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Over the past five years, fishermen in the Pacific Northwest have found themselves increasingly impacted by climate change.

As the oceans absorb greater amounts of carbon dioxide from human-caused emissions, the waters off Washington and Oregon have become more acidic. That has forced some commercial fisheries to relocate to places like Hawaii or get out of the business entirely, since the acidic waters make it more difficult for mollusks, crabs, and corals to grow their shells.

And things are only likely to get worse.

Not only is climate change making the ocean more acidic, it's warming the waters, causing sea levels to rise and even helping to unlock methane deposits that have been frozen on the seafloor, which in turn makes the oceans more acidic. And those plums of methane bubbling up through the water column could exacerbate global warming.

"It means global warming has come to our waters," said Paul Johnson, a University of Washington professor of oceanography who has researched the methane plumes.

Johnson first heard about the plumes about a decade ago from fishermen who spotted them on their sonars. He immediately assumed the culprit to be methane.

Johnson's labanalyzed the water temperature about a third of a mile down where the methane was frozen below the seafloor. The researchers found the ocean temperatures had warmed 0.3 degrees Celsius in the past 44 years, suggesting the warming was causing frozen deposits of methane — known as methane hydrates — to be converted into methane gas that was rising through the water column.

"In the deep ocean, that is a big deal," Johnson said of the rate of warming.

Then this month, Johnson and his team found a disproportionate number of the 168 bubbling plumes they observed off the Washington and Oregon coasts were at critical depths where methane hydrates are found.

"What we're seeing is possible confirmation of what we predicted from the water temperatures: Methane hydrate appears to be decomposing and releasing a lot of gas," Johnson said. "If you look systematically, the location on the margin where you're getting the largest number of methane plumes per square meter, it is right at that critical depth of 500 meters."

Methane has a 25 times greater capacity to warm the atmosphere than carbon dioxide over 100 years. It the second greatest greenhouse gas emitted from the United States, coming primarily from the oil and gas industry and to a lesser degree livestock and landfills.

As global temperatures warm, scientists are concerned that methane trapped in permafrost thawing in the higher latitudes, like in Alaska and Siberia, might be released into the atmosphere. More recently, attention has turned to the oceans.

Last year, researchers reported finding hundreds of methane plumes rising from the seafloor off the Atlantic coast — though there remains a debate over whether they came from methane hydrate or another source, such as decomposing organic material.

Then there is the West Coast.

Johnson and his colleagues concluded that warming — as a result of water from global warming hotspot off Siberia that travelled with ocean currents east across the Pacific Ocean — would theoretically destabilize methane deposits on the Cascadia subduction zone, which runs from northern California to Vancouver Island.

They calculated as much as 4 million metric tons of methane has been released since 1970 from the seafloor along the Cascadia subduction zone. That is roughly 220 million pounds of methane per year from the sediments off the Washington coast or about the same amount of methane released from the 2010 Deepwater Horizon disaster and 500 times the rate that is naturally released from the seafloor.

That figure, they projected, is expected to quadruple in the decades ahead.

But, according to Gavin Schmidt, a climatologist who is the director of NASA's Goddard Institute for Space Studies, that doesn't mean much when put in the bigger context of global warming.

"These are interesting but they are minor issues," Schmidt said of the plumes. "People have detected methane seeps all over the place off the continental shelves and mostly they are in equilibrium with what is going on. So if the water is warm, you will get a little bit more or a little bit less … The overall constraints are such that it's really hard to see how in the present climate situation how it could be a big effect."

But Kevin Schaefer, a research scientist with the National Snow and Ice Data Center who studies emissions from permafrost, was not as dismissive. The challenge, he said, was that so little is known about these methane plumes including just how much is being released on a global scale.

"It's a potential risk but I'm not sure the scientific community has a good estimate of how much this might be in the future," he said. "We don't know. It could have an impact. Whether it's radical, I would probably say not."

In the Pacific Northwest, it seems very little of the methane is reaching the atmosphere. Johnson said about 95 percent is being consumed by microbes as it makes its way to the surface.

"Methane is free lunch for a lot of bacteria," Johnson said. "As the methane percolates up through the sediments in the top 10 to 20 centimetersbacteria eat it like crazy. Then as it gets into the water column, there is another type of bacteria that eat it."

So far, the biggest threat from the methane plumes appears to be the local water quality, Johnson said. The bacteria convert the methane into carbon dioxide, producing lower-oxygen, more-acidic conditions — and that hurts local fisheries.

"It's not that it is going to immediately accelerate global warming. There is not enough coming out to make much of a contribution," Johnson said. "The fishing communities are concerned about ocean acidification and low oxygen in the water and it (methane) has a tendency to make that worse."