The tiny mountain pine beetle (Dendroctonus ponderosae) is approximately the size of a grain of rice. ((Natural Resources Canada))

The mountain pine beetle's infestation in western Canada is turning forests into a new source of greenhouse gases, according to new research to be published Thursday in the journal Nature.

Scientists from Natural Resources Canada said the beetle's ravenous spread through pine forests in British Columbia and Alberta is killing trees. The decaying trees are in turn releasing carbon into the atmosphere.

The cumulative impact of the beetle outbreak in the region will release 990 megatonnes of carbon dioxide-equivalent greenhouse gases from 2000 to 2020, said the NRCan's senior research scientist, Werner Kurz.

"That's equivalent to five times the annual emissions from the transportation sector in Canada," Kurz told CBC News. "So these are very big numbers."

Forests are traditionally seen as beneficial to keeping carbon trapped in the ground, with tropical rainforests in particular valued for their ability to absorb carbon dioxide from the atmosphere. Carbon dioxide is considered a leading greenhouse gas, or a gas that traps heat in the atmosphere and can lead to the warming of the planet.

But Kurz and his colleagues found the number of dead trees left behind in the mountain pine beetle's wake has changed the forests' role in the carbon cycle.

"This impact converted the forest from a small net carbon sink to a large net carbon source both during and immediately after the outbreak," they wrote in the journal Nature.

"The impact of the mountain pine beetle on B.C. is so large that the release of carbon dioxide in the affected areas is greater than the uptake of all the forest of B.C. together," said Kurz.

"The beetles are the driving force and we anticipate the forests will remain a carbon source for the next decade," he said.

Kurz suggests the only way to combat the problem is to log most of the dead trees and replant new ones as quickly as possible.

The Nature findings are interesting because the spread of the mountain pine beetle has been blamed, in part, on changing weather conditions that have allowed the beetle to migrate to regions it would normally find inhospitable, such as forests east of the Rockies like the Peace River valley.

Kurz said the new findings will need to be taken into account in future climate models.

The mountain pine beetle is as small as a grain of rice but in large swarms, it can take down a lodgepole pine from the inside.

In 2006 alone, 9.2 million hectares of forest in Western Canada were in an advanced stage of attack from the mountain pine beetle.

Kurz and his colleagues estimate the cumulative outbreak area by the end of the 2006 was 130,000 square kilometres, with timber losses estimated to be more than 453 million cubic metres and additional losses outside the commercial forest.