The balmier climate of recent years has helped the mountain pine beetle of western North America reach ever-higher latitudes and elevations, resulting in the largest-ever recorded outbreak of this insect. And a large-scale multidisciplinary effort to catalogue and predict the effects of the outbreak now reveals that the destruction of pine forests by growing beetle populations will further contribute to climate change. Although the news is bleak, says Werner Kurz, a forest ecologist with the Canadian Forest Service in Victoria, British Columbia, and lead author of the report, it might help forest managers make proactive and informed decisions and allow them to reverse the trend.

North America's temperate forests are one of Earth's greatest assets in combating climate change because they function as carbon 'sinks', soaking up carbon dioxide from the atmosphere. But explosions in mountain pine beetle (Dendroctonus ponderosae) populations are threatening to turn these forests into carbon sources rather than sinks. “There are literally several hundred million cubic metres of wood out there in the forests decomposing and releasing carbon dioxide back into the atmosphere,” says Kurz.

The only way to stop the beetle is to kill it and its host: that is, cut down and burn infected trees. “There is no chemical agent that one could spray to control them,” says Kurz. Another factor that makes beetle infestation difficult to contain is that, unlike forest fires — another cause of carbon-dioxide release into the atmosphere — infestation does not start from a single ignition point, but rather as many small clusters that then grow together and “bleed across the landscape”, says Kurz.

Kurz and his colleagues performed both aerial and ground surveys to document the extent of destruction in the south-central forests of British Columbia. They also drew on decades of plot measurements and historical data on forest-growth curves gathered by government bodies. The numbers show that by the end of 2006, the beetle outbreak area covered 130,000 square kilometres, resulting in an estimated loss of 435 million cubic metres of timber in commercial forests alone and many millions more in protected areas and parks.

The group then conducted 100 simulations to calculate the long-term range of the outbreak and its effects on trees, also taking into account factors such as forest fires and harvesting (see page 987). These models project that between 2000 and 2020, the beetle outbreak in forests in the south-central region of British Columbia will increase atmospheric carbon by 270 million tonnes. This equates to 990 million tonnes of carbon dioxide — more than the entire annual emissions reported by Canada in 2005.

Alarming though the situation is, Kurz believes that there is a “silver lining”. “I think there are things that we can do now that we're confronted with this challenge to try to mitigate the impacts of climate change,” he says. “That's really my life's effort these days.”

One possibile way to offset the impact of pine beetles on climate change, Kurz believes, would be to turn pellets from the affected timber into a biofuel, thus making use of some of the wood that would otherwise decompose and release carbon. And we can also replace lost trees and, in doing so, help safeguard against similar events in the future. Most of the mature pine forest is expected to fall victim to the pine beetle during the next few years, but quickly planting younger trees of varying species should make forests more resistant to insect attacks, helping to tip the scale back in favour of carbon uptake.