Planting forests won’t stop global warming

By Michael Marshall
19 June 2011 The UN is failing to accurately measure the global climate benefits of preserving forests. As well as providing homes for many species, trees store carbon dioxide that would otherwise warm the planet. With this in mind, the UN set up the REDD programme (Reducing Emissions from Deforestation and Forest Degradation) in 2008, which will pay poorer countries to preserve their forests based on how much carbon dioxide they store. What this fails to take into account is that forests also alter temperature in other ways. Those close to the poles are dark, and so absorb more sunlight than croplands would. But in the tropics, more water evaporates from forests than from unforested land, so they cool their surroundings. To get a fuller picture, Vivek Arora of Environment Canada and the University of Victoria, British Columbia, and Alvaro Montenegro of St Francis Xavier University in Antigonish, Nova Scotia, Canada, used a computer model to estimate the overall effect of reforesting [Nature Geoscience, DOI: 10.1038/ngeo1182]. They used what they admit are “somewhat extreme” scenarios in which half or all of the world’s croplands have been converted to forests by 2060. Foresting all or half the world’s cropland reduced global temperatures in 2100 by 0.45 °C and 0.25 °C respectively. Arora reckons that no more than 10 to 15 per cent of existing cropland is likely to be forested, so the effects will be even smaller. “The overall temperature benefits of any realistic afforestation efforts are expected to be marginal,” he says. […]