Permafrost underlies much of the far north, but this amalgam of ice and frozen soil is far from stable—it’s thawing as temperatures rise worldwide. That’s bad news because permafrost is a significant repository of carbon, which can be readily converted into carbon dioxide, a major greenhouse gas. Now, researchers have used satellite remote sensing to monitor one signature of permafrost thawing—ground subsidence—after a wildfire in eastern Siberia. Surprisingly, the team found that parts of Earth’s surface subsided more than others despite the relative homogeneity of the fire. This variation is likely due to differences in the thickness of the insulating active layer directly above the permafrost, the scientists suggest.
Kazuki Yanagiya and Masato Furuya, both geophysicists at Hokkaido University in Japan, focused on a 3,600-hectare swath of permafrost in eastern Siberia, Russia. The region, composed of low shrubs dotted with 3- to 5-meter-tall larch trees, burned in July 2014 in a wildfire of unknown cause.
Siberia has been plagued by many blazes recently, said Roger Michaelides, a geophysicist at the Colorado School of Mines in Golden not involved in the research, and there’s no sign of the fires abating. “With climate change, wildfire frequency and severity are expected to increase.”
Maps of Sinking Ground
The researchers used a remote sensing technique called interferometric synthetic aperture radar to generate maps of ground subsidence following the fire. Subsidence is a common outcome of thawing permafrost and can wreak havoc on built structures.
Wildfire triggered the measured subsidence, but only in an indirect way, said Furuya. “The fire itself doesn’t melt permafrost directly.” Rather, a blaze eradicates vegetation, which reflects and absorbs sunlight. When that insulating layer is lost, the ground heats up more readily, causing permafrost to thaw.
Using microwave data collected by two satellites—Sentinel-1 and the Japanese Advanced Land Observing Satellite-2—the team traced how the ground subsided in the burned region between October 2015 and June 2019 to centimeter-level precision.
A Bulkier Layer to the Rescue
Subsidence proceeded the fastest in 2015 and 2016, Yanagiya and Furuya found. That’s probably because of a negative feedback loop, the authors propose: Initial permafrost thawing bulked up the active layer—the layer above permafrost that freezes and thaws seasonally—which in turn provided additional thermal insulation against further thawing. This time-resolved look at how permafrost thawing proceeds after a fire is novel, said Yanagiya. “The detailed time series of deformation is very new.”
The researchers also found that east facing slopes tended to experience the most subsidence. That’s consistent with previous research and makes sense because these areas receive less intense sunlight, Yanagiya and Furuya suggest. Their active layers are therefore thinner to begin with and accordingly provide less insulation, the scientists propose.
A New Megaslump?
In total, the fire-scarred region lost roughly 3.5 million cubic meters of permafrost, the scientists calculated. For comparison, that’s about an order of magnitude less than the thawed volume of the nearby Batagaika megaslump, an enormous craterlike depression formed by thawing permafrost. In September 2019, Yanagiya and Furuya did fieldwork in Siberia and flew a drone over the Batagaika megaslump. “It’s huge,” said Furuya.
It’s possible that the burned area they studied might one day come to look like Batagaika, the researchers hypothesize. “We are kind of expecting it,” said Furuya.
These results were published in June in the Journal of Geophysical Research: Earth Surface.
—Katherine Kornei (@KatherineKornei), Science Writer