Methane bubbles are coming up from ocean vents off the Washington and Oregon coast, and a new study identified warming ocean temperatures one-third of a mile below the surface as likely responsible.
Frozen methane at this depth is believed to transform from a dormant solid to methane gas, a known major greenhouse that has been contributing to climate change.
Presented in Geochemistry, Geophysics, Geosystems, the study found that 168 bubble plumes have been detected in the past 10 years, a disproportionate count of which were found at a critical depth for methane hydrates’ stability. Some 14 were situated at the transition depth, where there are more plumes per unit area than on surrounding areas of the seafloor.
According to lead author and oceanography expert H. Paul Johnson, they saw an unusually high number of bubble plumes where methane hydrate would decompose with seawater warming.
"So it is not likely to be just emitted from the sediments,” he said, saying it seems to emerge from decomposing methane that has stayed frozen for millenniums.
The research sought evidence of bubble plumes off the coast, such as based on research cruise observation, earlier studies, and reports from local fisherfolk. Included in the data are bubble plumes that emerged at least 490 feet tall and clearly originating from the seafloor. Fishing boats first detected some 45 plumes, as their modern sonars chanced upon while seeking schools of fish.
Methane, a common fuel source, is a greenhouse gas. While it does not stay as long in the atmosphere as carbon dioxide, it is deemed initially far more damaging due to its effective heat absorption.
The Environmental Defense Fund (EDF) warned that post-release, methane is 84 times more powerful than carbon dioxide, and that 25 percent of the man-made global warming experienced today is from methane emissions.
The methane bubbles act as a potent greenhouse gas, too, if they emerge all the way up and reach the surface, although the study said they seem to be consumed on the way up, with marine microbes converting the gas into carbon dioxide and producing conditions that are lower-oxygen and more acidic in the deeper offshore water.
Climate change-related methane emissions have also been reported off the Atlantic coast and in Arctic permafrost.
Johnson said that further methane release could exacerbate the current environment effects in Washington and Oregon on local fisheries and biology, such as seafloor slopes – where gluing the sediment slopes in place is frozen methane – becoming destabilized.
A study last year from the University of Washington team calculated that methane hydrate decomposition will potentially release about 0.1 million metric tons of methane annually into the sediments off the coast of Washington at the rate the ocean is warming.
At present, co-author and oceanography associate professor Evan Solomon is analyzing the chemical composition of bubble plume samples emitted at about 500 meters deep off the Washington coast, seeing whether the gas comes from methane hydrates instead of other sources.
The United States government is currently tackling the methane issue. In August this year, the Environmental Protection Agency (EPA) proposed the first-ever rule of directly limiting methane emissions from the oil and gas industry, its leading sources.
