Forest snows keep northern soils relatively toasty, diminishing how much climate-warming carbon they can sequester

By Janet Raloff

Web edition: June 6, 2012

Arctic winters may be snowy and cold, but a deep blanket of snow can actually keep the soil surface fairly warm, a new study finds — at least in taiga, the conifer forests that may constitute almost half of the Arctic’s land cover.

Temperature plays a major role in determining not only plants’ uptake of climate-warming carbon, but also the soil’s potential for storing the element.

Scientists who develop computer programs to evaluate climate under changing conditions know this. Yet for convenience, their simulations have largely treated Arctic snows as if they blanket forest-free tundra, notes climate modeler Isabelle Gouttevin of the CNRS/University Joseph Fourier-Grenoble in France.

Her team has now quantified the impact of ignoring the taiga snows’ insulating capacity in climate simulations, and found that the oversight may make a substantial difference. At a depth of 50 centimeters, soil in wintry taiga can be 12 degrees Celsius warmer than computer simulations predict when all snow-covered Arctic terrain is treated like tundra, the researchers conclude June 2 in the Journal of Geophysical Research. Gouttevin’s team also finds that because forested soils heat up from a warmer baseline in spring, their summer temperature at 50 centimeters depth could be 4 degrees Celsius warmer than all-tundra simulations had assumed. 

Blustery winter winds sweeping across the relatively flat tundra compact the snow’s crust, eventually diminishing the whole blanket’s insulating capacity. The taiga’s tree canopy protects the surface, allowing winter snows to remain fairly airy and insulating. This means some taiga soil surfaces can remain around the freezing point all winter, regardless of how low air temperatures plummet. Gouttevin’s team confirmed such details in late winter at tundra field sites in Alaska and taiga locations in Finland.

Accounting for taiga’s insulating snows decreases the amount of carbon that Arctic soils can hold by some 64 billion metric tons, the scientists estimate. Soil warming associated with taiga snows throughout the Arctic also could annually increase by 22 percent the activity of microbes that release carbon-based greenhouses gases into the atmosphere.

After accounting for the greater insulating effect of forest covered Arctic snow, “there is a slight shift in the carbon cycle,” Gouttevin says, “towards less carbon storage by vegetation. This was quite a surprise.”

Accounting for the higher insulating value of taiga snow leads to such a dramatic shift in soil warming, “that decomposition of soil organic matter (greenhouse gas production) and permafrost thawing would be significantly greater at the global scale,” says forest ecologist Glenn Juday of the University of Alaska Fairbanks. “What elevates this result from the mundane story of another [computer] model with another parameter that needs fixing,” he adds, “is the huge pool of carbon stored in cold or frozen soils in the north — more than the atmosphere and land plants combined.”