In the last 200 years, atmospheric methane has more than doubled due to decomposing organic materials in wetlands and swamps and human aided emissions from gas pipelines, coal mining, increases in irrigation and livestock flatulence.

However, there is another source of methane, formed from decomposing organic matter in ocean sediments, frozen in deposits under the seabed.

"We understand that other greenhouse gases apart from carbon dioxide are important for climate change today," said Gavin Schmidt, the lead author of the study and a researcher at NASA's Goddard Institute for Space Studies in New York, NY and Columbia University's Center for Climate Systems Research. "This work should help quantify how important they have been in the past, and help estimate their effects in the future."

The study will be presented on December 12, 2001, at the American Geophysical Union (AGU) Fall Meeting in San Francisco, Calif.

Generally, cold temperatures and high pressure keep methane stable beneath the ocean floor, however, that might not always have been the case.

A period of global warming, called the Late Paleocene Thermal Maximum (LPTM), occurred around 55 million years ago and lasted about 100,000 years. Current theory has linked this to a vast release of frozen methane from beneath the sea floor, which led to the earth warming as a result of increased greenhouse gases in the atmosphere.

A movement of continental plates, like the Indian subcontinent, may have initiated a release that led to the LPTM, Schmidt said. We know today that when the Indian subcontinent moved into the Eurasian continent, the Himalayas began forming.

This uplift of tectonic plates would have decreased pressure in the sea floor, and may have caused the large methane release. Once the atmosphere and oceans began to warm, Schmidt added, it is possible that more methane thawed and bubbled out. Some scientists speculate current global heating could eventually lead to a similar scenario in the future if the oceans warm substantially.

When methane (CH4) enters the atmosphere, it reacts with molecules of oxygen (O) and hydrogen (H), called OH radicals. The OH radicals combine with methane and break it up, creating carbon dioxide (CO2) and water vapor (H2O), both of which are greenhouse gases. Scientists previously assumed that all of the released methane would be converted to CO2 and water after about a decade.

If that happened, the rise in CO2 would have been the biggest player in warming the planet. But when scientists tried to find evidence of increased CO2 levels to explain the rapid warming during the LPTM, none could be found.

The models used in the new study show that when you greatly increase methane amounts, the OH quickly gets used up, and the extra methane lingers for hundreds of years, producing enough global warming to explain the LTPM climate.

"Ten years of methane is a blip, but hundreds of years of atmospheric methane is enough to warm up the atmosphere, melt the ice in the oceans, and change the whole climate system," Schmidt said. "So we may have solved a conundrum."

Schmidt said the study should help in understanding the role methane plays in current greenhouse warming.

"If you want to think about reducing future climate change, you also have to be aware of greenhouse gases other than carbon dioxide, like methane and chlorofluorocarbons," said Schmidt. "It gives a more rounded view, and in the short-term, it may end up being more cost- efficient to reduce methane in the atmosphere than it is to reduce carbon dioxide."

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