Alaska's biggest tundra fire sparks climate warning

• New Scientist, 30 July 2009 by Tracey Logan
• http://www.newscientist.com/article/dn17537-alaskas-biggest-tundra-fire-sparks-climate-warning.html
• For similar stories, visit the Climate Change Topic Guide

The fire that raged north of Alaska's Brooks mountain range in 2007 left a 1000-square-kilometre scorched patch of earth – an area larger than the sum of all known fires on Alaska's North Slope since 1950.

Now scientists studying the ecological impact of the fire report that the blaze dumped 1.3 million tonnes of carbon dioxide into the atmosphere – about the amount that Barbados puts out in a year. What's more, at next week's meeting of the Ecological Society of America in Albuquerque, New Mexico, two teams will warn that as climate change takes hold tundra fires across the Arctic will become more frequent.

Tundra fires only take off once certain thresholds are reached, says Adrian Rocha of the Marine Biological Laboratory, Woods Hole, Massachusetts. "But projected changes in climate over the next century – increased aridity, thunderstorms, and warming in the Arctic – will increase the likelihood that these thresholds will be crossed and thus result in more larger and frequent fires."

Scarred surface

Rocha's team placed carbon dioxide and radiation sensors across the fire-scar and found that in the year after the fire, the most severely burned tundra emitted twice as much carbon as undamaged tundra normally stores away.

Pristine tundra takes up about 30 to 70 grams of carbon per square metre during the summer months, whereas the severely burned site lost about 40 to 120 grams per square metre. The team also found that the most severely burned terrain absorbed 71 per cent more solar radiation than normal, warming faster as a result and losing a layer of permafrost 5 to 10 centimetres deep.

"That may not seem like a lot," says Rocha, "but over the entire fire scar you're talking about 5 to 10 cm of water over a 1000 sq km area." Plus there's the double whammy of positive feedback: as tundra burns and emits carbon, it melts the permafrost – and that releases more carbon into the atmosphere. "Along with the melting ice in the permafrost, you're also exposing more old carbon that was stored in that freezer [as organic material] and is being allowed to decompose and reintroduce itself to the atmosphere."

Another team, led by Michelle Mack of the University of Florida, carbon-dated soil at the most severely burned sites. They found that organic matter accumulated over 50 years had been lost.

Long-term impact

Wetland ecologist William Bowden of the University of Vermont, Burlington, not at the ESA meeting, says that the fire blackened the surface and increased the amount of water in the soil. "Both factors should promote soil warming, permafrost thaw, and possible thermokarst formation," he says.

Thermokasts are areas of collapsed terrain where structurally important permafrost has thawed – a process that can damage the foundations of homes, roads, and pipelines. Permafrost melt will also increase the amount of greenhouse gases such as methane entering the atmosphere.

There are great similarities between plants and soils across the Arctic, adds Bowden, and lessons learned in Alaska are relevant to similar terrains in Canada and Russia.