When Being Green Raises the Heat By KEN CALDEIRA
The New York Times Tuesday, January 16, 2007 The notion that we can save the planet just by planting trees is a dangerous illusion. Ken Caldeira is a scientist at the Carnegie Institution’s department of global ecology
Trees don’t just absorb carbon dioxide — they soak up the sun’s heating rays, too. Forests tend to be darker than farms and pastures and therefore tend to absorb more sunlight. This has a warming influence that appears to cancel, on average, the cooling influence of the forest’s carbon storage. This effect is most pronounced in snowy areas — snow on bare ground reflects far more sunlight back to space than does a snowed-in forest — so forests in areas with seasonal snow cover can be strongly warming.
In contrast, tropical forests appear to be doubly valuable to the earth’s climate system. Not only do they store copious amounts of carbon, the roots of tropical trees reach down deep, drawing up water that they evaporate through their leaves. In the atmosphere, this water may form clouds that reflect sunlight back to space, helping to cool the earth. These findings have important policy implications.