Greenpeace called for a moratorium on new logging in areas of the forest that still have large, unfragmented blocks of older-growth trees, and warned it had similar fears about the boreal forests that stretch across Russia and northern Europe.

"Research is starting to show that the forest is tipping from being an annual carbon sink to being an annual carbon source," said Christy Ferguson, Greenpeace's forests campaigner in Toronto.

The "Turning Up the Heat" report, prepared by researchers at the University of Toronto, surveyed a variety of separate scientific studies on the boreal forest in recent years.

Canada's boreal forest, characterized by the predominance of conifers like pine and spruce, stretches in a vast curve across the country below the Arctic, from the Yukon territory in the northwest to the Atlantic coast of Newfoundland.

A 1993 study estimated it stored about 186 billion tonnes of carbon, equal to about 27 times what the world produces from burning fossil fuel each year.

Two-thirds of the carbon is stored in the forest's soil, which decays when the tree cover is removed.

Greenpeace says the carbon released as trees are harvested contributes to climate change. That, in turn, threatens the northern forest with problems such as insect outbreaks and increased forest fires that destroy more trees.

The global warming, which is often most apparent in the far north, also allows the permafrost to melt, releasing still more greenhouse gases.

That creates the scenario of a "carbon bomb" or the sudden release of massive amounts of carbon into the atmosphere, similar to what happened in 1997 when peat fires broke out in Indonesia, Ferguson said.

"It hasn't happened yet. That's good, and we can stop it," she said.

Greenpeace is not calling for a total ban on logging in the boreal forest, where many small communities are dependent on the lumber, plywood and paper industries for economic survival. In fact, protecting parts of the forest will help producers in the long run, the group said.

Canada's forest industry has argued that because harvested trees are replanted, carbon released through logging is eventually recaptured as the new trees grow.

The Canadian Forest Products Association said in October that its members had agreed to make the industry carbon neutral by 2015 without having to purchase carbon offset credits.

But Ferguson said the scientific studies indicate carbon neutral logging might not be possible, and the soil in some logged areas continues to release carbon for up to a decade after the trees are cut.

"As much as we would want it to be the way it works, that just isn't what happens," Ferguson said.