The Earth responds to climate change in interesting ways, sometimes producing what we call “feedbacks,” which can either amplify global warming or dampen it. Some of the feedbacks that can amplify warming have behavior that is hard to predict in the near future and leaves scenarios that are potentially worse than expected hanging over our heads. One of those is the release of greenhouse gas from thawing permafrost—and regions of shallow seafloor that were frozen land thousands of years ago when sea level was much lower.
The most dangerous-sounding issue involves methane hydrate, an ice-like substance full of methane that is present in some ocean sediments. Its sudden release is a suspect in some major extinction events. But how much we should worry about destabilizing methane hydrates or thawing permafrost is not perfectly clear.
A group of researchers led by Kathryn Sparrow headed to the Beaufort Sea in the Canadian Arctic to bring an improved measurement technique to bear on an important question: how much of the methane in the sea floor is ending up in our atmosphere?
A different group of scientists working offshore of Siberia has sounded alarms about plumes of methane it observed bubbling up from the seafloor there. Researchers are trying to work out how warming is affecting that bubbling methane and what it will do in the future.
Studies off Svalbard have found that methane bubble plumes there pre-date human-caused climate change. And, counterintuitively, those plumes have been shown to actually cause a net removal of atmospheric greenhouse gas by stimulating the growth of plankton that consume CO2. So there’s a lot to account for before projecting what the future holds for this particular feedback.
But back to Sparrow and her team working in the Canadian Arctic: they have essentially carbon dated the methane dissolved in the water by measuring radioactive carbon-14. As they moved from the shore toward deeper water, they were able to make measurements both at the bottom and in the surface water. This allowed them to calculate how much ancient methane was coming out of the seafloor and how much successfully reached the surface to be released into the atmosphere.
We don’t actually expect those two numbers to be the same. Microbes in the water can digest the methane and turn it into CO2. Currents may also drag the methane into the deep ocean.
The measurements did find plenty of ancient methane at the bottom, although there was also young methane produced by the microbial breakdown of dead plankton (another source is atmospheric methane that mixes into the water).
The surface water measurements are another story, though. Closer to shore, the team found a fair portion of ancient methane at the surface. But once the water got around 30 meters deep, the ancient methane all but disappeared from surface waters, accounting for 10 percent or less of the total methane dissolved in the water.
That indicates that things like hungry microbes are effectively preventing the methane that’s being released on the seafloor from reaching the atmosphere once the water is deep enough. Since most of the Beaufort Sea is more than 30 meters deep, that means Beaufort seafloor methane isn’t being directly dumped into the atmosphere, even if warmer water is causing the seafloor to produce more.
In other words, the news is good, assuming other areas of the Arctic Ocean are behaving similarly. The obvious next question is: how will these processes respond to continued global warming? If the seafloor releases more methane, will microbes keep up as the water they live in warms?
That’s one piece of the puzzle researchers will need in order to offer more precise estimates of how much greenhouse gas the natural world will release in response to our own emissions. More evidence of limits on the release of methane could at least partially defang this particular sleeping dragon.