Care needed with carbon offsets

By Jonathan Amos
Science reporter, BBC News, San Francisco

Trees planted in the tropics would have the biggest cooling effect

Planting forests to combat global warming may be a waste of time, especially if those trees are at high latitudes, new research suggests.

Scientists say the benefits that come from trees reducing atmospheric carbon dioxide can be outweighed by their capacity to trap heat near the ground.

Computer modelling indicates that trees only really work to cool the planet if they are planted in the tropics.

The research has been discussed at an American Geophysical Union meeting.

Planting more trees in high latitudes could be counterproductive from a climate perspective Dr Govindasamy Bala

"What we have found is in the so-called mid-latitude region where the United States is located and majority of European countries are located, the climate benefits of planting will be nearly zero," said ecologist Govindasamy Bala of Lawrence Livermore National Laboratory.

"[In] the seasonally snow-covered regions [at even higher latitudes], planting new trees could be actually counter-productive," he told BBC News.

Growing issue

Dr Bala and colleague Ken Caldeira, from the Carnegie Institution of Washington, used a computer model to determine the impact which forests in different parts of the planet would have on temperature.

Their analysis indicates that three key factors are involved:

• forests can cool the planet by absorbing the greenhouse gas carbon dioxide during photosynthesis
• they can also cool the planet by evaporating water to the atmosphere and increasing cloudiness; a deck of white clouds reflects incoming solar radiation straight back out into space
• trees can also have a warming effect because they are dark and absorb a lot of sunlight, holding heat near ground level

"Our study shows that tropical forests are very beneficial to the climate because they take up carbon and increase cloudiness, which in turn helps cool the planet," explained Dr Bala.

Interest has been growing in offsetting carbon emissions

The further you move from the equator, though, these gains are eroded; and the team's modelling predicts that planting more trees in mid- and high-latitude locations could lead to a net warming of a few degrees by the year 2100.

"The darkening of the surface by new forest canopies in the high-latitude boreal regions allows absorption of more sunlight that helps to warm the surface," Dr Bala said.

"In fact, planting more trees in high latitudes could be counterproductive from a climate perspective."

The study finds little or no climate benefit when trees are planted in temperate regions.

The scientists warn that many schemes designed to offset emissions of carbon by planting trees may not be appropriate.

"When you plant trees to slow down global warming, you have to be careful where you do it. I think our study shows clearly the climate benefits are maximised if you plant them in the tropics," Dr Bala told BBC News.

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