When permafrost thaws, microbes convert ancient organic matter in the frozen soil into climate-warming gases, like carbon dioxide and methane, potentially triggering a positive feedback loop that further melts the Arctic.

But the once-barren soil also spouts new—and larger—shrubs that can act as a carbon sink, and scientists have wondered what the net effect of permafrost thawing would be on atmospheric carbon balance.  A new study published this week in Nature suggests that changing landscape can counterbalance the release of permafrost carbon—but only for a little while.

"The greening of the Arctic will not compensate for the huge amount of permafrost carbon that will be released," says lead author Ted Schuur, an ecosystem ecologist at the University of Florida in Gainesville.

Permafrost contains approximately 98 petagrams (98 quadrillion grams, or 108 billion tons) of carbon—or one sixth the total amount currently in the atmosphere. In order to understand how rapidly this carbon would be released into the air, Schuur and his group have been measuring carbon dioxide absorption and emission by plants and soil at the Eight Mile Lake watershed in the northern foothills of the Alaska Range across what they call a "gradient of thaw." Then, they use radiocarbon dating to determine how long the greenhouse gas had been trapped as carbon in the permafrost.

Not surprisingly, the researchers found that more extensively thawed soils emit carbon at a higher rate than recently thawed soils. But when the researchers looked at sites that had been thawing for about 15 years, they found that the increased plant growth more than compensated for their old carbon emissions. After 15 years, however, these soils were losing much more old carbon than the new plants were able to soak up.

"This is the first real study to put numbers on these things," says David Lawrence, an Earth systems scientist at the National Center for Atmospheric Research who was not involved in the study. "Data like this are critical." He is eager to incorporate the new findings in the Community Land Model used for climate simulations.

Lawrence points out, however, that not all permafrost will thaw in the same way. The study was conducted on a type of ground called thermokarst, which allows water to drain away as it melts. In wetter areas, Lawrence explains, permafrost has the potential to turn into a peat land and remain a long-term carbon sink, but that could also lead to increased methane emissions.  "From a global warming perspective," he says, "that could still turn around and be positive feedback."

Schuur agrees with this assessment. "We can't say its going to be 15 years in every place," he says, "but we can say [that] in the initial decade of thaw permafrost will be acting as a carbon sink, and by 30 years it will be a carbon source."