"Fire activity more than doubled since the '70s across Canada," Flannigan says. And the mountain pine beetle, which has proliferated because of warmer winters, has killed vast track of boreal forest in British Columbia that is also becoming a source of CO2.

"I used to say this is what we'll expect with climate change," Flannigan says, pointing to the increasing rate of fires and insect infestation.

"Now I say climate change is here."

Flannigan says the new study builds on previous Canadian research, and provides a more detailed picture of how the forest's carbon budget is changing.

"It's a real concern," Flannigan says. "We were hoping our forests would be a carbon sink. But the more work we do the more we see they are a becoming a source because of things like disturbances."