Digging up the dirt on Arctic carbon

You never know what you'll find until you dig a little deeper. Scientists taking the measure of how much carbon the Arctic locks up beneath its tundra have done just that. Based on what they've found, they estimate that the Arctic could harbor an average of 60 percent more carbon that previous estimates have indicated.

Researchers are interested in the Arctic's carbon budget because projections of global warming suggest that the region's average temperature could warm by as much as 6 degrees Celsius (10.8 degrees Fahrenheit) by the end of the century, depending on the trajectory that emissions from human activities take. The concern: As the Arctic continues to warm and the permafrost thaws, significant amounts of carbon will find their way into the atmosphere as carbon dioxide or methane. These added greenhouse gases would serve to reinforce global warming.

The results come from a group led by University of Alaska at Fairbanks soil scientist Chien-lu Ping and published in today's edition of Nature Geoscience. Hints that the Arctic's tundra may lock up more carbon than older, widely cited estimates had indicated date back at least to 2005. For instance, a team led by Jeffery Welker at the University of Alaska at Anchorage reported at a meeting of the American Geophysical Union that in Greenland, it was finding between nine and 125 times the amount of carbon previously reported, depending on the type of Arctic landscape yielding the measurements.

The reason for wide differences between old and newer estimates? Researchers are literally digging deeper and in more places. The older estimates were based on measurements taken from the first 4 to 16 inches of soil, notes Dr. Ping's group. But as Arctic soils warm over the course of this century, the heat will reach deeper, enlarging the permafrost's active layer – the one subject to thawing and slow overturning. Ping's group reached depths of nearly 40 inches, and did so at 117 spots around the Arctic to include different Arctic soil types. Once the team took its measurements, it beefed up the sample size (using previously published results) to 139 locations. Past studies, by contrast, had looked at only a handful of soil types. And based on today's criteria, some of those older test locations no longer qualify as Arctic.

Once Ping's team had scooped up its data, it used soil maps from other parts of the Arctic to extend its estimate of carbon stocks beyond North America's Arctic.

The results highlight "the continual efforts by the scientific community to develop an accurate estimate of what's out there," says Dr. Welker in a phone chat. But, he adds, the next question is more complicated: How fast and how much of the tundra's carbon stocks might move into the atmosphere? It's a balancing act between the CO2 that tundra vegetation takes up via photosynthesis and the CO2 the soil gives off via respiration during the short but intense growing season. In addition, scientists increasingly are interested in CO2 given off through soil respiration in winter -- something that could be an important part of the Arctic's carbon equation.

At the moment, results from his and others' work points to the Arctic as a continued net source for atmospheric carbon. It's the combined effect of the tug of war between photosynthesis and respiration, plus decomposition of frozen organic material as the permafrost thaws. The Arctic's permafrost locks up far more carbon than the atmosphere currently holds, so even small changes in the region's status as a sink or source could have a significant effect on climate. Little wonder that scientists are digging deeper to pin down the details.

Note: Eoin O'Carroll is on vacation. He will return Sept. 2