The last few months have witnessed a flurry of debate in the scientific literature over the relative merits of using forests as carbon sinks to combat global warming. The broad-based consensus seems to be that, yes, trees can work - if planted in the right areas - but also that we shouldn't hang our hopes on them.
The latest salvo in the debate comes from Tom Gower of the University of Wisconsin, who has determined that Canada's boreal forest - far from hindering global warming's advance - may be spurring it on by releasing more GHG than it absorbs. "The boreal forest, at least in the north-central part of Manitoba, has gone from a weak carbon sink to a weak carbon source. It is now contributing to atmospheric (carbon dioxide) concentration," he said.Using a one million sq. km stretch of forest in Manitoba, Gower and his colleagues coupled their measurements of how carbon moved between the atmosphere and the trees with past records and computer models to examine how the forest's ability to store carbon dioxide has changed since mid-century. Their results showed that the forest's ability to store more carbon dioxide than it emitted had weakened over the last few years - to the point where it has now become a net emitter of carbon.
The culprit for this abrupt reversal, Gower explained, is forest fires. "The warmer climate has increased fire frequency and extent. Those wildfires have caused this transition in the boreal forest from a carbon sink to a carbon source." They have, in effect, caused a positive feedback loop: the more soil is exposed to sunlight as a result of trees burning, the speedier the decomposition process and the more carbon dioxide is released.
The real concern is that climate change may soon causing the large-scale thawing of permafrost around the world; second only in size to tropical rainforests, boreal forests cover a vast swathe of land in the upper latitudes of Alaska, Siberia, China, Scandinavia and the Yukon.
Gower's assessment is fairly grim, to say the least: "Based on our current understanding, fire was a more important driver than climate was. But if carbon dioxide concentration really doubles in the next 50 years and the temperature increases four to eight degrees Celsius, all bets may be off."