As alpine permafrost thaws, new sources of decaying organic matter become available to CO2-emitting microbes. Climate scientists and their models may be underestimating this stealthy source of carbon dioxide, according to a new study.

In a paper published Thursday in the journal Nature Communications, scientists presented evidence that Colorado's Front Range tundra emits more CO2 than it absorbs each year, making it a net carbon contributor -- potentially worsening the impacts of climate change.

Previous studies have suggested melting Arctic tundra is releasing CO2 that has been sequestered in the frozen soil for centuries.

"We wondered if the same thing could be happening in alpine terrain," lead researcher John Knowles said in a news release. "This study is a strong indication that that is indeed the case."

Now a scientist at the Institute of Arctic and Alpine Research and a researcher at the University of Arizona, Knowles conducted the study as a geography doctoral student at the University of Colorado, Boulder.

The carbon sequestering services of forests are well documented. Trees and other types of vegetation absorb CO2 via photosynthesis. When their leaves and branches fall to the ground and decay, the organic matter is broken down by microbes, releasing CO2 back into the air. But much of the carbon absorbed by trees is stored in the tree's root system and the surrounding soil -- more than is released by munching microbes -- allowing many forest ecosystems to serve as a carbon reservoir.

According to the latest study, tundra and melting permafrost feature a slightly different balancing act -- one that is less friendly to the warming climate.

When previously unavailable carbon-rich organic matter becomes available to hungry microbes, the ecosystem's greenhouse gas emissions increase. To quantify this dynamic, Knowles and his research partners measured surface-to-air CO2 transfer rates between 2008 and 2014 at Colorado's Niwot Ridge Long Term Ecological Research site.

Across the tundra landscape in Colorado's Front Range, scientists confirmed more carbon is emitted than absorbed over the course of each year. They also measured the release of old carbon during the middle of the winter. The discovery suggests scientists have underestimated year-round microbial activity.

"Microbes need it to be not too cold and not too dry, they need liquid water," said Knowles. "The surprise here is that we show winter microbial activity persisting in permafrost areas that don't collect much insulating snowpack due to wind stripping it away."

Alpine forests are likely to remain carbon sinks, but fields of treeless tundra may continue to release greater levels of greenhouse gas as the climate warms.

"Until now, little was known about how alpine tundra behaved with regard to this balance, and especially how it could continue emitting CO2 year after year," Knowles said. "But now, we have evidence that climate change or another disturbance may be liberating decades-to-centuries-old carbon from this landscape."

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