A 15-year study of Alaskan tundra has demonstrated that thawing permafrost is persistently releasing more carbon-based compounds than it absorbs, a historic reversal that is adding greenhouse gases to the atmosphere and exacerbating the planet's rising temperatures.
The paper, published March 30 in JGR Biogeosciences, showed that warmer weather has been causing more carbon-emitting microbial activity in the previously frozen soil, which overwhelmed the additional carbon stored by boosted plant growth. Its authors predict that as much as one-fifth of the carbon in permafrost may be released into the atmosphere by 2100 in the worst warming scenarios.
The research represents the longest study of carbon flows at a site with actively degrading permafrost, providing a rare view of the long-term effects of rising temperatures on tundra ecosystems.
Carbon actively cycles through ecosystems in a number of forms. Plants absorb carbon dioxide during photosynthesis, and they add organic forms of carbon to the soil when they decompose. Some organic carbon is broken down by microbes, insects and other animals and reconverted into carbon dioxide and methane, which are both greenhouse gases.
Permafrost, subsurface soil that has been frozen for at least two years, stores more carbon than other soil because the sub-freezing temperatures suppress the microbes and creatures that break down organic carbon. The Earth's permafrost contains nearly twice as much carbon as its entire atmosphere.
As global warming has warmed the Arctic much faster than the rest of the world, the permafrost in the area has been thawing and emitting more carbon. The hotter climate promotes more plant growth, which takes carbon dioxide out of the atmosphere, but it also leads to more soil activity that releases greenhouse gases.
"This organic matter that's been previously frozen is now being eaten by soil microbes," said Ted Schuur, a professor of ecosystem ecology at North Arizona University. "That adds to the greenhouse gases that are already there making things warmer."
Global warming is also expected to release "huge amounts" of carbon dioxide stored in permafrost via increased lightning strikes in the Arctic, according to the authors of a recent study from Nature Climate Change.
Since 2004, Schuur, the recent study's lead and senior author, has been monitoring the carbon flow in and out of the soil in an area just outside Denali National Park in Alaska. The surrounding tundra ecosystem was once a carbon sink in the far past, but that is no longer the case.
He and his colleagues used 3-meter-tall eddy covariance towers to track the movement of gases and other particles through the space above permafrost. By combining the tracking with the direction of wind and other weather conditions, they could estimate whether more carbon is entering or exiting the soil.
"If we're going to accelerate climate change as we warm the north, is that happening this year or next year, or is it like 10 years or 100 years?" Schurr said. "Measuring how fast it's coming out is the key point here."
The period from 2004-18 included both carbon-positive and -negative years, but the researchers concluded that the region ultimately lost about 782 grams of carbon per square meter, a 1.6% decrease. The relatively long time period of the study allowed the team to see past the variability between individual years and recognize a persistent trend, Schuur said.
If the carbon-release trend continues or worsens, the authors estimate that the land's carbon content will fall an additional 8% to 20% by the end of the century — a range comparable to the 5% to 15% of expected carbon loss for all Arctic permafrost by 2100, a 2015 estimate from Schuur and other researchers.
The Northern Arizona professor said the similarity between the projections is a good sign that the most recent findings are generalizable and likely apply to many other permafrost ecosystems.
The authors also focused on time periods when the conditions were hot and either very wet or very dry, the two climate directions the future Arctic might take. Carbon loss was dominant in both scenarios: With high rainfall, plants grew more but soil microbes became even more active, and plants suffered worse under dry conditions.
Near the eddy covariance towers in Alaska, Schuur is also deliberately heating areas of permafrost in ongoing research to simulate future warming and learn how the ecosystem reacts. The tests seem to confirm that more carbon will exit the permafrost, he said. They are also pointing to how melting ice in the soil is causing changes in moisture — leaving some areas very wet while others are dry.
Monitoring the Arctic and creating more accurate predictions for the carbon it will emit could inform future economic policy. Schuur said he is working with University of Arizona economist Derek Lemoine to determine the economic benefits of more precisely knowing the Arctic's future greenhouse-gas contributions, including a more effective rollback of human-based emissions.
"Right now, the Arctic is remote and, I would say, understudied," Schuur said. "If you put sensors up there and you have monitoring stations and you know, 'Hey, we're releasing a lot or releasing a little,' then you can adjust your own policies to act more efficiently."
The professor said he believes the best way to reduce emissions tied to permafrost is to "put our own house in order" and cut emissions from human activity, which accelerate the warming of the Arctic and the thawing of permafrost.
The study, "Tundra underlain by thawing permafrost persistently emits carbon to the atmosphere over fifteen years of measurements," published March 30 in JGR Biogeosciences, was authored by Ted Schuur, Chris Ebert, Justin Ledman, Elaine Pegoraro, Heidi Rodenhizer and Christina Schaedel, Northern Arizona University; Rosvel Bracho and Gerardo Celis, Northern Arizona University and University of Florida; Fay Belshe, Jason Vogel and Elizabeth Webb, University of Florida; Marguerite Mauritz, Northern Arizona University and University of Texas at El Paso; Cesar Plaza, Northern Arizona University, Rey Juan Carlos University and Spanish National Research Council; Vladimir Romanovsky, University of Alaska Fairbanks; Dave Schirokauer, Denali National Park and Preserve; and Meghan Taylor, Northern Arizona University and Yale University.