Charlie Paull/ MBARI
It was around 20,000 years agoThe world was so cold that large glaciers soaked up enough water from the ocean to lower sea levels by 400ft. As the sea receded, the newly exposed land became permafrost. This is a mixture of earth & ice that now sprawls across the northernmost regions. As the world warmed, the climate we now enjoy (For the moment), sea levels rose again, submerging the coastal edges of that permafrost, which remain frozen below the water.
It’s a huge, hidden climate variable that scientists are racing to understand. They are fully aware of the dangers associated with the destruction Terrestrial permafrost is a significant source of carbon entering the atmosphere. When it thaws microbes consume the organic matter, releasing carbon dioxide (if it is quite dry) or methane (if it melts into a pond). This feedback loop can lead to more permafrost melting, which produces more emissions that heat the planet. even more permafrost. That’s an extra-big problem because the Arctic is Now warming Four times as fast as the rest of the planet.
Yet submarine permafrost is largely unstudied, owing to its inaccessibility—renting out time on a research vessel is not cheap anywhere, much less in the Arctic, and it’s much harder to reach for drilling samples. It is now alarming. Paper published last week in the Proceedings of National Academy of Sciences, an international team of scientists give us a rare look at what’s going on down there. The team used oceanic robotics, which look like torpedoes off the coast of northern Canada, and mapped the seafloor using sonar. The scientists repeated this several times over the course of nine years to get a sense of how the topology of the seafloor might be changing and found that it’s undergoing massive upheaval.
The result is the worrisome image shown above—a massive sinkhole indicating that the subsea permafrost has thawed and collapsed. This sinkhole is one of many pockmarks that researchers discovered on the seafloor. Scientists have already documented this powerful phenomenon. Also known as thermokarst, on land. Permafrost is soil suspended in a frozen water matrix. When it thaws, the land shrinks, causing massive holes in the Arctic landscape. And as these images of the seafloor show, it’s also happening underwater.
“I think it’s just absolutely remarkable that there are places on the seafloor where changes of this scale are happening at this rate,” says Monterey Bay Aquarium Research Institute marine geologist Charlie Paull, a coauthor on the paper. The entire survey covered an area that’s half the size of Manhattan and tallied 40 holes. (See the image below to see a small portion of the area. The giant one, he says, “is equivalent to a whole New York City block composed of six-story apartment buildings.”
What is the reason for this? Because temperatures are rising, permafrost in land is thawing. But, Paull says, there’s no evidence that seafloor temperatures are rising enough to initiate thaw. So it’s likely that thaw isn’t starting from above, but from below. Submarine permafrost forms a thick wedge of perhaps several hundred meters, says Paull. Below that is relatively warm groundwater which can degrade permafrost. “If it’s pure permafrost ice, it’ll produce voids that subsequently collapse,” he says. “And so we’re inferring that the voids that we’re seeing developing in this environment are a consequence of the long-term warming that the seafloor in this area has experienced.”
That long-term bit is important. Unlike rapid thaw on land, the degradation of this submarine layer has probably been unfolding over much longer timescales—a lingering effect of how much the world has warmed since the last ice age. “I think that the biggest lesson from this paper is a reminder of how long these systems take to respond,” says Ben Abbott, who studies permafrost at Brigham Young University but wasn’t involved in the research. “You could maybe misinterpret that to say, ‘Oh, there’s nothing to worry about.’ I actually take exactly the opposite conclusion. We cannot change the direction of the system once it is in motion. It’s not a car with a steering wheel—this is more like a boulder that you push off the top of a mountain.”
Although this undersea melting is likely to be caused by long-term processes scientists are concerned that it could accelerate as the Arctic is warming so rapidly. Ocean circulation patterns may change, bringing in more warm water. “So the kind of long-term change that we’re observing right now could be accelerated quite soon by human interference on the climate,” says Abbott.
Two major unknowns are the amount of submarine permafrost and its greenhouse gas content. Scientists can’t take samples of every square foot of Arctic seafloor, so instead they look backward, comparing how much land was exposed during the last glacial maximum thousands of years ago to how much is exposed today. This gives them an idea about how much permafrost was formed and then submerged by the glaciers melting and the sea rising to its present level. There may be as many as 200,000. 775kilom2 of submarine permafrost, sequestering perhaps hundreds of gigatons of organic carbon and tens of gigatons of trapped methane.
Methane is an organic compound. Climate change is a serious concernIt is what it is 80 times more potent a greenhouse gas as CO2 (although it disappears from the atmosphere much faster). In terrestrial permafrost, it’s produced when microbes have wet organic material to chew on—and obviously the seafloor is rather wet. Submarine methane can also be produced from underground deposits of natural gas, which have leaked upwards and become trapped in a latticework containing frozen water, a substance known as methanehydrate. (It’s basically gassy ice; it’ll even ignite.) These molecules are “sitting there just waiting for a temperature threshold to be crossed, and then they can be released quite dramatically,” says Abbott.
Submarine permafrost releases methane, and microbes in the sediments and water column convert it into CO.2. “That’s been described as this ‘microbial cap’ that is protecting us from this methane release because it transforms that very potent greenhouse gas, methane, into a less potent gas, CO2,” says Abbott. “But there are questions about if we have a massive collapse of subsea permafrost—like what’s described in this paper—maybe we’re going to get more methane released in bubbles. That bubbling short-circuits that microbial oxidation of the methane and releases it straight to the atmosphere.”
It’s hard to say exactly how the thaw might play out because submarine permafrost is a sort of time capsule and can’t be compared directly to permafrost on land. “It’s this ancient, legacy environment that has been essentially cut off by expansion of those seawaters,” says Merritt Turetsky, an ecologist at the University of Colorado Boulder who studies permafrost but wasn’t involved in the new paper. But when you combine potential undersea releases with the volume of gases already known to be released from the land—which Turetsky says is like “like adding another industrialized nation to the world”—the climate effect could be substantial.
“We like to say what happens in the Arctic doesn’t stay in the Arctic,” she says. Think of the region as Earth’s freezer, which has locked away carbon for millennia. “We’re now pulling the plug of the freezer out of the wall,” Turetsky says. “We’ve got this hot spot of warming that has been a climate-Cooling hero for thousands of years but now has the potential to actually release all of that stored carbon back into the atmosphere.”