Warming ocean temperatures found a third of a mile below the surface, in a dark ocean where not much lives, may not draw much attention. But this is precisely the depth where frozen pockets of methane “ice” transition from a dormant solid to a powerful greenhouse gas.
Now researchers believe subsurface warming could be causing more methane gas to bubble up off the Washington and Oregon coast. A new study shows that of 168 bubble plumes observed within the past decade a disproportionate number were seen at a critical depth for the stability of methane hydrates.
“We see an unusually high number of bubble plumes at the depth where methane hydrate would decompose if seawater has warmed,” says lead author H. Paul Johnson, professor of oceanography at the University of Washington. “So it is not likely to be just emitted from the sediments; this appears to be coming from the decomposition of methane that has been frozen for thousands of years.”
Methane has contributed to sudden swings in Earth’s climate in the past. While it is unknown what role it might contribute to contemporary climate change, recent studies have reported warming-related methane emissions in Arctic permafrost and off the Atlantic coast.
Of the 168 methane plumes in the new study, about 14 were located at the transition depth—more plumes per unit area than on surrounding parts of the Washington and Oregon seafloor.
If methane bubbles rise all the way to the surface, they enter the atmosphere and act as a powerful greenhouse gas. But most of the deep-sea methane seems to get consumed during the journey up. Marine microbes convert the methane into carbon dioxide, producing lower-oxygen, more-acidic conditions in the deeper offshore water, which eventually wells up along the coast and surges into coastal waterways.
“Current environmental changes in Washington and Oregon are already impacting local biology and fisheries, and these changes would be amplified by the further release of methane,” Johnson says.
A Lot of Gas
Another potential consequence is the destabilization of seafloor slopes where frozen methane acts as the glue that holds the steep sediment slopes in place.
