Did Planetary "Belch" Cause Prehistoric Warming?

Hillary Mayell
for National Geographic News
December 27, 2001
Around 55 million years ago the Earth belched a massive bubble of
methane gas from beneath the sea floor, causing a 100,000-year period of
global warming, report NASA scientists.

The study suggests that the expulsion of methane trapped in the seafloor bed all those years ago caused the planet's temperature to rise by up to 13 degrees Fahrenheit (7 degrees Celsius). The resulting thousand-century period of global warming is called the Late Paleocene Thermal Maximum.

Could it happen again today? Computer models at least raise the possibility that if the oceans warm substantially a similar scenario could arise in the future.

"The study makes you take a broader view of climate change, looking at greenhouse gases other than carbon dioxide, like methane, ozone, black soot particles, chlorofluorocarbons, and so on as an explanation for global warming," said Gavin Schmidt, the lead author of the study. Schmidt is a researcher at NASA's Goddard Institute for Space Studies in New York and Columbia University's Center for Climate Systems Research.

The study used computer models to better understand the role of methane in climate change. Methane is a greenhouse gas that is 20 times more potent as a heat-trapping gas than carbon dioxide, the greenhouse gas getting most of the attention as the major culprit of global warming. The amount of atmospheric methane has more than doubled in the last 200 years.

"Rice paddies and other massive irrigation projects are a huge source of methane right now," said Schmidt. Other sources include livestock flatulence, emissions from gas pipelines, coal mining, and to a lesser extent, the natural decomposition of organic materials in wetlands and swamps.

Late Paleocene Thermal Maximum

Methane is also formed in a crystalline form as organic matter sinks to the ocean floor and is frozen in deposits under the seabed. Generally, cold temperatures and high pressure keep methane stable beneath the ocean floor. The scientists believe that 55 million years ago the movement of continental plates could have disturbed the status quo.

"We know that when the Indian subcontinent moved into the Eurasian continent, the Himalayas began forming. The shifting of tectonic plates would have decreased pressure in the sea floor, and may have caused the large methane release," said Schmidt. Once the atmosphere and oceans began to warm, Schmidt added, it is possible that more methane thawed and bubbled out.

Other hypotheses proffered by scientists to explain the Late Paleocene Thermal Maximum include an extraterrestrial effect from comet impacts and extreme volcanism.

"It's difficult to pin down exactly what happened because we don't really know much about global ocean circulation at the time; the role atmospheric chemistry played, and other factors. The neat thing about this particular study is that you can take a hypothesis, and work it all the way through, discarding hypotheses that are completely out to lunch," said Schmidt. "The methane explosion hypothesis answers a lot of questions."

Warming Oceans

Most of the methane frozen under the seafloor occurs along the continental margins; "the water can't be too deep or too shallow—a lot of methane hydrate deposits are under water 1,000 to 2,000 meters deep, where the water exerts an incredible amount of pressure," said Schmidt.

Could global climate change warm the oceans enough to change the methane from the crystalline form back to a gas that erupts from the belly of the ocean? Is there a threshold effect, where in waters below 4 degrees Celsius the deposits remain stable and above 4 degrees Celsius they become unstable? What would be the effect of a slow leak of frozen methane? Understanding the role methane plays in current greenhouse warming could have broad policy implications.

"We can't get too fixated on just one gas like carbon dioxide when we think about global warming. It's complicated and surprises happen—they happened in the past and they can happen now. We need to look at climate change from a much broader perspective," said Schmidt.

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