Bad news for buyers of ecological Christmas presents and seekers of environmental quick fixes: offsetting carbon emissions by planting extra trees might not work as well as we had all hoped. It always seemed too easy, didn't it?

New research presented at a meeting of the American Geophysical Union suggests that the carbon reducing benefits of the trees is outweighed by their tendency to trap heat close to the ground - especially when they are planted in the normally snowy, higher latitudes.

So while they cool the planet by taking carbon out of the atmosphere, they also warm it, like a winter blanket.

Ecologist Govindasamy Bala of Lawrence Livermore National Laboratory told the BBC: "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."

For the trees to have an impact they need to be planted in the tropics, he said.

Dr Bala and his colleague Ken Caldeira at the Carnegie Institution of Washington simulated the environmental impact of planting trees at different latitudes in a computer model.

They found that trees have two cooling effects: reducing atmospheric carbon and evapourating water, increasing cloudiness at the right height to reflect incoming solar radiation. In the tropics this produces a significant cooling effect, the researchers said.

But because they are dark, the trees also increase heat by trapping solar radiation close to the ground. This effect is most pronounced in areas that would otherwise be reflecting solar energy.

Dr Bala concludes: "When you plant trees to slow down global warming, you have to be careful where you do it. "

The Green Party argues that rather than paying to plant trees, people should think about the amount of carbon their lifestyles generate.

Siân Berry, Green Party principal Speaker said: "Offsetting isn't an excuse not to reduce burning fossil fuels - the first thing we need to do is to burn less.

"But if flying or whatever you are offsetting is absolutely unavoidable, then it's better to invest in a scheme that provides clean technologies to the Global South so they can avoid making the same mistakes we have made."

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