Study offers new explanation for Siberia’s permafrost craters

Mysterious craters that first appeared in Siberian permafrost a decade ago were caused by climate change-induced pressure shifts that explosively released frozen methane underground, a new study reports. The research offers a new view of the origins of craters first observed on Russia’s Yamal Peninsula in 2014.

The new study reveals that the unusual geology of the region, combined with global warming, triggered a process that led to the release of methane from methane hydrates present in permafrost.

“There are very, very specific conditions that allow this phenomenon to occur,” said Ana Morgado, a chemical engineer at the University of Cambridge and one of the study’s authors. “We are talking about a very specialized geological space.”

The research was published in Geophysical research letters.

The case of the permafrost explosion

The Yamal Peninsula is a low-lying landmass jutting into the Kara Sea from north-central Russia. In 2014, there were reports of a crater about 70 meters (230 feet) in diameter at its widest point, suddenly appearing in the permafrost. Over the next decade, more craters were discovered on both the Yamal Peninsula and the neighboring Gydan Peninsula.

Numerous explanations for the craters have emerged over the past 10 years, attributing the explosions to a buildup of methane underground due to melting permafrost, or to the craters’ proximity to natural gas reserves.

But the authors found that warming permafrost alone would not be enough to cause an explosion. The new explanation states that surface warming leads to a rapid change in pressure deep in the ground, causing the release of explosive methane.

“We knew something was causing the methane hydrate layer to break down,” Morgado said. “It’s a bit like detective work.”

Osmosis causes explosions

The researchers solved the puzzle from top to bottom, first looking at a fundamental question: Were the explosions caused by physical or chemical processes?

“There are only two ways to cause an explosion,” said Julyan Cartwright, a geophysicist at the Spanish National Research Council and one of the study’s authors. “Either a chemical reaction occurs and you have an explosion, like dynamite exploding, or you inflate your bike tire until it explodes, that’s physics.”

In this case, he said, there was no evidence that the explosions were caused by chemical reactions, so they must have had a physical source. “And then you have to think: What is the pump that inflates your bike tire?” he said.

The authors say the pump was osmosis, which is the way a fluid moves to equalize the concentration of dissolved substances. Salt water is a classic example. If there is a barrier allowing water to pass but not salt, pressure can build up on the salty side as water flows through it.

The thick, clayey permafrost of the Yamal Peninsula acts as an osmotic barrier and warming is changing it. This 180 to 300 meters (590 to 980 feet) thick layer remains permanently frozen throughout the year. An “active layer” of topsoil above thaws and refreezes seasonally.

Scattered across the tundra and sandwiched in the permafrost are unusual meter-thick layers of high-salinity unfrozen water called crypogegs, kept liquid by a combination of pressure and salinity. Beneath the cryopegs is a layer of crystallized methane-water solids, called methane hydrates, which are kept stable by high pressure and low temperature.

But warmer temperatures destabilize these layers. Climate change caused the active layer to melt and expand downward until it reached the cryopeg, releasing water that moves via osmotic pressure within the cryopeg, the researchers found.

But there isn’t enough space in the cryopeg to hold the extra meltwater forced by osmosis, so the pressure increases. The increasing pressure creates cracks in the ground that rise from the cryopeg to the surface. The pressure gradient then reverses: the cracked soil causes a sudden drop in pressure at depth. This pressure change damages the methane hydrates beneath the cryopeg, causing a release of methane and a physical explosion.

The period before the explosion can last decades, according to the study. This timeline corresponds to increasing global warming starting in the 1980s.

The new explanation depends on how global warming and the region’s geology interact to create these explosions, unique to the Yamal Peninsula.

“This could be a very rare phenomenon,” Morgado said. “But the amount of methane released could have a pretty big impact on global warming.”