How trees might not be green in carbon offsetting debate
· Planting may worsen warming, say scientists
· Environmental gestures 'useless' outside tropics
The Guardian (
It may have become the penance of choice for the environmentally conscious individual, but planting trees to offset carbon emissions could contribute to global warming if they are planted outside the tropics, scientists believe.
They argue that most forests do not have any overall effect on global temperature but, by the end of the century, forests in the mid and high latitudes could make their parts of the world more than 3C warmer than would have occurred if the trees did not exist.
Govindasamy Bala, an atmospheric scientist at the Lawrence Livermore Laboratory in the
Dr Bala and his colleague, Ken Caldeira of the department of global ecology at the Carneige Institute in Standford, used a computer model to show that, outside a thin band around the equator, forests end up trapping more heat than they help to get rid of through a cut in carbon dioxide. Planting trees above 50 degrees latitude - the equivalent of Scandinavia or
The scientists said that the results, published yesterday in the Proceedings of the National Academy of Scientists, are explained by the way in which the sun's rays are absorbed or reflected by different parts of the world.
Dr Bala said that trees at lower latitudes have a dual role. "It is a win-win situation in the tropics because trees in the tropics, in addition to absorbing carbon dioxide, promote convective clouds that help to cool the planet. In other locations, the warming from the albedo effect [the amount of sunlight reflected back into space] either cancels or exceeds the net cooling from the other two effects."
The results follow increasing criticism from climate scientists of the benefits of forestry schemes to offset carbon emissions. Kevin Anderson, a scientist with the Tyndall Centre for Climate Change Research, warned recently that offsetting was a dangerous delaying technique that helped people "sleep well at night when we shouldn't sleep well at night".
Environmental groups have also been debating the issue. When Dr Bala's preliminary findings were discussed at the American Geophysical Union's annual meeting in
He added that forestry projects were difficult to manage. "There are a lot of reasons why buying credits can be very fleeting, from a consumer point of view. You don't know what's going to happen to your forest in 10 years. All that effort you made to store that carbon can disappear in a heartbeat."
Dr Caldeira warned that chopping down trees outside the tropics was not a good idea. "Preservation of ecosystems is a primary goal of preventing global warming, and the destruction of ecosystems to prevent global warming would be a counterproductive and perverse strategy."
Dr Bala added: "Apart from their role in altering the planet's climate, forests are valuable in many other aspects. Forests provide natural habitat to plants and animals, preserve the biodiversity, produce economically valuable timber and firewood, protect watersheds and indirectly prevent ocean acidification."
Planting trees to neutralise carbon emissions has become a big business: £60m worth of trees have been bought this year, up from £20m in 2005. By 2010 the market is expected to reach £300m.