It was big news: One of the largest potential contributors to global warming and ocean acidification is potential no longer, and hasn’t been for decades. A paper published last month in Geophysical Research Letters by University of Washington researchers shows that it’s taking place right now, off the coast of Washington and presumably other seacoasts around the world.
Thanks to warming sea water, methane hydrates (a.k.a. clathrates) — that's methane encased in frozen water molecules and trapped in seafloor sediment a third of a mile deep — are melting and releasing large plumes of a greenhouse gas about 20 times more potent than carbon dioxide.
Researchers have, for years, viewed methane deposits in Arctic sediments and permafrost as climate time bombs, But they neglected deposits at more temperate latitudes. Until last year only three methane seeps had been confirmed below the shelf break on the entire Atlantic seabord.
The deposits are neglected no more. Last year another group of scientists identified 570 seeps along the north Atlantic coast. The UW researchers calculate that each year the deposits off Washington alone release as much methane as the amount that escaped in the sensational Deep Horizon oil well blowout in the Gulf of Mexico five years ago.
And the phenomenon is not confined to Washington’s waters. The amount of methane frozen in various ocean sediments varies according to the amount of biomass, from algae to whales, teeming in the waters above. When these organisms die, they sink to the bottom, there to be consumed by bacteria that release methane and carbon dioxide in the digestive process, just as you and I do.
But if warming waters are melting the top layer of what’s called the “methane reservoir” along this coast, they’re surely doing the same to mid-latitude sediments around the world. It’s a big discovery with big implications for climate and ocean health. And but for an inquisitive Seattle fisherman and a diligent undergraduate, we still wouldn’t know about it.
Five years ago, Peter Lathourakis was in his second season fishing for hake off Washington’s continental margin when he noticed mysterious clouds of something moving on his sonar fish finder. At first he thought the clouds were alive: “I said, Jesus Christ, that’s a lot of fish!” He quickly realized it wasn’t fish.
Lathourakis wasn’t the first fisherman to spot the plumes. “Apparently fishermen have known about these methane hydrate plumes for a while,” he says. Some guessed that they were gas plumes. Others didn’t. “I’ve seen a few fishermen go over them and try to fish the bubbles," says Lathourakis. "At least one I talked to said he nearly lost his net because of the buoyancy.”
The clouds excited Lathourakis for a different reason: He’s a geology nerd. He snapped a photo of the fish-finder image with his cellphone and sent it to UW’s geology department, hoping someone there could tell him more about the plumes. “The [department's] secretary didn’t know what to do with it and passed it to me,” recalls UW marine geologist Paul Johnson. “She thought it looked like a thermal vent” — an undersea fissure in the earth’s crust where geothermally heated water and other gases steam out. Johnson knew better.
He called Lathourakis. “That’s really interesting," he told Lathourakis. "Do you have more?” Lathourakis continued snapping photos as he passed over the plumes, eventually sending images and coordinates for about 30 of them.
Johnson had a research cruise coming up and was too busy to follow up on the plumes. But an undergraduate had just started working in his lab, so he delegated. "I said, ‘As a project, why don’t you download all the CTD casts for that area.'” CTD refers to the trio of conductivity (a measure of salinity), temperature and depth. These most basic physical characteristics of seawater are taken routinely on research cruises.
The undergraduate in question was then-sophomore Una Miller (below). She gamely downloaded 6,836 casts (each set of readings is called a cast) from NOAA’s database. That's a huge data set, but one loaded with what Johnson calls “a lot of garbage,” such as readings taken at the wrong depths. Miller and another student winnowed the set down to about 3,000 solid readings and built what Johnson calls a “really striking” four-dimensional model of the results.
“We said, ‘Oh, let’s look for temperature trends,’” recalls Miller. “We thought they might not be strong enough to be picked up in data. I was a little surprised to see such a clear signal emerge from what was just a lot of numbers.”
Five hundred meters below the surface, at the methane-hydrate margin, the water was clearly warming, at the rate of about seven-thousandths of a degree Celsius per year. That may not sound like much, but it adds up to four-tenths of a degree Celsius, nearly one degree Fahrenheit, over the 43 years tallied. And that’s enough to melt the hydrates.
The model the researchers developed shows that the warming effect extends 30 meters into the sediment, destabilizing the hydrates 13 meters deep along a kilometer-wide horizontal stretch. If that trend continues, Washington’s offshore sediments could release as much as 36 million tons of methane by 2100.
So far only a small share of that methane — Johnson estimates between 1 and 10 percent — has reached the surface and contributed directly to greenhouse warming. Much of it gets oxidized by bacteria on the way up, a process that consumes available oxygen and contributes to acidification in a part of the ocean where pH is already falling rapidly.
Dire though these effects are, the new findings leave us better off in one way: Scientists who’ve focused on the ocean’s lower depths and volatile surface will no longer neglect the waters and sediments in between, where a clear and present peril lies.
