Off the West Coast of the United States, methane gas is trapped in frozen layers below the seafloor. New research from the University of Washington shows that water at intermediate depths is warming enough to cause these carbon deposits to melt, releasing methane into the sediments and surrounding water. Researchers found that water off the coast of Washington is gradually warming at a depth of 500 meters, about a third of a mile down. That is the same depth where methane transforms from a solid to a gas. The research suggests that ocean warming could be triggering the release of a powerful greenhouse gas.

"We calculate that methane equivalent in volume to the Deepwater Horizon oil spill is released every year off the Washington coast," said Evan Solomon, a UW assistant professor of oceanography. He is co-author of a paper to appear in Geophysical Research Letters.

While scientists believe that global warming will release methane from gas hydrates worldwide, most of the current focus has been on deposits in the Arctic. This paper estimates that from 1970 to 2013, some 4 million metric tons of methane has been released from hydrate decomposition off Washington. That's an amount each year equal to the methane from natural gas released in the 2010 Deepwater Horizon blowout off the coast of Louisiana, and 500 times the rate at which methane is naturally released from the seafloor.

"Methane hydrates are a very large and fragile reservoir of carbon that can be released if temperatures change," Solomon said. "I was skeptical at first, but when we looked at the amounts, it's significant."

Methane is the main component of natural gas. At cold temperatures and high ocean pressure, it combines with water into a crystal called methane hydrate. The Pacific Northwest has unusually large deposits of methane hydrates because of its biologically productive waters and strong geologic activity. But coastlines around the world hold deposits that could be similarly vulnerable to warming.

"This is one of the first studies to look at the lower-latitude margin," Solomon said. "We're showing that intermediate-depth warming could be enhancing methane release."