In 2007, the largest Arctic tundra wildfire on record released around 2.1 million tonnes of carbon to the atmosphere, adding to the levels of greenhouse gas CO2. That’s close to how much carbon tundra plant growth across the whole Arctic absorbs in one year, noted Michelle Mack at the University of Florida. With human-caused, or anthropogenic, climate change seemingly causing more fires, further CO2 release may contribute to more warming. Other consequences will also have a big local impact, Mack told Simple Climate. “Fire on this landscape will change many things, and that’s frightening for me because I do think that the increasing fires are driven by anthropogenic climate change,” she said. “That people emitting carbon from cities, factories and automobiles very far to the south are influencing this wilderness area where people still practise subsistence livelihood is disturbing to me.”
For about a decade, Mack has been a regular at the Toolik Field Station on the North Slope of the Brooks Range of mountains in far northern Alaska. After the fire started in July 2007 at Anaktuvuk River, a plume of smoke could be seen drifting through the air from the Toolik Field Station 15 miles to the southeast. “When it started it was characteristic of these tundra fires – very small – just a couple of hectares from a lightning strike,” Mack said. “Normally a fire like that would just go out and there would just be a little blackened spot. It wasn’t until August that the weather conditions were such that the fire blew up and burned a really large area. At that time you could see it from space. People in local villages like Anaktuvik Pass and other coastal villages were getting smoked out. People were miserable.”
The fire scarred 1000 square kilometres of tundra, equal to the total area of all recorded fires on the North Slope since 1950. After it had stopped, and the following winter’s snows had melted the next June, Mack and a team of US scientists began to look at the damage it has caused. But before they could do so, they had to figure out how to best analyse the tundra fire’s footprint in the soil. Seeing that tussocks of sedge plants had been able to survive, the researchers exploited them.
“Sedge has really dense leaf bases,” Mack said. “As organic soil accumulates, these tussocks grow up to stay in the light so they don’t get covered by other plants and by mosses. The dense bases of the tussocks didn’t burn, but the organic soil burnt out from between them.” Travelling widely across the Arctic tundra, the researchers collected data on unburned tussocks to compare with those at the fire site. “We developed this methodology so that we could relate the height of the tussocks to the depth of the soil, carbon concentration, bulk density and nitrogen concentration.”
The tussocks revealed that, while wildfires in grasslands mostly burn plants, the tundra fire burned much of the peat-like soil, down to an average depth of 6 cm. Those conclusions, revealed in top journal Nature on Thursday, were backed up by Mack’s husband and co-worker Ted Schuur, who gauged the age of the soil burnt from moss fossils on the new surface. Analysing how much carbon in the moss has decayed from a radioactive type to its more abundant form can show how old the fossils, and therefore the new surface, are. The average date of formation of the uncovered surface was 1970 – and soil in Alaska has been shown to deepen quickly enough to deposit 6 cm in 37 years. “70 per cent of the carbon loss from the fire came from that soil carbon,” Mack said. “You’ve removed a very old stock of carbon from the landscape. Like burning old growth tropical forests, that’s changing the net balance of carbon on the Arctic landscape very rapidly.”
Curiosity and concern
Climate change has created a warmer Arctic, meaning more plants can grow there and absorb CO2 from the atmosphere. However, charcoal and pollen from lake sediments have suggested that tundra wildfires were frequent 10,000 years ago, and could be again as the landscape heats up. Fires in tundra and forests further south in Alaska have already been shown to become more common as temperatures rise, Mack adds. Consequently, she warns that more frequent wildfires could undermine any chance an Arctic turning from white to green might have of slowing global warming.
Mack is also troubled by what more wildfires would mean for local people. “There are reports that not only are there health issues with smoke, but that fires change the migratory patterns of caribou, they interfere with subsistence activities like hunting or berry picking,” she said. These worries temper her enthusiasm at having been able to visit new areas of tundra due to the fire, and studying events previously unseen by modern humans. “I look at these soil profiles that have been accumulating for 7,000 years, and they’ve never had an event like this, where they burn and charcoal is deposited,” she enthused. “But even though I’m excited by fires at the same time I’m worried about the people who live here, a tundra system that hasn’t burned in thousands of years.”