By Tyler Irving
Posted September 2011
It seems logical enough: if deforestation raises COemissions, then afforestation — planting more trees — should be an effective way of combating climate change. But a new Canadian climate model shows that while this is true, the effect is smaller than you might expect.
Previous climate models have been somewhat unrealistic when it comes to afforestation, says Alvaro Montenegro of St. Francis Xavier University, one of the co-authors of the study, which is based on a computer simulation called the Canadian Earth System Model (CanESM1). Unlike previous models, CanESM1 accounted for the dynamics of forest growth, a slow process that can take decades. It also accounted for the fact that in areas like the Prairies, low rainfall precludes forest growth, even if agriculture were to cease. Finally, while forests do reduce atmospheric carbon, they are also darker than most cropland. This means that they absorb more sunlight and thus generate more heat than open fields.
According to the model, global average temperatures will have increased between 1850 and 2100 by about 3 C. Replacing 100 per cent of the world’s cropland with forest — an unrealistic scenario — would only reduce this by 0.45 C. Montenegro cautions that this is only one study, but says that the results point to the importance of decreasing emissions, rather than trying to compensate for them. “What the paper says is that if we continue to emit the way we are, planting trees, even over a very large fraction of our present day crop land, will do very little to reduce global temperature.” The study is published in the June issue of Nature Geoscience.
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