If an explosion of gas opens a great cavity in Siberia's permafrost, and no one is around to hear it, does it make a sound?
Well, the methane gases it releases certainly send ripples around the world, and now a team from the United Kingdom and Spain has traced its source.
Scientists first noticed these mysterious craters emerging in 2014, when they encountered a hole in the Yamal Peninsula in Siberia, about 30 meters across (98.4 feet) and more than 50 meters deep, surrounded by ejecta that hinted at its explosive origins.
Since then, many more of these holes have burst forth from the surface of the Yamal and Gydan peninsulas, and chemical engineer Ana Morgado from the University of Cambridge says the massive amounts of methane they release could have a big impact on global warming.
"There are very, very specific conditions that allow for this phenomenon to happen," says Morgado. "We're talking about a very niche geological space."
The team found that it's not just a case of gas from the melting permafrost expanding and bubbling up due to warmer temperatures; that's definitely happening, but it wouldn't be enough for such big, forceful bangs.
"There are only two ways you can get an explosion," geophysicist Julyan Cartwright from the Spanish National Research Council says. "Either a chemical reaction happens, and you have an explosion, like dynamite blowing up, or you pump up your bicycle tire until it blows up – that's physics."
And because there were no lights or combustion products reported from investigations into any of the explosions – which would signify a chemical reaction taking place – the researchers deduced the groundbreaking pressures must have a physical source.
This, they propose, is osmosis: the tendency of a fluid to move in a way that equalizes the concentrations of the substances dissolved within.
As the permafrost warms (seasonally and, at present, for longer periods of time due to climate change), the surface soil, bustling with all the goings-on of life – thaws and expands downwards, with fresh meltwater trickling through the permafrost.
Usually, this fluctuation is shallow, but due to climate change it is penetrating further into the ground. Here, the researchers report, it reaches a layer of salty water called a cryopeg, which typically evades freezing, due to its salinity, and pressure from above.
And ordinarily, the cryopeg is nested carefully above a layer of methane hydrates – crystallized, hydrogen-bonded water and methane gas – which is maintained with the high pressure and low temperature afforded by the cryopeg.
But as meltwater rushes in, the cryopeg, with its low pressure and high salt content, not only absorbs it as it flows down from the surface, but acts as a kind of pump via osmosis. As this subterranean swell mounts, the pressure forms cracks in the permafrost above.
The stable sanctuary of the methane hydrates now ruptured, they are released as methane gas that belches up in a physical explosion.
These soil fractures were thought to occur across timescales of millennia, but the study found these osmotic forces can speed up the process to occur within decades – and thus lines up with global warming that began to accelerate in the 1980s.
"This might be a very infrequently occurring phenomenon," Morgado says. "But the amount of methane that's being released could have quite a big impact on global warming."
This research was published in Geophysical Research Letters.