The biggest wildfire to burn on treeless Arctic tundra triggered a dramatic permafrost thaw that unfolded over several years, new research finds.
A study published in the Nature journal Scientific Reports examines the effects of the 2007 Anaktuvuk River fire, which burned about 400 square miles of land on the North Slope, more than all previous North Slope tundra fires combined.
Long after the flames were extinguished on the surface, ice locked in the frozen earth below continued to melt, the study found, causing the land to slump and a pattern of angular ridges to emerge, said the study.
The scientists, from the U.S. Geological Survey, the University of Alaska Fairbanks and other institutions, used LiDAR technology to measure the precise elevation of the land surface that had once burned but later was covered with new vegetation, capturing changes that occurred between 2009 and 2014. LiDAR is a system that uses lasers and sensors, usually on aircraft, to get precise topographical measurements.
By that time, seven years after the fire, 34 percent of the burned area measured by LiDAR had developed thermokarst, the irregular surface features created when permafrost ice melts, hollowing out sections of land. By comparison, less than 1 percent of the adjacent unburned territory had developed thermokarst as of 2014, the study said.
The LiDAR data revealed subtle creases and wrinkles in the fire-stricken area, in sharp contrast to the much smoother surface of the unburned areas -- and a sign to researchers of the ice-wedge thaws that caused the thermokarst.
"Our findings indicate that fire disturbances, projected to increase in frequency, magnitude, and severity in a warming Arctic will play a major role for permafrost degradation and associated landscape change and ecosystem shifts in tundra regions in the future," the study says.
LiDAR was crucial to detecting the permafrost changes, said lead author Benjamin Jones, a USGS research geographer who has studied the Anaktuvuk River fire for several years. "It would be difficult to actually quantify it in detail without the LiDAR," he said.
Without LiDAR, surface changes caused by underlying permafrost thaw are detected by using aerial photography or high-resolution satellite images, Jones said. "You basically look for changes in surface characteristics," he said. That often means the small ponds of water that pool on top of the tundra after ice melts, he said.
But at the Anaktuvuk River fire site, the cracks and depressions are so interconnected that surface water created by ice melt is channeled away and does not pool, making the changes more subtle -- possibly too subtle to catch the eyes of observers, Jones said.
"This could have gone largely unnoticed without the LiDAR," he said.
Not all of the 2007 fire area was surveyed by LiDAR. The technology is expensive, Jones said, and the 2009 data was obtained through cooperation with the Alaska Department of Transportation, which was using LiDAR for a road planning project. Ultimately, Jones said, the study was able to examine about 300 square kilometers of the Anaktuvuk River burn area, he said.
Along with the LiDAR data, evidence of thaw comes from temperature measurements taken 1 meter below the surface in the fire area. In the burned land, mean annual ground temperature has increased by 1.8 degrees Fahrenheit since 2009, the study found, compared to a fairly unchanged ground temperature in the unburned areas.
The Anaktuvuk River fire's intensity might have played a part in the yearslong permafrost-thaw cycle. The fire subjected some sections to severe burns, destroying nearly 30 centimeters of the insulating organic matter in some places. Lack of the protective layer may have caused the thaw process to start at the top and work its way down over the years, Jones said, resulting in a time lag between the fire and the thermokarst formation.
Researchers can't say whether future tundra wildfires will result in similarly extensive permafrost thaw and ground slump, Jones said. The Anaktuvuk River fire happened to burn in an area where permafrost is especially ice-rich; other tundra sites might not undergo similar effects after a fire.
The 2007 fire, noteworthy for its size and location, has been the subject of numerous studies. Some have focused on the carbon released by the event, estimated to be 2.1 million metric tons, similar to the amount of greenhouse gases emitted annually in the city of Miami.
Other studies have focused on the return of vegetation to the site, which happened quickly enough to surprise some researchers -- though the mix of plants that emerged after the fire differed from those found in the area before it.
Since 2007, there has not been any Arctic Alaska wildfire to rival the Anaktuvuk River event.