Excerpt from this story from Inside Climate News:
The slowdown of a key ocean current could release methane that is frozen in layers of organic seabed sediments along some of the world’s coastlines, a new study shows.
Cold temperatures and high pressure on sea floors currently sequester about one-sixth of the world’s methane, a potent but short-lived greenhouse gas, in an ice-like form called methane hydrate, or clathrates. Sudden thawing of those clathrates could result in a surge of methane emissions that would spike the planet’s fever. The new research, published today in the Proceedings of the National Academy of Sciences, shows that some of the shallower layers in the Atlantic Ocean could be more vulnerable than previously thought to warming that could release that methane, and that such events have happened in the distant past.
The trigger for such warming and thawing, according to the study, is a large inflow of fresh, frigid water from melting Arctic ice, which can disrupt the Atlantic Meridional Overturning Current, a slow ocean heat pump, pushing cold water in the Arctic deep down and southward, and warm water to the surface and northward.
Temperature, density and salinity contrasts drive the pump. But in recent decades, the influx of water from rapidly melting Arctic ice, especially the Greenland Ice Sheet, appears to be weakening the current, which could warm the ocean at depths of 300 to 1,300 meters to destabilize methane hydrates buried 20 to 30 feet deep in the seabed.
It’s happened before, said University of California, Santa Barbara climate scientist Syee Weldeab, who led the detailed analysis of ocean and coastal conditions in the Gulf of Guinea. The study covers a 500-year slice of time during the Eemian Age, about 125,000 years ago.
The global average temperature was about 1 to 2 degrees Celsius warmer than now, due to changes in the amount of sunlight reaching Earth. The new study’s analysis of trace elements in the shells of ancient ocean microorganisms shows that was enough to free some methane from its icy state, he said.
The main message of the study is that researchers need to consider temperature changes greater than they are currently looking at, Weldeab said, due to the possibility of unexpected feedbacks that can magnify warming like changing currents or methane releases brought on by higher temperatures.
