The result is the worrisome image shown above – a massive sinkhole indicating that the permafrost on the seafloor has thawed and collapsed. The sinkhole is one gigantic among dozens of pits that researchers have found on the ocean floor. Scientists have recorded this dramatic phenomenon, called thermokarst, on land. Because permafrost is made up of soil suspended in a matrix of frozen water, when it thaws, the land shrinks, digging huge holes in the Arctic landscape. It’s also happening underwater, as these seafloor images show.

“I think it’s absolutely remarkable that some parts of the ocean floor are changing at this rate and on this scale,” said Charlie Paul, a marine geologist at the Monterey Bay Aquarium Research Institute and co-author of the paper. The survey covered an area half the size of Manhattan, with a total of 40 holes. (You can see part of the area in the image below.) The gigantic “equivalent to an entire New York City block made up of six-story apartment buildings,” he said.

Illustration: Eve Rensten

Why is this happening? On land, permafrost is melting due to rising temperatures. But, Paul said, there’s no evidence the seafloor has warmed enough to start thawing.So thawing is most likely not from above, but from the following. The permafrost on the seafloor formed a thick wedge that could be several hundred meters, Paul said. Relatively warm groundwater flows underground, which can degrade permafrost. “If it’s pure permafrost ice, it creates voids that then collapse,” he said. “We therefore infer that the voids we see in this environment are the result of long-term warming of the seafloor in the region.”

That long Bits are important. Unlike rapid thaws on land, the degradation of this seafloor may have unfolded over longer timescales — a lingering effect of how much the world has warmed since the last ice age. “I think the biggest lesson of this paper is a reminder of how long these systems take to respond,” said Ben Abbott, who studies permafrost at Brigham Young University but was not involved in the study. “You might misinterpret it as, ‘Oh, nothing to worry about.’ I actually came to the exact opposite conclusion. Once the system is up and running, we have very little ability to change its direction. This is not a car with a steering wheel— – It’s more like a boulder you push off the top of a mountain.”

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While this seafloor melting may be driven by long-term processes, scientists fear it could be accelerating because the Arctic is warming so rapidly now. Ocean circulation patterns may also change, bringing in more warm water. “So long-term changes of the kind we observe now may soon be accelerated by human interference with the climate,” Abbott said.

The two biggest unknowns are how much seafloor permafrost exists, and how much greenhouse gas it holds. Scientists can’t sample every square foot of the Arctic seabed, so they look back and compare the area of ​​land exposed during the last glacial maximum thousands of years ago with the area of ​​land exposed today. This gave them an idea of ​​how much permafrost may have formed and then submerged when glaciers melted and sea levels rose to where they are now. Estimates vary, but there may be about 775,000 square miles of seafloor permafrost, sequestering tens of gigatons of organic carbon and billions of tons of methane.