Global warming could be melting ancient greenhouse gases under Oregon coast

Plume2_nolabels-e1418151769899.jpg
This sonar image captured bubbles rising from the seafloor off the Washington coast, where global warming has raised water temperatures and possibly caused gases frozen underwater for millennia to melt. ( Brendan Philip / University of Washington)

by  Kelly House

Greenhouse gases 25 times more powerful than carbon dioxide are bubbling up from beneath the ocean along Oregon and Washington, fueling global warming and contributing to changes in water chemistry that have devastated the northwest shellfish industry.

Scientists with the University of Washington believe abnormally warm water off the Pacific Coast is causing the gaseous plumes by vaporizing methane that had been frozen for thousands of years in deep ocean sediments.

The vapors are dissolving into the water and bubbling up into the atmosphere, potentially causing problems in both.

The scientists published their findings in the American Geophysical Union’s journal, Geochemistry, Geophysics, Geosystems.

“We’re not predicting an apocalypse, but we are saying global warming is coming to the marine waters off Oregon and Washington,” said H. Paul Johnson, the University of Washington oceanography professor who led the study.

PlumesMap.jpgThis map pinpoints 168 methane bubble plumes researchers located off the Oregon and Washington coasts.

Johnson worked on the study along with oceanography professor Evan Solomon, doctoral student Marie Salmi and research assistant Una Miller. The study builds upon research a University of Washington and Oregon State University team conducted last year.

In that study, scientists found that water 500 meters below the ocean surface has warmed by three-tenths of a degree Celsius over the past four decades – enough to melt methane frozen in ocean sediment.

Their work joins a growing body of research suggesting climate change might not happen in a slow and steady fashion. Rather, the earth’s warming could allow trapped greenhouse gases to escape, creating a snowball effect in which the earth could warm faster over time.

Other researchers have discovered methane plumes along the Atlantic and Norwegian coasts and in the Arctic tundra.

Under cold, high-pressure conditions, methane interacts with water by crystallizing into an ice-like solid. As temperatures warm, it takes more and more pressure to produce crystals. Under lower-pressure conditions, chemical bonds break and the methane reverts to its gaseous state.

When it shows up in the air, methane prevents solar energy from leaving earth’s atmosphere. Over time, this heats the planet in a process known as the greenhouse effect. Left unchecked, the release of greenhouse gases into Earth’s atmosphere is expected to cause catastrophic changes in the planet’s climate.

Previous models from University of Washington and Oregon State University scientists estimated present ocean warming trends are melting 100,000 metric tons of methane each year off the Washington coastline alone. Johnson said the same warming effect is happening from northern Vancouver Island down to Mendocino, California.

Only some of that gas leaves the sediment to alter ocean and air chemistry.

The research published last week backs up those models. Of 168 methane plumes discovered off the coast within the past decade, a high number originated at 500 meters below the surface. That’s the upper limit of depths at which methane could crystallize under cold, high-pressure conditions. As the ocean warms, methane requires deeper water to crystallize.

Solomon said it’s likely there are more plumes out there, yet to be discovered.

“Every time we go out on an expedition, we discover new seep sites,” he said.

Most of the resulting gas bubbles chemically react with water to create ocean-born carbon dioxide that contributes to the acidic seas that have plagued the Northwest shellfish industry. It’s unclear how significant that contribution might be, Solomon said.

A tiny fraction of the gas bubbles up to the surface, where it contributes to the greenhouse effect.

Although the research suggests seawaters warmed by global climate change are causing the plumes, more work is needed to know with certainty. The researchers’ next step is to analyze the plumes’ chemical makeup to find out.

If their hypothesis checks out, Johnson said, it’s reasonable to expect the methane melt to accelerate in the coming years. Arctic sea water that has warmed by as much as 2 degrees Celsius is heading for Oregon, but will take years to get here.

“That warming is already in the bank,” Johnson said. “We’re just waiting for it to reach us.”

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