Scientists have found evidence that during past periods of global warming, methane gas was released from the ocean floor. They found fossils from bacteria that fed only on methane that lived on the ocean floor near Santa Barbara 70,000 to 12,000 years ago. They could only have survived if large amounts of methane were released from the bottom of the ocean at that time. Methane is one of the major greenhouse gases. What caused this methane to suddenly be released? And could it happen again?

Kai-Uwe Hinrichs of the Woods Hole Oceanographic Institution (WHOI) looked for fossil bacteria that could have lived only with high concentrations of methane, and found evidence for an abrupt, catastrophic release of methane, which had once been trapped as methane hydrate below the ocean floor. From drilling cores from the ocean bottom off the coast of southern California, they now know that large quantities of methane were released several times during the past 60,000 years, leading to global warming.

Researchers think increased bottom water temperatures could cause the release of significant amounts of methane hydrate in shallow waters. According to some estimates, there are 10,000 billion tons of methane stored beneath the ocean. Humans have only produced about 200 billion tons of the greenhouse gas carbon dioxide. If even a small part of the stored methane escapes into the atmosphere, the resulting greenhouse warming would be catastrophic. The next question is: what caused the water near the ocean floor to warm up?

“It was a surprise to find this sort of evidence,” says Hinrichs. He plans to look for similar evidence elsewhere to find out if this process happened at other locations around the world during the same period.

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