Long after tundra wild fire flames have been extinguished the permafrost continues to rapidly thaw

“Fire has been largely absent from tundra for the past 11,000 or so years, but the frequency of tundra fires is increasing, probably as a response to climate warming.  If the frequency of these fires remains at long intervals, 80 to 150 years, then the tundra has time to recover. If these fires occur more frequently, say every 10 years or so, then the landscape cannot recover.” Syndonia “Donie” Bret-Harte, an ecosystem ecologist at the University of Alaska Fairbanks Institute of Arctic Biology.

A new study, prepared by scientists that have been mapping permafrost in Alaska, was recently published in the journal Science Direct. The United Sates Geological Survey summarizes the grim findings.

Using statistically modeled maps drawn from satellite data and other sources, U.S. Geological Survey scientists have projected that the near-surface permafrost that presently underlies 38 percent of boreal and arctic Alaska would be reduced by 16 to 24 percent by the end of the 21st century under widely accepted climate scenarios. Permafrost declines are more likely in central Alaska than northern Alaska.

Northern latitude tundra and boreal forests are experiencing an accelerated warming trend that is greater than in other parts of the world.  This warming trend degrades permafrost, defined as ground that stays below freezing for at least two consecutive years. Some of the adverse impacts of melting permafrost are changing pathways of ground and surface water, interruptions of regional transportation, and the release to the atmosphere of previously stored carbon.

“A warming climate is affecting the Arctic in the most complex ways,” said Virginia Burkett, USGS Associate Director for Climate and Land Use Change. “Understanding the current distribution of permafrost and estimating where it is likely to disappear are key factors in predicting the future responses of northern ecosystems to climate change.”

In addition to developing maps of near-surface permafrost distributions, the researchers developed maps of maximum thaw depth, or active-layer depth, and provided uncertainty estimates.  Future permafrost distribution probabilities, based on future climate scenarios produced by the Intergovernmental Panel on Climate Change (IPCC), were also estimated by the USGS scientists.  Widely used IPCC climate scenarios anticipate varied levels of climate mitigation action by the global community.

These future projections of permafrost distribution, however, did not include other possible future disturbances in the future, such as wildland fires. In general, the results support concerns about permafrost carbon becoming available to decomposition and greenhouse gas emission.

The research has been published in Remote Sensing of Environment.  The current near-surface permafrost map is available via ScienceBase.

Human caused climate change has altered the Arctic tundra ecosystem in so many ways that unprecedented fires have recently begun burning there. These fires may portend the release of billions of tons of methane and carbon dioxide into the atmosphere. This process is known as a positive feedback loop and tundra fires are contributing additional carbon dioxide to the atmosphere.  Because arctic soils do not have drainage due to frozen soil, any melt that occurs in the summer creates a moist environment so that the carbon is protected from devastation by wildfire.

Institute of Arctic Biology at the University of Alaska Fairbanks explains the impact of recent tundra fires in Alaska.

Tundra soils store huge amounts of carbon hundreds to thousands of years old. Layers of organic soil insulate the permanently frozen ground, called permafrost, below and restrict fires to aboveground plants and plant litter leaving the carbon stored in soils relatively intact.

As arctic summers get warmer and dryer, so too do the soils, which are highly flammable and able to burn more deeply when dry. This allows fires to burn more deeply into the ground. When aboveground plant materials burn, that not only releases carbon into the atmosphere, it also speeds thawing of the permafrost below. The once-frozen organic material in the thawing permafrost then begins to decompose, releasing additional carbon and amplifying climate warming.

Rapid (a,b) and delayed (c) thermokarst development following the Anaktuvuk River tundra fire. (a) Active layer detachment slides occurred in the first few years following the fire (2009) but were largely stabilized five years post-fire (2012). (b) Retrogressive thaw slumps were also triggered immediately following the fire (2009). They have expanded locally but remained relatively uncommon and have begun to stabilize four years post-fire (2011). (c) Widespread ice-wedge degradation was not evident in the first few years following the fire (2010) but noticeable following the fifth year post-fire (2012). Image pairs show the same location but from slightly different perspectives and in different years. 

A  recent study published in the Nature journal Scientific Reports examined the effects of the 2007 Anaktuvuk River Tundra Fire, North Slope, Alaska, an enormous tundra wildfire which was triggered by a bolt of lightning striking a hillside and burning 400 square miles of tundra. With the insulation gone, gullies, pits, and cracks have formed. In, addition, underground chunks of ice that were thousands of years old are now melting, and the Anaktuvuk River hills are rougher than they were before the fire.  This fire released half as much carbon in smoke as all other arctic plants worldwide sucked in that year. Arctic Wire summarizes the study.

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.

The study holds warnings for future impacts to tundra, where once-rare wildfires are expected to occur more often as the circumpolar north continues to warm.

“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.

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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.

In a previous study, scientists from the University of Alaska Fairbanks found that the Anaktuvuk River fire released 50 years' worth of carbon storage from the tundra forest into the atmosphere.