Thawing Permafrost in the Arctic Will Speed Up Global Warming
There's more carbon locked up in the high north's permafrost than the combined total released into the atmosphere by humans. The big melt going on right now in the Arctic could trigger a train of catastrophic events across the world.
On the hill above Toolik Field Station, Gaius Shaver has been running experiments for more than 30 years to determine how fertilizer changes the tussock tundra's floral medley. His test plots, north of Toolik Lake, are accessible only by a mazelike wooden walkway that, at points, is little more than a web of two-by-sixes. As a rule, nobody ventures off the walkway, lest they crush someone else's experiment. On an overcast morning Shaver, clad in mud boots, a Carhartt jacket, and blue jeans, confidently makes his way along the slippery walkway and up to his plots. These are very different from the nearby untouched tundra. Because nitrogen and phosphorous are scarce in the frozen tundra in forms useful to plants, yet crucial to their growth, Shaver wanted to see what would happen if he added the nutrients to small plots. It's been more than two decades since he started the study, which has transformed the plots from calf-high grass-dominated tussocks to knee-high shrub-covered patches. In one section covered with clear plastic--a makeshift greenhouse--birches are thriving and waist high, but few, if any, other species abound. The clear lesson: When more food is available, shrub growth takes off, shading out plants like cranberries and cotton grass.
As the North Slope has warmed during the past half century, it has become shrubbier. While this rise in woody vegetation has been a boon for songbirds and moose--mammals once scarce as far north as Toolik, now regularly spotted there--it may help spur warming. "Shrubs tend to feed back into the local and regional climate," says Michelle Mack, a University of Florida plant ecologist. During the summer, birch, willow, and alder reflect solar radiation, driving up atmospheric temperature. In winter, tall, branched woody plants trap snow that would otherwise blow across the tundra. This actually keeps the ground warmer and may allow soil microbes to remain active for longer, cycling nutrients (providing food for shrub growth) and releasing greenhouse gases.
Disturbances like thermokarsts could also expand shrub cover. As with the older thermokarst at lake NE-14, decades after these features form "you see shrubs, not tussocks," says Mack. Once shrubs move in, she adds, it could be difficult for lost permafrost to become reestablished.
More woody material also means more carbon is being stored above ground, instead of in the soils. While plants may trap carbon for hundreds of years before it's cycled back into the atmosphere, permafrost can store it for tens of thousands of years.
Since trees act as carbon sinks, soaking up CO2, a shrubbier tundra might seem like a plus--but it actually exhales more carbon than tussock tundra. Although the plants in Shaver's fertilized plots have more biomass and take up more carbon dioxide than those in the control plots, that gain is offset by the loss of carbon and nitrogen from deep soils; the nutrient-rich plots saw a net loss of nearly 2,000 grams of carbon per square meter over 20 years, Mack and colleagues reported in Nature in 2004. "The tundra is moving toward a shrubbier community, which means it will hold less carbon overall," says Mack. The release of that carbon into the atmosphere, in turn, will feed back into more warming.
All of that wood creates more potential kindling. "We know that 12,000 years ago, when the tundra was more shrubby, there were more fires," says Mack. Now, for the first time since the woolly mammoth and sabertooth tiger went extinct, big wildfires are again raging on the North Slope.
"As an undergrad I took an Arctic seminar and was taught that there are no cumulus clouds in the Arctic, no thunderstorms," John Hobbie, a founder of Toolik Field Station, recalls of his education in the 1950s. Today in the warmer modern Arctic, thunderstorms are regular summer events. "One day this year we had 272 lightning strikes on the North Slope," says Syndonia Bret-Harte, an ecologist at the University of Alaska-Fairbanks and Toolik's associate science director. Most of the flashes that hit the wet tundra don't catch fire.
But the summer of 2007 was an exceptionally dry year, and in July a strike hit the grass near Toolik, smoldered for a few weeks, then exploded. Bret-Harte recalls watching the fire from camp. "When the wind was not blowing, you could see this big wall of smoke," she says, "and that was awesome and beautiful but disturbing at the same time. When the wind shifted and brought the smoke into camp, then it was like being in this thick, acrid fog. It was gross." The Anaktuvuk River fire, the largest ever recorded on the North Slope, burned until October, ultimately consuming nearly 350 square miles and releasing 2.2 million metric tons of carbon into the atmosphere--the amount the entire country of Barbados emits annually.
Walking around the severely burned area two years later, the previously scorched earth dusts my boots and pant legs. Already some cotton grass has come back, and the green stems and white tufts growing out of singed tussocks contrast sharply with the blackened terrain. Bret-Harte estimates that plants now cover about half of the ground. But, she adds, "I haven't seen any of the normal mosses and lichens coming back."