Forests have long been thought of as an ally in the fight against global warming, but a new study suggests that Canada’s boreal forest may in fact be releasing more greenhouse gases than it absorbs.
“The boreal forest, at least in the north-central part of Manitoba, has gone from a weak carbon sink to a weak carbon source,” said Dr. Tom Gower of the University of Wisconsin, whose paper is being published Thursday in the journal Nature. “It is now contributing to atmospheric (carbon dioxide) concentration.”
Gower and his fellow researchers studied a million-square-kilometre stretch of forest around Thompson, Man. The team took field measurements of how carbon moved between the forest and the atmosphere and then used computer modelling and forestry records to suggest how that cycle has changed since the 1950s.
Trees absorb carbon dioxide as they grow and release it when they burn or decompose.
Although results varied for individual years depending on the severity of the forest fire season, Gower found that the forest once absorbed, on average, slightly more carbon than it emitted – about five or 10 grams per square metre of forest per year.
Now, however, the direction of that flow has reversed. On average, the forest actually emits about two grams per square metre per year.
“(The forest) is actually contributing to rising carbon emissions,” Gower said.
The cause, he said, is forest fires. Climate change, according to most models, leads to increased forest fires because it creates hotter and drier conditions.
“The warmer climate has increased fire frequency and extent,” said Gower. “Those wildfires have caused this transition in the boreal forest from a carbon sink to a carbon source.”
Not only do the fires themselves release vast quantities of carbon dioxide, they expose soil to sunlight, which speeds decomposition and more carbon dioxide release. If enough warming occurs to melt permafrost, that may release more carbon yet.
In fact, Gower said we may already be in a feedback loop of increased carbon emissions from the boreal forest.
“Climate change is what’s causing the fires,” he said. “If it was left unchecked, it could become a feedback.”
Boreal forests around the world are second in size only to tropical rainforests. They cover the upper latitudes of every Canadian province, as well as the Northwest Territories, the Yukon, Alaska, Siberia, China and Scandinavia.
The size of the forest, as well as the fact that those regions are expected to experience the most severe climate changes, suggests that even a small shift in the carbon cycle of that ecosystem could have serious effects on the entire planet.
If carbon concentrations in the atmosphere continue to increase, Gower said, climate change will become an increasingly important contributor to forest fires.
“Based on our current understanding, fire was a more important driver than climate was. But if carbon dioxide concentration really doubles in the next 50 years and the temperature increases four to eight degrees Celsius, all bets may be off.”