As the frozen soil in the Arctic thaws, bacteria will break down organic matter, releasing long-stored carbon into the warming atmosphere.
At the same time, plants will proliferate, nurtured by balmier temperatures, more nutrients from decomposing soil and the increasing abundance of the greenhouse gas they depend on for growth.
These connected but contrasting changes have raised a question for scientists who study the causes and consequences of global climate change: Will the shrubs and incipient forests spreading across the Arctic compensate for the permafrost's rising release of carbon, blunting its impact on a warming planet? Or, with twice as much carbon locked up in the permafrost as now present in the atmosphere, will the lush growth become overwhelmed -- like a kitchen sponge put down to stem a water main break?
Researchers led by a University of Florida ecologist may have an answer. In a paper set to appear May 28 in the journal Nature, the team reports experimental results suggesting tundra plant growth may keep up with rising carbon dioxide initially.
But if thawing continues in a warmer world, the permafrost will spew carbon for decades, and the plants will become overwhelmed -- unable to sop up the excess carbon despite even the most vigorous growth.
"At first, with the plants offsetting the carbon dioxide, it will appear that everything is fine, but actually this conceals the initial destabilization of permafrost carbon," said Ted Schuur, a UF associate professor of ecology and lead author of the paper. "But it doesn't last, because there is so much carbon in the permafrost that eventually the plants can't keep up."
Schuur noted most of the 13 million square kilometers, or roughly 5 million square miles, of permafrost in Alaska, Canada, Siberia and parts of Europe remain frozen. However, thawing already occurring around its southern edges is expected to expand this century.
Should that occur, this study suggests the permafrost could lose in the range of 1 gigaton of carbon, or 1 billion tons, per year – about the same order of magnitude as being added by current deforestation of the tropics, another large biospheric source, Schuur said.
While burning fossil fuels contributes considerably more carbon, about 8.5 gigatons annually, that process can at least in theory be controlled – whereas once the permafrost thaw begins, it sets up a self-reinforcing loop far from human activity and potentially difficult to stop.
Again, no time.