Submarine surveys of the seafloor beneath the Arctic Ocean have revealed deep craters appearing off the Canadian coastline. The scientists involved attribute these to gasses released as permafrost melts. The causes, so far, lie long before humans started messing with the planet’s thermostat, but that could be about to change.

For millions of years, soil has been frozen solid over large areas of the planet, both on land and under the ocean, even where snow melts at the surface to leave no permanent ice sheet. Known as permafrost, this frozen layer traps billions of tonnes of carbon dioxide and methane. It is thought the sudden melting of similar areas around 55 million years ago set off the Palaeocene-Eocene Thermal Maximum, when temperatures rose sharply over the space of a few thousand years.

Now the permafrost is melting again, revealed in plumes of bubbles coming to the surface in shallow oceans, the collapsing of Arctic roads, ruined scientific equipment, and great craters that suddenly appeared in Siberia. For the first time, scientists have revealed in Proceedings of the National Academy of Sciences what all this is doing to part of the Arctic Ocean’s seafloor.

Dr Charles Paull of Monterey Bay Aquarium Research Institute and co-authors ran four surveys of the storied Beaufort Sea between 2010 and 2019 using autonomous underwater vehicles assisted by icebreakers at the surface. They restricted their observations to depths between 120 and 150 meters (400-500 feet) as in most places this captures the permafrost's outer margin.

The paper reports numerous steep-sided depressions up to 28 meters (92 feet), along with ice-filled hills up to 100 meters (330 feet) wide known as pingos. Some of these, including a deep depression 225 meters (738 feet) long and 95 meters (312 feet) across, appeared between successive surveys, rather than being long-standing features. Others expanded in the time the team were watching.

The depressions are the result of groundwater ascending up the continental slope. Sometimes the groundwater freezes from contact with colder material, causing the ground surface to heave upwards and produce pingos.

“We know that big changes are happening across the Arctic landscape, but this is the first time we’ve been able to deploy technology to see that changes are happening offshore too,” Paull said in a statement. “This groundbreaking research has revealed how the thawing of submarine permafrost can be detected, and then monitored once baselines are established.”

The research was possible because the Beaufort Sea, once too icebound for research like this, is melting fast. That trend is, the authors agree, a consequence of human emissions of Greenhouse gases. The same goes for the widespread disappearance of permafrost on land.

However, the extra heat those gasses put into the global system has yet to penetrate to the depths Paull and co-authors were studying. Here, temperatures operate on a much slower cycle, buffered by so much water, and are still responding to the warming that took place as the last glacial era ended. At the current rate, it would take more than a thousand years to produce the topography the team observed.

“There isn’t a lot of long-term data for the seafloor temperature in this region, but the data we do have aren’t showing a warming trend,” Paull said. “The changes to seafloor terrain are instead being driven by heat carried in slowly moving groundwater systems.”

The natural melting of Ice Age permafrost releases gasses that warm the planet, part of a reinforcing interglacial era cycle, but the effect is slow enough to present little problem for humans or other species. As human-induced atmospheric heat permeates the oceans at these levels things could accelerate dramatically, and the authors see their work as establishing a baseline so we know if that occurs.