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2007 Arctic wildfire released 50 years of stored carbon into atmosphere

Doug O'Harra

A massive, unprecedented fire that swept across more than 400 square miles of Alaska's North Slope in 2007 spewed more carbon into the air in a few months than the entire Arctic tundra ecosystem normally absorbs in an average year.

Some of the carbon released from the soil last existed in a living plant more than 50 years ago.

The Anaktuvuk River fire, the largest ever seen in the world’s tundra, dumped an estimated 2.1 teragrams -- or about 2.3 million U.S. tons -- of carbon into the atmosphere over just a couple of months, according to a new paper published this week in the journal Nature.

This single event -- which has exposed extensive permafrost to thawing, creating sinkholes and gullies known as thermokast -- will almost certainly help hurry the rate of climate warming in the Arctic and has already begun to alter how scientists view the Arctic’s carbon budget.

“Fire has been largely absent from tundra for the past 11,000 or so years, but the frequency of tundra fires is increasing, probably as a response to climate warming,” explained study co-author Syndonia “Donie” Bret-Harte, an ecosystem ecologist at the University of Alaska Fairbanks Institute of Arctic Biology in this story by UAF’s Marie Gilbert.

“If the frequency of these fires remains at long intervals, 80 to 150 years, then the tundra has time to recover. If these fires occur more frequently, say every 10 years or so, then the landscape cannot recover.”

Such an enormous short-term dump of greenhouse gas shows that wildfires have the potential to abruptly cancel out any theoretical upside from climate warming in the Arctic, the scientists say.

“The amount of carbon released into the atmosphere from this fire is equivalent to the amount of carbon stored by the global tundra biome,” said lead author Michelle Mack, a biologist from the University of Florida in Gilbert’s story. “This was a boreal forest-sized fire.”

It amounts to a stunning reality check on climate change dynamics -- and a wake-up call to anyone hoping that the rise in greenhouse gas concentrations might someday get stalled by the flourishing of carbon-hungry Arctic plant growth.

The thinking goes that as higher temperatures and more rain allow more shrubs and trees to spread into the north, and growing seasons lengthen, the “greening” of the Arctic might have the potential to slowly remove an ever larger share of greenhouse gases like carbon dioxide from the air and help stabilize the home planet’s climate by acting as a sort of carbon “sink.”

Thus, a greening Arctic and faster-growing boreal forest might store enough carbon to begin to offset emissions from human vehicles, ships, power plants and factories.

But this new study from Nature shows that a single hot sunny summer that dries out Alaska’s tundra -- perhaps baking it into flammable tinder ready to ignite with a single lightning strike -- can shift the dynamics of the carbon cycle as fast as flames licking through an expanse of parched and crumbling moss.

“Our results show how rapidly a single tundra fire -- even one that burned relatively surficially -- can offset local and biome-scale (carbon) uptake,” the authors wrote. “At both local and regional scales, fire emissions of this magnitude can instantaneously offset or reverse (carbon) cycling processes hypothesized to feedback negatively to warming such as greening of the Arctic.”

Biggest tundra fire on record derails the carbon budget

The natural presence of certain gases like carbon dioxide in the air helps keep the Earth warm enough for life to exist. Solar radiation warms the planet, and these gases slow the loss of that heat back into space. They act somewhat like glass in a greenhouse, hence the nickname.
 
After thousands of years of relative stability, Earth’s carbon dioxide concentration has been rising over the past century, in sync with the burning of fossil fuel by industrialization and the emissions of modern life. The Earth’s CO2 level is now more than 393 parts per million, according to the latest measurements posted by the Mauna Loa Observatory in Hawaii.

Scientists say such concentrations will accelerate overall warming of the atmosphere and eventually bake the planet to average temperatures not seen in millions of years -- leading to climate change on a scale not seen during the rise of human civilization. Predictions for the coming century include dramatic rises in sea level as ice fields and glaciers melt, big shifts in temperatures that could alter growing seasons and displace agriculture, thawing of permafrost, loss of summer sea ice in the Arctic, plus a slew of more extreme weather events like Category 5 hurricanes, heat waves, droughts, floods and cold snaps.

This human-triggered increase in CO2 concentrations has also prompted scientists to take closer looks at the Earth’s natural carbon cycle, and some surprising details have emerged about the role played by Far North ecosystems in the suck and spew of greenhouse gases.

Charred tundra on Alaska's North Slope. (Photo courtesy: Photo courtesy Adrian Rocha, Marine Biological Laboratory)

Generally, summer growth and green-up absorb carbon and sometimes store it in peat, dirt and permafrost. Natural decay and wildfires release it back to the air.

A study published last month reported that the world’s forests play a much larger role in this carbon two-step than anyone realized, especially tropical rain forests in South America, Africa and Southeast Asia. The great boreal forests that encircle the Northern Hemisphere — including the vast spruce-birch woodlands of Alaska — are thought to account for about 22 percent of the total.

“Our results imply that clearly, forests play a critical role in Earth’s terrestrial carbon balance, and exert considerable control over the evolution of atmospheric carbon dioxide,” said A. David McGuire, co-author and professor of ecology at the UAF’s Arctic biology institute, in a recent article.

But Alaska’s share in the forest carbon drawdown has been flagging in recent decades --mostly due to increases in wildfire, McGuire told Alaska Dispatch in an email.

Rather than absorb carbon, Alaska’s boreal forests lost 6 teragrams of carbon per year during the 1990s, and 14 teragrams of carbon per year in the 2000s.

“The estimated losses occurred primarily because of increased fire activity that occurred in the 1990s and the 2000s in comparison with earlier decades,” McGuire said. “Most of the losses occurred from ecosystems in interior Alaska that were impacted by fire  (between the Alaska and the Brooks Ranges)."

It’s a small amount when compared to the 2,400 teragrams absorbed by the world’s forests, with boreal forests sucking up about one fifth of the total.

“Alaska itself seems to be a pretty small player in the total sequestration of carbon by the world's forests,” McGuire added. “However, the estimated losses of soil carbon in Alaska due to wildfire and climate might be a harbinger of things that could happen in Canada and Russia."

If climate change triggers more losses -- through wildfires, insect infestation, permafrost thaw  -- then the “capability of the world's ecosystems to offset some of the anthropogenic carbon emitted to the atmosphere (from fossil fuel burning and tropical deforestation) will be diminished,” he said.

Lightning strike and a paradigm shift

The Anaktuvuk River Fire was ignited by lightning in July 2007. The warmest summer temperatures in at least 19 years, combined with about a quarter of the normal precipitation, helped it spread. Scientists watched a “wall of smoke” from the Toolik Field Station, only 15 miles to the south, Gilbert said.

Such warm, dry conditions were maintained by high pressure over the North Slope that appeared to be related to that summer's record retreat of sea ice, the authors explained in a supplemental discussion of their research.

“Normally we would expect the fire to go out in the moist soil, but this summer was so dry that the fire didn’t go out and strong winds in September caused it to burn a very large area,” said Bret-Harte, in this story.

When snowfall put the fire out in October, the fire had burned across 257,000 acres -- doubling the total acreage burned on Alaska’s North Slope over the past 50 years, the authors said.

“This fire was an order of magnitude larger than the average fire size in the historic record for the North Slope … and remotely sensed indices of severity were substantially higher than for other recorded tundra burns.”

The fire soon became the focus of an intensive research investigation by scientists at the Institute of Arctic Biology at the Univesity of Alaska Fairbanks. 

“Now, scientists are amassing growing evidence that major events precipitated by warming -- such as fires and the collapse of slopes caused by melting permafrost -- are leading to the loss of tundra in the Arctic,” Alaska writer Bill Sherwonit wrote in this article that described some of the impacts of the Anaktuvuk River fire. “The cold, dry, and treeless ecosystem -- characterized by an extremely short growing season; underlying layers of frozen soil, or permafrost; and grasses, sedges, mosses, lichens, and berry plants -- will eventually be replaced by shrub lands and even boreal forest, scientists forecast.”

Here’s another discussion about research into the fire.

“The fire was on the tundra where small fires occasionally burn but large ones are quite rare,” according to the IAB’s website for the project. “The fire was large enough that it appears to include several complete 1st-3rd order watersheds."

Mack, Bret-Harte and the study’s six other authors tackled the task of figuring out how the fire had tinkered with the tundra’s carbon budget. They sampled 20 burned sites in 2008 and 11 unburned sites from the region, and then calculated the loss of carbon into the air.

What they found suggested that this single fire -- covering an area as large as Cape Cod and visible from space -- canceled out the entire Arctic uptake of carbon for the year.

The combustion released soil carbon to the air 30 to 50 times faster than other processes -- such as a surge in thawing permafrost or decomposition triggered by temperatures rising up to 5 degrees Celsius, the scientists said.

The total released -- 2.3 million tons -- is about the same amount of carbon that would be produced collectively by the exhaust of more than 19,000 cars, assuming each dumped 120 tons while motoring 12,000 miles over the course of a year.

“These fires could be a radical and very rapid positive feedback to atmospheric carbon dioxide,” said Mack, the lead author, in this story.

Contact Doug O'Harra at doug(at)alaskadispatch.com