The result is the disturbing image shown above – a large sinkhole showing that the sea permafrost has thawed and collapsed. This sinkhole is one of dozens of pits that researchers have found on the seafloor. Scientists have documented this violent phenomenon, called thermokarst, on land. Because permafrost is made up of soil suspended in a matrix of frozen water, as it thaws, the soil shrinks, forming large holes across the Arctic panorama. And like these images of the seabed, it also takes place underwater.
“I think there are places on the seafloor where changes of this scale are happening at this rate,” said marine geologist Charlie Paull of the Monterey Bay Aquarium Research Institute, a co-author of the paper. The entire survey covers an area half the size of Manhattan and calculates 40 holes. (You can see a portion of the area in the image below.) The giant is “equivalent to an entire New York City block of six-story apartment buildings,” he said.
Why is this happening? On land, the permafrost is thawing as temperatures are rising. However, Paull said, there is no evidence that seafloor temperatures have risen enough to start melting. So it’s possible that the thaw doesn’t start from above, but from below. The submarine’s permafrost forms a wedge shape perhaps several hundred meters thick, Paull said. Below is a relatively warm groundwater, which can degrade the permafrost. “If it’s pure permafrost, it will create gaps and then collapse,” he said. “And so we deduce that the gaps that we’re seeing growing in this environment are a consequence of the long-term warming experienced by the seafloor in this region.”
That long-term bit is important. Unlike the rapid thaw on land, this class of submarines’ degradation probably took place over a much longer period of time – a lasting effect of how much the world has warmed since then. since the last ice age. “I think the biggest takeaway from this paper is a reminder that systems are not very efficient,” said Ben Abbott, who studies permafrost at Brigham Young University but was not involved in the study. How long does this take to respond. “You can get that wrong by saying, ‘Oh, there’s nothing to worry about.’ I actually came to the opposite conclusion. Once the system is in motion, we have little chance of changing its direction. It’s not a car with a steering wheel — it’s like a rock that you push off the top of a mountain.”
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While this seafloor thawing could be driven by long-term processes, scientists fear it could accelerate because the Arctic is currently warming so quickly. The ocean’s circulation patterns could also change, bringing in more warm water. “So the kind of long-term change we’re seeing now could be accelerated by human intervention in the climate,” Abbott said.
The two big unknowns are how much permafrost exists in the submarine and how much greenhouse gases it stores. The scientists couldn’t sample every square foot of the Arctic seafloor, so they instead looked in reverse, comparing the amount of land exposed during the last glacial peak thousands of years ago with the extent contact today. This gives them an idea of how much permafrost may have formed and then sank as glaciers melted and sea levels rose to present levels. Estimates vary, but there may be 775,000 square miles submarine permafrost, sequestering perhaps hundreds of gigatons of organic carbon and tens of gigatons of trapped methane.
