrost," commented co-author Professor Mary Edwards of the University of Southampton’s School of Geography. "As the climate warmed, the permafrost thawed, forming thermokarst lakes. When the permafrost around and under the lakes thawed, it would have released organic material into the lakes – primarily dead plant material – which would be a good energy source for methane-producing bacteria. Under such conditions, carbon which had been locked in the ground for thousands of years could rapidly be converted into potent greenhouse gases: methane and carbon dioxide."

The group’s calculations suggest that the lakes contributed 33 to 87 per cent of the methane increase from northern sources. The team examined lakes in Siberia and northern Alaska that currently release methane. They gathered samples of permafrost and thawed them in the laboratory to measure the levels of methane that permafrost soil can produce immediately after thawing. Using geological data they were able to reconstruct the pattern of lake formation since the end of the last ice age.

"Thermokarst lake formation is a source of atmospheric methane today, but it was even more important during the warming that took place at the end of the ice age," continues Professor Edwards. "It is possible that, with global warming, large releases from these lakes may occur again in the future."