Since time immemorial, methane and oil have seeped from beds buried beneath the ocean sediment off the California coast. The methane bubbles up and out of the sea, adding to the store of greenhouse gases in the earth's atmosphere. The oil floats up as well and, over time, breaks down into tar that settles back into the marine sediment layer. Now new research into that tar residue seems to show that such seeps are influenced by ocean temperatures, and therefore by the very global warming they help to engender.

Tessa Hill of the University of California, Davis, and her colleagues studied the amount of tar in samples taken from the seafloor of the Santa Barbara channel off the California coast. Because the oil and gas escape at the same time, measuring tar levels in such samples provides a good estimate of how much methane was released. Digging into the sediment to a time corresponding to the last 32,000 years, the scientists found that tar levels peaked at the same time as global temperatures: roughly 16,000 to 14,000 years ago and 11,000 to 10,000 years ago. "These findings indicate that petroleum seeps are 'activated' during time periods of warming, and increase their seepage of methane and oil," Hill says.

Currently, such seeps worldwide account for the release of 30 teragrams (roughly 33 million tons) of methane every year, or roughly 15 percent of the natural sources of greenhouse gas. But tar levels during warming periods were as much as three times higher than at present, meaning methane releases could potentially triple. Because methane is a potent heat-trapping gas, this increasing seepage could result in even greater warming--another positive feedback loop not unlike the loss of albedo (reflectivity) at the earth's poles as their snow cover melts.

Hill and her team suspect that the effect has to do with melting methane hydrates--ice lattices that imprison the gas--that ultimately release the hydrocarbons trapped within and beneath them. The hydrates are stable at lower temperatures and lower pressures as well as at moderate temperatures and higher pressures, Hill explains, but they may not remain stable as the ocean continues to warm. The paper presenting the finding is published online this week in Proceedings of the National Academy of Sciences.