Arctic Permafrost Melt Releasing Carbon Dioxide At Unprecedented Rate
February 12, 2013

Arctic Permafrost Thaw Releasing Carbon Dioxide At Unprecedented Rate

Lawrence LeBlond for - Your Universe Online

Editor's note (Feb 16): This story earlier reported that thermokarst failures were occurring do to permafrost melt. However, permafrost doesn't melt, it thaws. The changes have been made to this story to reflect that.

Researchers studying Arctic thermokarst failures in Alaska were alarmed to find climate-warming carbon dioxide gas may be releasing into the atmosphere at an unprecedented rate. This release is being caused by thawing of the Arctic permafrost, which has kept ancient carbon locked away for millennia; only to be exposed as more warmth and sunlight erodes the long-frozen soils, causing collapse and release of carbon.

George Kling, an ecologist and aquatic biogeochemist at the University of Michigan, said these areas where the permafrost is thawing are causing land surfaces to erode and collapse, exposing long-buried soils to sunlight. This sunlight increases bacterial conversion of the exposed soil carbon into CO2 by at least 40 percent compared to carbon that remains in the dark.

The study was led by Rose Cory, of the University of North Carolina, and published online (Feb 11) in the Proceedings of the National Academy of Sciences (PNAS).

Prior to this study, little has been understood about the transformation of organic Arctic carbon into CO2. Instead, much of the work on permafrost degradation has focused more on the widespread, slow thawing of the icy soils from the top down, said Vladimir Romanovsky, a geophysicist at the University of Alaska, Fairbanks (UAF).

Romanovsky, who was not involved in the research, said in an interview with Science Magazine that thawing transfers carbon into surface waters, but in recent years scientists have recognized there is more to the permafrost story than previously considered. As the melting has progressed in the Arctic, more soil erosion has occurred, causing large holes and landslides across the tundra. The permafrost underneath is exposed, begins to melt, causes the land to slump, and creates features known as thermokarst failures.

Thermokarst failures have the potential to release rapid amounts of carbon-rich organic material, as streams of water from the melting ice cut deep channels into the soil and transport carbon into rivers and the ocean. But what happens to those huge rivers of carbon has not been well studied, said Cory.

"Until now, we didn't really know how reactive this ancient permafrost carbon would be – whether it would be converted into heat-trapping gases quickly or not," said Kling, a professor in the U-M Department of Ecology and Evolutionary Biology.

"What we can say now is that regardless of how fast the thawing of the Arctic permafrost occurs, the conversion of this soil carbon to carbon dioxide and its release into the atmosphere will be faster than we previously thought," Kling said in a statement. "That means permafrost carbon is potentially a huge factor that will help determine how fast the Earth warms."

Massive stores of organic carbon have been frozen in the Arctic permafrost for thousands of years, and if thawed and released as CO2, this vast carbon warehouse has the potential to double the amount of greenhouse gas in the atmosphere on a timescale similar to that of humankind´s contribution from the burning of fossil fuels.

To study the effects of the permafrost-thaw, Cory and her colleagues collected samples of water running out of seven Alaskan thermokarst failures into nearby lakes and streams. They analyzed the samples for colored dissolved organic matter, which contains light-absorbing molecules from the breakdown of plant polymers such as lignin.

"The general idea is that the closer to the terrestrial source, the more color the carbon will have," Cory says.

At six of the seven sites, Cory and her colleagues irradiated the samples with UV light to mimic the effects of sunlight.

Surprisingly, they discovered the samples from the thermokarst failures had lower levels of colored dissolved organic matter than seen in the reference sites where slow, top-down melt occurred. This suggests the carbon in the deeper soils exposed by thermokarst failure is significantly different from the carbon draining from top-down melting above the permafrost.

The deeper carbon was about 40 percent more susceptible to photochemical and biochemical degradation into CO2 than was carbon from the top active layer, meaning the carbon coming out of the thermokarst sites is more reactive than the carbon draining from the active layer above the permafrost, Cory explained.

This creates the potential for positive feedback for more warming, she said.

“As the Earth warms due to the human-caused release of heat-trapping gases into the atmosphere, frozen Arctic soils also warm, thaw and release more carbon dioxide. The added carbon dioxide accelerates Earth's warming, which further accelerates the thawing of Arctic soils and the release of even more carbon dioxide,” according to the study.

“We know that in a warmer world there will be more of these thermokarst failures, and that will lead to more of this ancient frozen carbon being exposed to surface conditions," Kling said. "While we can't say how fast this Arctic carbon will feed back into the global carbon cycle and accelerate climate warming on Earth, the fact that it will be exposed to light means that it will happen faster than we previously thought."

Cory said her team is now working on scaling up their findings. But with data still limited, it will be a challenge to determine how widespread the effect is.

In some areas of Alaska and Siberia, scientists have begun to conduct more thorough analyses on thermokarst failures, noted Romanovsky in the Science Magazine interview. "But these features are still very localized, and the question of how important this is for the entire northern hemisphere is still unclear."