LONDON: 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.

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.

The scientists said that the results, published on Monday in the Proceedings of the National Academy of Scientists, are explained by the way in which the sun’s rays are absorbed or reflected by different parts of the world. Forest canopies, being relatively dark, absorb most of the sun’s heating rays that fall on them, warming the surface of the earth all around.

Dr Bala said that trees at lower latitudes have a dual role. “It is a win-win situation in the tropics because trees in the tropics, in addition to absorbing carbon dioxide, promote convective clouds that help to cool the planet. In other locations, the warming from the albedo effect (the amount of sunlight reflected back into space) either cancels or exceeds the net cooling from the other two effects.”

The results follow increasing criticism from climate scientists of the benefits of forestry schemes to offset carbon emissions. Kevin Anderson, a scientist with the Tyndall Centre for Climate Change Research, warned recently that offsetting was a dangerous delaying technique that helped people “sleep well at night when we shouldn’t sleep well at night”.

Environmental groups have also been debating the issue. When Dr Bala’s preliminary findings were discussed at the American Geophysical Union’s annual meeting in San Francisco, a spokesman for Greenpeace USA said that the charity had always encouraged limiting the number of trees which countries and firms use to mitigate climate change.—Dawn/The Guardian News Service