TOOLIK FIELD STATION, ALASKA—For nearly 3 months in the hot, dry summer and fall of 2007, the biggest arctic tundra fire in Alaska's history—the Anaktuvuk River fire—raged on the North Slope, a large area that contains the Arctic National Wildlife Refuge and the National Petroleum Reserve. Scientists have been concerned that such fires—which seem to be on the rise—could contribute to global warming by burning deep into tundra soils, thereby releasing carbon that thousands of years' worth of plants have stored there. But a new study finds that the Anaktuvuk River fire burned only the newest, topmost layer of the soil, leaving the tundra's ancient stores of carbon intact below. That's a small victory when it comes to climate change, says ecologist Michelle Mack of the University of Florida, Gainesville. Although the fire released a lot of carbon, "it's not radically changing the carbon system, as far as we can tell."
To find out just how much soil carbon the Anaktuvuk River fire burned, Mack and her team helicoptered into sites throughout the burned area in 2008. Some 80% of the normally green tussock cottongrass—which grows in dense clumps—had survived the fire, becoming "a forest of knee-high black pillars." Tussocks normally keep a predictable amount of leaves above the soil, so Mack was able to estimate how much soil—and therefore how much soil carbon—the fire had burned and released by measuring the height of the burned tussocks. Overall, she found that the fire, which eventually charred 1000 square kilometers, liberated 2 billion kilograms of soil carbon, "an impressive amount of carbon for a fire."
Despite the dramatic blaze, the Anaktuvuk River fire burned much more shallowly than Mack had imagined, sweeping across the landscape instead of smoldering down like peat fires in the tropics. Radiocarbon dating of soil samples showed that the fire consumed a soil layer that represents on average 37 years of plant growth and no more than 50 years. "It's not burning as much as it could burn," Mack told a gathering of journalists earlier this week here, only about 40 kilometers south of the burn site. She thinks the deeper soils may not burn because they are moister.
In fact, Mack and colleagues found that the fire left at least 12 centimeters of unburned organic soil on top of the permafrost in the region. Although the fire seems to have led to some thawing already, Mack thinks the remaining soil should be enough to insulate the region's permafrost and prevent a massive degradation.
This is better news than expected for climate change, says Mack. She estimates that arctic tundras, which are found around the world, including in Siberia and Scandinavia, store an amount of carbon equal to nearly half of what's currently in the atmosphere. So if tundra fires were to add massive amounts of carbon into the atmosphere, they could contribute to a feedback loop that could amplify global warming, which in turn could lead to more tundra fires by increasing temperature and lightning strikes.
Fire ecologist Philip Higuera of the University of Idaho, Moscow, writes via e-mail that he is surprised that the fire did not burn older plant material. "If these results hold up" after more sampling, he writes, "it suggests that the carbon released from this fire was [laid down] by plants growing only several decades ago" and could be replaced by future plant growth within decades instead of making an important contribution to global climate change.