Ancient Methane Burst: More Slow Hiss than Big Burp

Scripps-led study suggests wetland growth rather than widespread melt-off caused methane release 12,000 years ago

Some 11,600 years ago, one geological period gave way to another in dramatic fashion. As the last glacial period ended, Greenland, for example, experienced a 10* C (18° F) temperature increase over a 20-year stretch.

And levels of methane, a gas highly efficient at warming the atmosphere, began to spike as well, increasing by about 50 percent in fewer than two centuries.

For years, that occurrence has been a source of concern for scientists. What if, some wondered, it had been caused by a catastrophic melt of the stores of methane frozen in Arctic permafrost or encased at the bottom of the sea? What if that meant such a giant "burp" could happen again in the midst of the planet's current warming trend? A sudden influx of methane into the atmosphere would accelerate the greenhouse effect to an extent that could exceed the worst-case climate change scenarios for the next century envisioned by scientists.

But a finding published last month by a team led by Scripps Institution of Oceanography at UC San Diego could allay those fears. An expansion of wetlands and not a large-scale melting of frozen methane deposits appears to be behind the spike.

The finding is expected to come as a relief to climate watchers concerned that a huge acceleration of global warming might have been touched off by the melting of solid methane deposits called clathrates. In addition to what a repeat of that event would mean for current global warming trends, some evidence suggests the worldwide melting of methane triggered continental shelf collapses as undersea structures became unstable.

Vasilii Petrenko, then a Scripps graduate student in the laboratory of Scripps geoscientist Jeff Severinghaus, led a multi-year analysis of ice from the vast ice sheets of Greenland. With an international team of researchers, he measured the amount of carbon-14 isotopes in methane from air bubbles trapped in glacial ice and determined that the methane spike was more chemically consistent with an expansion of wetlands than a meltdown. Wetland regions, which produce large amounts of methane from bacterial breakdown of organic matter, are known to have spread during prehistoric warming trends.

"This is good news for global warming because it suggests that methane clathrates do not respond to warming by releasing large amounts of methane into the atmosphere," said Petrenko, now a postdoctoral fellow at University of Colorado, Boulder.

Vast stores of methane clathrate exist in seafloor deposits and in permafrost. Cold temperatures and the intense pressure of the deep ocean stabilize these masses and keep methane from entering the atmosphere. Scientists have estimated that a melting of only 10 percent of the world's clathrate deposits would create a greenhouse effect equal to a tenfold increase in the amount of carbon dioxide in the atmosphere. For comparison, the warming trend observed in the last century has taken place with only a 30 percent increase of atmospheric carbon dioxide.

The research team collected what may be the largest ice samples ever for a climate change study. The researchers cut away 15 tons of ice from a site called Pakitsoq at the western margin of the Greenland ice sheet to collect the ancient air trapped within. Methane exists in low concentrations in this air and only a trillionth of any given amount contains the carbon-14 isotope that the researchers needed to perform the analysis. Levels of carbon-14, which has a half-life of 5,730 years, were too high in the methane to have come from clathrates, the researchers concluded.

"This study is important because it confirms that wetlands and moisture availability change dramatically along with abrupt climate change," said Severinghaus. "This highlights in a general way the fact that the largest impacts of future climate change may be on water resources and drought, rather than temperature per se."

The results appear in April 24 editions of the journal Science. In addition to Petrenko and Severinghaus, researchers from the Australian Nuclear Science and Technology Organisation (ANSTO), Oregon State University, the National Institute of Water and Atmospheric Research in New Zealand, the Technical University of Denmark and the Commonwealth Scientific and Industrial Research Organisation in Australia contributed to the report.

—Robert Monroe

May 2009