Just how much Arctic permafrost will thaw in the future and how fast heat-trapping carbon dioxide will be released from those warming soils is a topic of lively debate among climate scientists.

To answer those questions, scientists need to understand the mechanisms that control the conversion of organic soil carbon into carbon dioxide gas. Until now, researchers believed that bacteria were largely responsible.

But in a study published online in Science on Aug. 21, 2014, University of Michigan researchers, including Professor George Kling, show for the first time that sunlight, not microbial activity, dominates the production of carbon dioxide in Arctic inland waters.

Last year, the same team reported in the Proceeding of the National Academy of Sciences that recently exposed carbon from thawed Alaskan permafrost is extremely sensitive to sunlight and can quickly be converted to carbon dioxide.

Worldwide, permafrost soils contain twice the amount of carbon that's in the atmosphere. So thawing permafrost is a special concern for climate modelers trying to predict the timing and extent of future warming due to the ongoing buildup of carbon dioxide and other greenhouse gases.

But soil carbon does not instantly turn into carbon dioxide gas when permafrost thaws. It must be dissolved in water and chemically processed before it gets released into the atmosphere as carbon dioxide. Until now, scientists believed that bacteria were largely responsible for converting dissolved organic carbon into carbon dioxide gas in Arctic streams, lakes and rivers.

To test that assumption, Rose Cory, first author of the Science paper and an assistant professor in the U-M Department of Earth and Environmental Sciences and her colleagues analyzed water samples collected from 135 lakes and 73 rivers on the North Slope of Alaska over a three-year period. They compared the levels of sunlight-induced carbon processing—also called photodegradation, photochemical oxidation or photochemical processing—to carbon conversion due to bacterial respiration.

They found that photodegradation of carbon exceeded bacterial respiration by up to 19-fold, accounting for 70-to-95 percent of the carbon processed in Arctic lakes and rivers. They determined that photochemical processing of soil carbon accounts for about one-third of all the carbon dioxide released from surface waters in the Arctic.

"Carbon in thawing permafrost soils may have global impacts on climate change, yet controls on its processing and fate have been poorly understood," said study co-author Kling, an ecosystem ecologist and an aquatic biogeochemist in the Department of Ecology and Evolutionary Biology. "Our study shows that photochemical processing of soil carbon is an important, newly measured component of the Arctic carbon budget."

Understanding how permafrost carbon is converted into carbon dioxide and incorporating photochemical processing into climate models "is critical for predictions of how the Arctic C [carbon] cycle will respond to and perhaps amplify climate change," the authors of the Science paper conclude.

Michigan News press release