Wednesday, April 11, 2007

How Trees Might Not Be Green in Carbon Offsetting Debate

by Alok Jha for 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. 0410 07

Govindasamy Bala, an atmospheric scientist at the Lawrence Livermore Laboratory in the US, has shown that only tropical rainforests are beneficial in helping slow global warming. The problem is that while the carbon dioxide forests use for photosynthesis indirectly helps cool the Earth by reducing the amount of greenhouse gases in the atmosphere, forests also trap heat from the sunlight they absorb.

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 Siberia in the northern hemisphere - can also cover up tundra normally blanketed in heat-reflecting snow.

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