科学美国人60秒:Methane Plumes Bubbling Along Northwest Coast

The Northwest is famous for its microbrews. But the region’s bubbles aren’t just for beer. Scientists have found plumes of the potent greenhouse gas methane bubbling up from the seafloor off the coast of Oregon, Washington and British Columbia.

The methane comes from rotting organic matter, that is, the waste and dead bodies of land and marine organisms. “They fall into the sediments and they decompose. It’s like a compost pile.” Paul Johnson, a marine geologist at the University of Washington.

Methane leaks out of this great compost pile all along the continental margin. But Johnson says some plumes may originate in vast layers of frozen methane called hydrates. Scientists worry that rising ocean temperatures may destabilize these deposits, freeing up even more methane. And Johnson thinks he already sees evidence that it’s happening.

Over the past few years, Johnson’s team compiled a map of more than a hundred Northwest methane seeps. In a new study in the journal Geochemistry, Geophysics, Geosystems, the researchers report a spike in the number of plumes emanating from depths of about 500 meters. [H. Paul Johnson et al, Analysis of bubble plume distributions to evaluate methane hydrate decomposition on the continental slope]

That’s the shallowest depth where frozen methane is stable—and the first place scientists expect it to start bubbling out.

But Johnson says we’re not facing a climate catastrophe just yet. Most of the methane released at the seafloor gets consumed by bacteria before it can escape into the atmosphere. Unfortunately, that’s still bad news for Northwest ecosystems. When bacteria feast on methane, they also consume precious oxygen and produce carbon dioxide—which makes seawater more acidic.

“We have a bad problem out here with ocean acidification and anoxia. There are fish kills along the Oregon coast and now they are spreading up here along the Washington coast. This is making that problem worse.”

After all, these hydrates seem to be belching out lots of gas.

“And we calculated that the amount of methane that’s released from this hydrate decomposition is equivalent to the Deep Horizons spill in the Gulf of Mexico, every year.”

As warming continues, Johnson says that figure could increase by a factor of four or five. In short, the bubbly waters of the Northwest won’t go flat anytime soon.

—Julia Rosen