 Environment - January 9, 2008 |
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Climate change could reverse role of forests
Trees might emit carbon dioxide instead of store it |
By Kevin Ma
Staff Writer
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Canada’s trees might lose their ability to fight climate change because the world is getting warmer, according to a new study.
Researchers believe that greenhouse gas emissions like carbon dioxide, particularly from fossil fuel use, are heating up the planet. Working in our favour are our oceans and forests, says Hank Margolis, professor of forestry and ecology at the Université Laval in Quebec, which act as sinks or stores of these gases. "We’ve had a discount on our fossil fuel emissions because the land and ocean have been absorbing about half of what we’re emitting."
Researchers had hoped a warmer world would make forests more effective sinks because they would grow longer, Margolis says, but a study he co-authored in Nature last week suggests this isn’t happening. Forests are emitting more carbon as the world warms and could, given the right conditions, become carbon sources.
Sinks are sunk?
Forests act as carbon sinks in two ways, says Chris Henschel, conservation and climate change spokesperson with the Canadian Parks and Wilderness Society in Ottawa. The trees turn carbon into wood, taking it out of circulation for the life of the tree, and leave some of it in the soil when they decay. Canada’s boreal forest is thought to contain some 186 billion tonnes of carbon, he notes, equivalent to about 27 years of fossil fuel emissions, and Canada has long planned to use its forests as carbon credits in international climate change agreements.
But forests are very dynamic systems, says Graham Stinson of the Canadian Forest Service in B.C., and one big fire or pine beetle outbreak could change one from a carbon sponge to a spout. "It could be substantial in either direction," he says.
Spring and fall temperatures in the north have risen about 1.1 and 0.8 degrees respectively in the last 20 years, according to the Nature study, resulting in a longer growing season. Margolis, working with researchers around the world under the Global Carbon Project, used ground and satellite readings to determine how this affected northern forests.
They found that the forests were indeed absorbing more carbon in the spring due to warmer temperatures, but were also releasing more in the fall due to greater decay. The warmer fall ate up all but 10 per cent of the spring gains, suggesting warmer weather would mean less absorption if the fall warms up faster than the spring, Margolis says. That’s what happening in North America, he notes, but the opposite is occurring in Asia, making the future tough to predict.
Urgent action needed
"Our study is not good news," Margolis says. "It means our projections on how long these carbon sinks will stay active is probably too optimistic."
The study suggests that forests could become carbon sources if the world keeps warming, Stinson says, creating a risk of a positive feedback cycle: more warmth releasing more gas producing more warmth. His own research suggests forest fires and insect outbreaks, both predicted to be more common in a warmer world, could have similar effects.
Other research suggests we might be seeing similar loops right now in arctic ice (warming melts ice, exposing water to more light, meaning a warmer north) and the oceans (warming means warmer water absorbs less gas, meaning more warming), Henschel says. "It’s a troubling trend and it calls attention to the need to act quickly before we get beyond a tipping point." Nations like Canada will have to cut their carbon emissions and preserve their forests to stop this trend, he says.
The study is available in the Jan. 3 issue of Nature. |
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