From The Economist:
Trees are good. Good enough to hug. Trees have a nifty biochemical strategy called photosynthesis that enables them to take carbon dioxide in through their leaves, and swap that nasty gas for oxygen, a nice one. They use the carbon thus sequestered to make molecules like cellulose, and thus more tree.
That is why some rich people who love to burn things containing carbon, such as petrol and aircraft fuel, have recently started paying others to plant trees on their behalf. Burning adds oxygen to carbon, making carbon dioxide. And carbon dioxide makes the world warmer. A warmer world will mean higher sea levels. So if people burn things without offsetting the carbon dioxide thus produced, their holidays in the Maldive islands will disappear, along with the islands themselves.
This chattering-class environmental picture is not necessarily wrong, but it does include many assumptions. One of them, that planting trees will make the world cooler than it would otherwise be, is the subject of a newly published study by Govindasamy Bala, of the Lawrence Livermore National Laboratory, in California, and his colleagues. Dr Bala has found, rather counter-intuitively, that removing all of the world’s trees might actually cool the planet down. Conversely, adding trees everywhere might warm it up.