Arctic permafrost holds about twice as much carbon as the atmosphere, making its ultimate fate a key factor in the severity and pace of climate change impacts.

Now a study reports that old assumptions of how permafrost carbon breaks down in Arctic lakes and rivers may be wrong. Instead of bacteria being responsible for the conversion of carbon into carbon dioxide, the real culprit in many cases is sunlight, according to the research supported by the National Science Foundation.

The findings could change estimates of how much permafrost soil carbon—which constitutes about half the world's soil carbon in total—is transformed to greenhouse gases and released to the atmosphere. They also suggest that warming could exacerbate the release of CO2 from Arctic waters by allowing more vulnerable areas to be exposed to sunlight over time.

"No one knows how much carbon from permafrost soils will be released to the atmosphere as carbon dioxide, but to answer that question, we have to know how it's going to happen," said Rose Cory, an aquatic chemist and lead author of the study, published in Science in late August. "If we consider only what the bacteria are doing, we'll get the wrong answer."

Bacteria activity is still a major player in breaking down carbon in soil directly and in dark areas of waterways. However, a lot of carbon from melting permafrost is washed out of soils into Arctic rivers and lakes. It is in those rivers and lakes that sunlight is playing a larger role than previously believed, Cory said in an email.

She and other scientists sampled 135 lakes and 73 rivers near the Kuparuk River Basin in Alaska between 2011 and 2013 and compared how the samples behaved with and without light in terms of the rate of conversion of dissolved organic carbon to CO2. They found that the breakdown of dissolved carbon via sunlight was responsible for about one-third of all carbon dioxide released from Arctic surface waters. Sunlight also was at least twice as fast as bacteria in converting dissolved carbon into CO2 in these waters. The sunlight effects had not been documented before, said Cory.

While lakes and rivers do not constitute the majority of CO2 release from the Arctic, they play a disproportionate role. This is because much of the terrestrial Arctic is still a carbon sink, because of the uptake of CO2 from plants, explained Cory. Thus, much of the CO2 release there is countered by plants.

Calling for carbon budgets
She said that climate change could intensify the sunlight process, if lakes and coastal waters become ice-free earlier in the season with ongoing warming. Currently, most lakes in the Alaskan Arctic are covered with ice in May, but that could change with time.

"There is more, and intense, sunlight in May through June compared to later in the summer in the Arctic ... warmer weather could cause the lakes to be ice-free sooner, causing more CO2 to be released," she said.

Byron Crump, a study co-author and an Oregon State University microbial ecologist, said in a statement that a chief reason why the sun is so effective at carbon breakdown is that most of the fresh water in the Arctic is shallow, allowing light to reach the bottom of any river. "Also, there is little shading of rivers and lakes in the Arctic because there are no trees," Crump said.

It is unclear how much the phenomenon is Arctic-specific. Some prior research has suggested that carbon dioxide emissions from all lakes on Earth are less than what was found in the Arctic, and future factors could shift the importance of sunlight, noted Lars Tranvik, a Swedish scientist, in an accompanying Science article.

However, much prior work focused on deep lakes, not shallow areas, said Cory. "Because there are many [more] shallower than deep lakes, it is likely that our findings apply outside the Arctic," she said.

The paper fits into a body of research on carbon cycling in melting permafrost. Last year, for instance, scientists reported that the moisture level in frozen soil could make a big difference with greenhouse gas release (ClimateWire, July 29, 2013).

Sally MacIntyre, a professor at the University of California, Santa Barbara, who did not participate in the work, said the paper is significant, especially since recent analyses show that some of the initial estimates on fluxes from Arctic streams and lakes may be too conservative.

"The quantity of carbon expected to be released from thawing permafrost is high, with emissions from Arctic waters expected to be equal to those from land-use change in other regions of the world. Accurate carbon budgets are required to motivate appropriate public policy," MacIntyre said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500