That is the same amount of greenhouse gas emissions that Canada is committed to reducing by 2012 under the Kyoto Protocol, and would effectively doom that effort to failure, the study says.

By the end of 2006, the mountain pine beetle -- Dendroctonus ponderosae -- had ravaged 130,000 square kilometres (more than 50,000 square miles) of forests in western Canada.

While not the first outbreak in the last decade, the latest is ten times larger than any previous attacks.

The tiny beetle, the shape and size of a grain of rice and native to the western part of North America, lays its eggs under the bark of mature lodge-pole pine and jack pine trees.

Once the insects are embedded, a tree's fate is sealed.

Healthy forests are normally carbon sinks, meaning that they absorb more carbon dioxide -- the number one greenhouse gas -- than they give off.

When trees die, however, they release large amounts of pent up CO2 into the atmosphere, and leave fewer living plants to soak it up.

Previous climate change models of northern forests have failed to take the impact of such insect infestations into account, the study says.

A team of researchers led by Werner Kurz of the Canadian Forestry Service measured the cumulative and individual effect on the carbon cycle of forest fires, logging and tree deaths due to beetles.

The impact of the beetles alone has "converted the forest from a small net carbon sink to a large net carbon source both during and immediately after the outbreak," the authors conclude.

Kurz calculated that by 2020 the beetle outbreak will have released 270 megatonnes of C02 into the air, more than "the total average sink of all of Canada's managed forest over the last decade."

This points up the often unpredictable interplay of factors influencing global warming, and how a single element can be both cause and effect.

Earlier research has shown that pine beetles have thrived in Canada in recent years due warmer weather. "Climate change has contributed to the unprecedented extent and severity of this outbreak," concludes the study, published in the British journal Nature.

Now the beetles are speeding up that very process.

Forests are only one of the Earth's major carbon sinks. Recently scientists have found that the Southern Ocean and the North Atlantic have lost some of their ability to absorb C02.

They fear that the same could be true of all the planet's oceans, which together absorb a quarter of man-made carbon dioxide emissions every year.

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