Alaska Wildfires Release Decades of Carbon

Three things you need to know:

—In the past decade, climate change has caused wildfires in Alaskan forests to burn more often and more severely.

—These fires are burning layers of moss and peat in the soil, which releases greenhouse gases that had been stored over decades and centuries and even millennia.

—These forests, which previously stored immense quantities of carbon, are now releasing more than they are saving.

Wildfires burning in Alaska's boreal forests are now releasing more greenhouse gases than they are storing. Credit: US Fish and Wildlife Service.

The debrief:

During the past decade, forests in Alaska and northern regions of Canada have become more susceptible to wildfires. With climate changes leading to warmer and drier spring and summer seasons, the growing seasons have become longer, the ground has been drying out, and more of the permafrost that lies beneath the forest floor has been melting. It all makes for more conducive burning conditions. Now, a new study shows that Alaska wildfires are also burning deeper into the ground and releasing immense quantities of greenhouse gases that had been stored away in the soil for years.

According to research published this week in Nature Geoscience, Alaska wildfires burning later in the summer burn deeper, freeing up more soil carbon, than early season fires. As all this biomass burns, and then smolders, it releases vast amounts of carbon dioxide (CO2) and other greenhouse gases.

“A fire that burns for a very brief time, sometimes hours, can release carbon that has been stored in the soil for decades, or even centuries,” says Merritt Turetsky, an ecologist at the University of Guelph in Ontario, Canada, who worked on the new study. According to the study, also led by scientists from the University of Maryland, the U.S. Forest Service, Michigan Tech, and the U.S. Geological Survey, twice as much carbon was released from Alaska wildfires between 2000 and 2009 than was released during the previous five decades.

What’s more, says Turetsky, is that it now appears these wildfires are causing Alaskan forests to release more CO2 into the atmosphere than they are taking out, which is just the opposite of what has typically been observed in these regions.  

Why this science matters:

For years, researchers have been observing changes to forest ecosystems in Alaska, including a dramatic increase in the number of wildfires and their severity. Whereas decades ago, an extreme year of wildfires in Alaska occurred only once every ten years or so, more recently they have been happening every three or four years. With this new finding that wildfires are also burning deeper into the carbon-rich soil, Turetsky says the research “contributes to the body of knowledge that the northern systems are receiving the brunt of climate change.”

In the past, these northern forest systems have had a unique ability to take large amounts of CO2 out of the atmosphere — including some of the man-made emissions in recent centuries. Between the trees and plants that consume CO2 through photosynthesis, and the moisture in the forests that promotes the accumulation of carbon in the ground, the area has been ideally suited to carbon storage. So much so, in fact, that half of the entire world’s soil carbon is locked in the ground in northern boreal forests.

But this legacy of stored carbon, which has accumulated in Alaska forests for centuries, now appears to be threatened by the increasing severity of seasonal wildfires. Turetsky and her colleagues estimate that already in the last decade, fires consumed 16 percent of the stored carbon in the region.

If global warming continues, fires are projected to burn more frequently and deeper into the soil, and as a result northern forests may continue on this new trajectory of pumping more greenhouses gases into the atmosphere than they take up. This is an example of a “positive feedback” that natural scientists often talk about in relation to climate change — warming leads to an increase in fires, which leads to more greenhouse gases in the atmosphere, which leads to more warming, and so on.