IT’S BEEN CALLED the Arctic methane time bomb: a vast reservoir of frozen methane buried beneath the Arctic Ocean floor that, if were to melt and somehow reach the atmosphere, could trigger catastrophic climate change. It’s the ultimate end-of-life-as-we-know-it scenario. But could it actually happen?
For nearly a decade now, scientists have debated the idea. Most agree that the volume of frozen methane, known as methane hydrate, trapped beneath the Arctic Ocean floor is indeed staggering – possibly containing more energy than all other known fossil fuels on the planet. But there has been no consensus on the likelihood that climate change could unlock this massive methane cache, or that the methane could ever reach the surface, making it one of the biggest unknowns in climate science.
A new study published earlier this month adds a new twist to the debate, suggesting methane seeps on the ocean floor could actually have a net cooling effect on the atmosphere, rather than contributing to global warming as previously assumed. Researchers from Germany, Norway and the United States Geological Survey found carbon sequestration rates at the water surface above large methane seeps near Norway’s Svalbard Island were up to 2,000 times greater than the rate of methane being emitted.
The reason? The upwelling methane gas carries with it nutrient-rich waters that support more plankton growth above. Plankton acts much like trees, sucking up carbon dioxide from the atmosphere through the process of photosynthesis. The carbon consumed by the plankton bloom far outstripped the trace amounts of methane that reached the surface.
The results, published in the Proceedings of the National Academy of Sciences, came as a complete surprise, says USGS biogeochemist John Pohlman, the paper’s lead author. “It just so happens that the equipment we use to measure methane levels also measures carbon dioxide. But once we’d made the observation and thought about it, it just made sense.”
The next step for the researchers is to determine whether the results found near Svalbard are applicable elsewhere, or limited to waters with a similar plankton population. If the findings prove to be widespread, it raises the notion that methane seeps might actually help mitigate climate change, which hadn’t been considered before.
Methane is about 25 times more potent a greenhouse gas than carbon dioxide. A single cubic meter (1.3 cubic yards) of methane hydrate contains more than 160 cubic meters (210 cubic yards) of methane, which some scientists fear could be unlocked by global warming.
The potential risk was first identified in 2007, when an expedition led by University of Alaska-Fairbanks researcher Natalia Shakhova found elevated methane in water samples collected above the Siberian Shelf, which is believed to be the world’s largest store of hydrate. The methane levels were up to 10 times higher than those taken the previous year from the same locations. The researchers returned in 2008 and observed frozen rings of gas on the seabed and methane plumes bubbling to the surface over hundreds of square kilometers of the shallow waters.
In a paper published in the journal Geophysical Research Abstracts, Shakhova’s team suggested an abrupt release of up to 50 gigatons (49 billion long tons) of methane from the Siberian Shelf was imminent – a disastrous scenario that would increase the methane concentration in the atmosphere by more than a factor of 10. Not surprisingly, the report set off alarm bells. But it was also met with deep skepticism from a number of prominent scientists, many of whom openly questioned the paper’s assumptions.
Two recent studies have added further fuel to the debate. In the first, a paper published in the Geological Society of America Bulletin last month, researchers from the Canadian Geological Service described 139 methane seeps found on a remote island in Canada’s far north that was once part of the ocean floor. The seeps – some as tall as 2 meters (6.5 feet) and resembling giant anthills – were dated to 110 million years ago, a time when the planet’s temperature rose dramatically.
Another, published in December 2016, cited evidence linking the largest extinction in the Earth’s history, some 250 million years ago, with a massive methane release. The study, by researchers from Canada, Germany, Italy and the U.S. and published in Palaeoworld, was based on the gas content found in Permian-era rocks and fossils. The team found elevated carbon dioxide, likely caused by a major volcano, but also extremely high methane levels, which the researchers suggested were the real culprit that drove temperatures nearly 59F (15C) higher.
But Carolyn Ruppel, a geophysicist who oversees the USGS Gas Hydrates Project, is quick to point out that there is no evidence yet that methane can travel from the ocean floor to the atmosphere. “Right now, the [data] suggests that very little methane emitted from seafloor seeps at water depths greater than around 100 meters reaches the sea-air interface,” she wrote in email while on board a research vessel. Most methane released from the seabed, Ruppel explained, either oxidizes to carbon dioxide or dissolves in the water column long before reaching the surface.
Ruppel, who recently co-authored a paper refuting the “methane burp” scenario, added that even under a worst-case scenario for global warming, it could take thousands of years for the frozen layers beneath the seabed to melt and hydrates to be released in large volumes. “As a proportion of future [greenhouse gas] emissions,” she wrote, “the amounts attributable to gas hydrate are likely to remain very small for centuries or more to come.”
Likewise, David Archer, a University of Chicago researcher who has studied methane hydrate extensively, thinks the doomsday scenario is far-fetched. “I think 50 gigatons is just a wild estimate,” Archer said, adding that there are more pressing climate issues than sub-sea methane hydrate that we should be worried about. “If I was going to invest a finite research effort, I would put it into melting permafrost or the changing jet stream, which are much nearer-term concerns.”