for National Geographic News
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.
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.
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