"Understanding the behaviour of global atmospheric methane is important because it's the third strongest greenhouse gas after carbon dioxide and water vapour," says biogeochemist Dr. Michael Whiticar, part of the Canada-U.S. team that conducted the study. "Atmospheric methane concentrations have increased about 250 per cent in the last 250 years, and they continue to rise about one per cent a year."



Methane in the Earth's atmosphere is an important greenhouse gas with a global warming potential of 25 over a 100-year period. This means that a methane emission will have 25 times the impact on temperature of a carbon dioxide emission of the same mass over the following 100 years. Methane has a large effect for a brief period (about 10 years), whereas carbon dioxide has a small effect for a long period (over 100 years). Because of this difference in effect and time period, the global warming potential of methane over a 20 year time period is 72.

The Earth's methane concentration has increased by about 150% since 1750, and it accounts for 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases.

The team, which included PhD student Hinrich Schaefer, studied the concentration and carbon isotope fingerprints of the methane in the ice off Pakitsoq, Greenland. The technique enabled researchers to investigate the changes in sources and sinks of atmospheric methane - perhaps analogous to today's rapid rise in the Pleistocene and Holocene ages.

"Methane is a gas that makes a significant contribution to global warming but has gone largely unnoticed by the public and some policy-makers," says Schaefer, now pursuing postgraduate studies at Oregon State University. "Its concentration has more than doubled since the industrial revolution, from things like natural gas exploration, landfills and agriculture. We need to know whether rapid increases of methane in the past have triggered global warming or have just been a reaction to it."

According to Whiticar, the study shows that massive destabilization of methane hydrates along the oceanic shelves and the corresponding release of methane to the atmosphere can't be responsible for detonating the rise in greenhouse gases 12,000 years ago.

"Our work supports the belief that wetlands and permafrost layers are responsible for the enhanced sources of methane to the atmosphere. This is critical knowledge because again, today, we see rapid retreating of our northern permafrost boundaries due to Arctic warming."

The findings of the team's research were featured in an article in the Aug. 25 edition of the international journal Science. Funding for the Canadian aspects of the work was provided by the Canadian Foundation for Climate and Atmospheric Sciences, the Natural Sciences and Engineering Research Council and the Canada Foundation for Innovation, with collaborations with Oregon State University, the University of Colorado, and the Scripps Institution of Oceanography at the University of California at San Diego.