It’s perhaps the best known and more worrisome of climate feedback loops: As the planet warms, permafrost—landscapes of frozen soil and rock—begins to thaw. And when it does, microbes consume organic matter, releasing CO2 and methane into the atmosphere, leading to more warming, more thawing, and even more carbon emissions.
But here’s something you’ve probably never heard of, and it’s something not even the UN’s Intergovernmental Panel on Climate Change has really considered: thermokarst. That’s the land that gets ravaged whenever permafrost thaws rapidly. As the ice that holds the soil together disappears, hillsides collapse and massive sinkholes open up. Climate scientists have been working gradual permafrost thaw into their models—changes that run centimeters deep over decades or centuries. But abrupt permafrost thaw happens on the scale of meters over months or years. That shocks the surrounding landscape into releasing potentially even more carbon than would have if it thawed at a more leisurely pace.
Today in the journal Nature Geoscience, researchers argue that without taking abrupt thaws into account, we’re underestimating the impact of permafrost thaw by 50 percent. “The amount of carbon coming off that very narrow amount of abrupt thaw in the landscape, that small area, is still large enough to double the climate consequences and the permafrost carbon feedback,” says study lead author Merritt Turetsky, of the University of Guelph and University of Colorado Boulder.
Less than 20 percent of northern permafrost land is susceptible to this kind of rapid thaw. Some permafrost is simply frozen rock, or even sand. But the kind we’re worried about here contains a whole lot of water. “Where permafrost tends to be lake sediment or organic soils, the type of earth material that can hold a lot of water, these are like sponges on the landscape,” says Turetsky. “When you have thaw, we see really dynamic and rapid changes.”
That’s because frozen water takes up more space than liquid water. When permafrost thaws, it loses a good amount of its volume. Think of it like thawing ice cubes made of water and muck: If you defrost the tray, the greenery will sink to the bottom and settle. “That’s exactly what happens in these ecosystems when the permafrost has a lot of ice in it and it thaws,” says Turetsky. “Whatever was at the surface just slumps right down to the bottom. So you get these pits on the land, sometimes meters deep. They’re like sinkholes developing in the land.”
“Essentially, we’re taking terra firma and making it terra soupy,” Turetsky adds.
As the earth turns to soup, the landscape begins to scar. The process is so rapid and so violent, Turetsky says, that sometimes when she returns to a site she’s monitoring to check her temperature and methane sensors, she’ll find they are gone. “When you come back in, it’s a lake and there’s three meters of water at the surface. You have to probably say goodbye to your equipment,” she says.
When these lands thaw, they play host to a number of processes. As ice turns to liquid water, trees flood and die off. Thus more light reaches the soil, further accelerating thawing. This is in contrast to gradual thaw, when the plant community largely stays the same as the ice thaws. Defrosted soil at the surface gets thicker and thicker, but it doesn’t catastrophically collapse.