"Since 1999, and especially in the last five years, the forests have shifted from being a carbon sink to a carbon source," Werner Kurz, senior research scientist at the Canadian Forest Service in Vancouver, told a Chicago Tribune correspondent.

Rising global temperatures are drying out forests making them more susceptible to fires, which release massive amounts of carbon into the atmosphere, the report said.

Adding to the problem is an infestation of pine beetles, which have killed off thousands of acres of evergreen forests in British Columbia and Alberta. Normally, the insects were kept mostly in check by freezing winters, but warming in recent years has allowed them to proliferate, said Jim Snetsinger, British Columbia's chief forester.

"Once those infested trees are killed by the pine beetle, they are no longer sequestering carbon -- they are giving it off," he said.

Canada has 7 percent of the world's forests, and stores an estimated 186 billion tons of carbon, "equivalent to 27 years worth of global carbon emissions from the burning of fossil fuels," the Tribune said.