|February 04, 2020|
Permafrost is thawing so fast, it's gouging holes in the Arctic
|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.
In addition, when you think of permafrost regions, you might think of featureless tundras, but most is actually boreal forest. These northern forests have recently seen an unprecedented number of wildfires. "Much of the boreal forest burns more and more often, and when the ecosystem burns, it can actually accelerate the permafrost thaw," says David Olefeldt of University of Alberta, coauthor on the paper. Without cover from these trees to shade it, the soil warms ever more intensely.
Abrupt warming also exacerbates emissions from permafrost. In a gradual thaw, the warming top layers of the soil open up to hungry microbes, which consume nutrients and give off CO2. "In the summer, permafrost---the top layer at least---thaws, and then cracks can build," says Northern Arizona University biogeochemist and plant ecophysiologist Christina Schaedel, who collaborates with the authors of this new paper, but wasn't involved in the work. In the fall it freezes back up, creating a cycle in which soil layers get mixed down to the bedrock, concentrating carbon at the bottom. "With abrupt thaw, you're exposing deeper layers to much warmer temperatures, and deep layers in permafrost can contain very high amounts of carbon," Schaedel says.
This can become particularly problematic from an emissions standpoint if the collapsed land forms a pond of water with low oxygen content, and with a layer of rich carbon at the bottom. The microbes that thrive in this kind of environment produce methane as a byproduct, a far more potent greenhouse gas than CO2.
Here's an important consideration: When permafrost melts abruptly, it doesn't just release carbon and then retire. That ecosystem can heal and begin sequestering carbon again. If the land has thawed, then collapsed and become inundated with water, new trees can't grow. Instead, that ecosystem is likely to become dominated by mosses and grass-like sedges. Because the plant material is waterlogged, decomposition actually slows as it forms peat---thick, mucky, layers of organic matter.
"So this rapid post-thaw peat accumulation that happens is eventually how it recovers some of the carbon that was lost," says USGS research geologist Miriam Jones, coauthor on the new paper. "But I will say that in the permafrost, carbon has accumulated over millennia. And so upon thaw, it's rapidly lost within years to decades."
It will take centuries or millennia, depending on the ecosystem, to sequester all that carbon again. And of course in the Arctic, which is warming twice as fast as the rest of the planet, the composition of vegetal species that make up some of its ecosystems are transforming, in turn changing how they sequester carbon.
The more closely scientists can parse what happens when permafrost thaws rapidly, the better they account for how these landscapes emit greenhouse gases---and eventually sequester some, too. The bad news is, the emissions could be the equivalent of an entire industrialized nation's greenhouse output. The better news is, it won't be as much as humanity's global toll. "Even though these are hot spots of carbon release, it's going to take decades for those hot spots to become large enough to seriously impact the climate system," says Turetsky. "But this is still something we need to take seriously."
And it's something that needs far more research. Any climate modeling comes with inherent uncertainties---there's no way to perfectly represent such complex systems. The uncertainty here is projecting how much land might succumb to abrupt thawing, says University of Alaska Fairbanks permafrost geophysicist Vladimir Romanovsky, who wasn't involved in the work. Scientists have only begun to study these rapid thaw events, which often happen at extremely small scales.
"It's very difficult in this particular case to use the past to predict the future," says Romanovsky. "That's understandable, and definitely there are some ways to try to narrow down this uncertainty. But that uncertainty will be there for forever, because of the limitation of all the models to predict this process in the future, in particular the entire area of permafrost existence."
What's clear, though, is that ecosystems in the Arctic are literally in upheaval. And the faster we cut emissions, the less they will suffer.