A group of U.S. researchers have found that wildfires — fuelled by climate change — may be turning boreal forests into sources of carbon dioxide.

The boreal forests — found in northern Canada, Alaska, Siberia, China, Scandinavia and elsewhere — make up the second largest type of forest in the world behind the tropical rainforest.

Scientists have historically believed that the boreal forests act as a carbon sink, as trees absorb carbon emissions and reduce them in the atmosphere.

But new research from the University of Wisconsin-Madison, published in the Nov. 1 issue of the journal Nature, has found that the forests may be emitting more carbon than they are absorbing.

Lead researcher S. Tom Gower said that any climate-related changes in the boreal forests could likely resonate well beyond their boundaries because of their size and the fact that they are expected to experience the most warming as temperatures rise.

The research team used data from 1948 to 2005 and focused on one million square kilometres of boreal forest in Manitoba, Saskatchewan and eastern Alberta. They created a computer model to simulate the carbon balance for that region over those decades.

The simulation found that any "ecophysiological" changes from global climate change — for example, higher temperatures — balanced out, while wildfires led to the increase in the forest's carbon dioxide emissions.

The study said that fires shift the carbon balance in the forests in two key ways.

First, burning organic matter quickly releases large amounts of carbon dioxide. Secondly, when fire destroys the forest canopy, more sunlight is able to reach and warm the ground, speeding decomposition and thus carbon dioxide emissions from the soil. It could be further shifted if the warmed soil heats up enough to melt underlying permafrost in some areas, releasing stored carbon.

Gower stressed that these findings don't mean that climate change is not affecting the forests.

He explained that hotter and drier summers, an effect of global warming, increase the frequency, intensity and size of the fires.

"Climate change is what's causing the fire changes. They're very tightly coupled systems," he said.