Thermokarst lakes may cool atmosphere
Thermokarst lakes like these, which are found in ice-rich regions of northern Siberia and Alaska, began cooling instead of warming the atmosphere roughly 5,000 years ago, a team led by University of Alaska Fairbanks researchers has discovered.

FAIRBANKS — Lakes formed by melting permafrost, which have been widely considered a troubling source of greenhouse gases, also store organic material that helps offset the effects of climate change, according to a newly published study.

A team led by University of Alaska Fairbanks researcher Katey Walter Anthony concluded that thermokarst lakes, which occur when permafrost melts and the surface depression is filled with melted fresh water, are "climate coolers" because they collect more atmospheric carbon over time than they release.

Many scientists have eyed the lakes as a significant contributor to climate change, since thawing permafrost emits methane, one of the most potent greenhouse gases. Most researchers believe such gases are the key driver of global warming.

But the research team found that other factors allow the lakes to store even more carbon over millennial time periods than they release. Thawing yedoma permafrost, which is rich in organic material, helps act as a fertilizer in thermokarst lakes. That rich environment boosts the growth of mosses and other plants, according to the study, and those help store carbon rather that releasing it into the atmosphere.

"Initially they were a large source of ancient methane, but over time they switched roles," Walter Anthony said.

Most thermokarst lakes were born about 10,000 years ago, eventually blanketing about 70 percent of the terrain in parts of Northern Siberia and Alaska. 

Decomposition initially caused the steady release of methane from those lakes. However, Walter Anthony said much of the organic material in that submerged permafrost was eventually consumed as it thawed, with the methane release decreasing over time.

About 5,000 years ago, a tipping point occurred, according to the study. That's when organic life growing in the lakes began collecting more carbon than was released through methane.

"The role of lakes switched from being climate warmers to climate coolers," Walter Anthony said. "For the last 5,000 years, the lakes have become an increasing climate cooler on the landscape."

Miriam Jones, a U.S. Geological Survey geologist who also worked on the project, said the plant-rich environment present in thermokarst lakes was stunning. 

The research team went to eroded Siberian coastlines to see cross-sections of those lake beds — a process Jones compared to seeing slices of a layer cake — and was amazed at how well-preserved mosses remained, thousands of years after their birth.

"They looked like something that had died last winter and never decomposed," she said.

The discovery of those underwater reservoirs of plant life increases the peat carbon pool estimate in permafrost regions by 50 percent, according to the study.

The team's findings are published Wednesday in the scientific journal Nature. Walter Anthony, a UAF assistant research professor, was joined on the project by Jones, UAF professor emeritus F. Stuart Chapin and Guido Grosse, a geoscientist from the Alfred Wegener Institute in Germany.

The research team used previously published data, field observations in Siberia, radiocarbon dating, atmospheric modeling and other factors to reach the conclusions. 

Despite their stabilizing effect on climate change, the researchers said thermokarst lakes may have only a short-term benefit. After such lakes drain, sediments that were previously underwater often refreeze. Future warming projections make it likely that some of that ancient trapped carbon will eventually be released when those areas thaw again, they say.

Grosse said the fate of that newly exposed permafrost could be the next step in their research.

"I think that's one of the next big goals we need to study, what's going to happen with that carbon," Grosse said. "How vulnerable is it to change in the near-term time scale — the next 100 years, the next 200 years?"

Contact staff writer Jeff Richardson at 459-7572. Follow him on Twitter: @FDNMbusiness.