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Global Warming Feedback Loop Caused by Methane, Scientists Say

Elizabeth Svoboda
for National Geographic News
August 29, 2006
 
In the ongoing debate over global warming, climatologists usually peg
carbon dioxide as the most dangerous of the atmosphere's heat-
trapping gases.

But methane, a greenhouse gas 20 times more potent than carbon dioxide, might be even more problematic.

According to Tessa Hill, a geologist at the University of California, Davis, more methane is released into the atmosphere from ocean deposits during periods of warming than previously thought.

This expelled methane increases temperatures and releases more methane, creating a positive feedback loop.

The research appears tomorrow in the early online edition of the Proceedings of the National Academy of Sciences.

Ocean Deposits

To assemble her portrait of methane's historical effects on Earth's climate, Hill analyzed samples of preserved tar from ancient undersea rock layers off the California coast (map of California).

"Oil and gases are constantly seeping out of the seafloor and bubbling through the water column, releasing methane into the atmosphere," she explained.

Oil slicks are left on the ocean's surface during this process. As the oil gradually evaporates, a thick black tar remains behind and sinks, becoming a part of ocean-bottom sediments.

Measuring the amount of tar in a sediment layer is a convenient way to assess how much methane seepage took place during a period in climatic history.

Hill found that tar levels in the sediments peaked at two critical intervals: from 14,000 to 16,000 years ago, and from 10,000 to 11,000 years ago.

These correspond to the most dramatic warming periods in Earth's recent history, when glacial eras gave way to more moderate temperatures.

"These petroleum seeps appear to be activated by periods of climate change," Hill said. "If the Earth is already in a mode of warming, they 'turn on' and become more active, which promotes further warming."

Methane hydrate—a solid form of methane embedded in glacier deposits—is the catalyst that touches off this warming cycle, Hill believes.

As glaciers begin to melt, methane hydrate is expelled from lattices of frozen water molecules.

"When methane hydrate is released, you have a lot of sediment movement. Landslides can occur, and you get big pockmarks in ocean sediments where bubbles of methane come out," Hill said.

"When you cause that much disturbance, you create an avenue for more petroleum-derived methane and oil to come to the surface."

(Related story: "Plants Exhale Methane, Add to Greenhouse Effect, Study Says" [March 2006].)

Catastrophic Results

In the past, geologists have proposed that much of the methane in the atmosphere is a result of emissions from swampy wetland areas such as the Florida Everglades, located on the southern tip of the peninsula (map of Florida).

But if large amounts of unaccounted-for methane begin emanating from ocean sources during the current warming period, the effects could be catastrophic, says Arlene Fiore, a physical scientist with the National Oceanic and Atmospheric Administration in Princeton, New Jersey.

"Methane is a very potent greenhouse gas, so the climatic implications of adding more of it to the atmosphere are grave," Fiore said. "A massive methane release could also affect the atmosphere's ability to cleanse itself."

Typically, pollutants such as nitrogen dioxide and carbon monoxide are neutralized when they bind to hydroxyl (OH) molecules that occur naturally in the lower atmosphere, Fiore says.

But if too much methane were released within a short period of time, it could bond with many of those molecules, leaving fewer to mitigate the effects of other emissions.

(See National Geographic magazine's "Global Warning: Signs From Earth.")

Despite methane's potential to accelerate global warming, the University of California's Hill says that no reliable cleanup strategy has yet been developed.

"We can't control this particular source of methane," she said. "The wiser thing to do would be to think about controlling our overall greenhouse gas emissions. There's a lot more carbon dioxide than methane in the atmosphere."

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