The following correction was printed in the Guardian's Corrections and clarifications column, Monday April 16 2007

We stated in error in the article below that the land above 50 degrees latitude in the northern hemisphere is where Scandinavia and Siberia sit, but those countries begin around the 60th parallel. The British Isles begin at 50 degrees of latitude, a line that joins the southern tip of Cornwall to Luxembourg, and passes south of Frankfurt and Prague.





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 yesterday 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. In contrast, grassland or snowfields reflect a lot more of the sun's rays back into space, keeping temperatures in open areas lower.

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. "What we see here is another reason to limit this," he said.

He added that forestry projects were difficult to manage. "There are a lot of reasons why buying credits can be very fleeting, from a consumer point of view. You don't know what's going to happen to your forest in 10 years. All that effort you made to store that carbon can disappear in a heartbeat."

Dr Caldeira warned that chopping down trees outside the tropics was not a good idea. "Preservation of ecosystems is a primary goal of preventing global warming, and the destruction of ecosystems to prevent global warming would be a counterproductive and perverse strategy."

Dr Bala added: "Apart from their role in altering the planet's climate, forests are valuable in many other aspects. Forests provide natural habitat to plants and animals, preserve the biodiversity, produce economically valuable timber and firewood, protect watersheds and indirectly prevent ocean acidification."

Planting trees to neutralise carbon emissions has become a big business: £60m worth of trees have been bought this year, up from £20m in 2005. By 2010 the market is expected to reach £300m.

Alternatives

Ways to cut carbon include:

· Replacing CO2-producing energy with human energy technologies ... a project in India has replaced diesel pumps with people-operated pumps for irrigation.

· Introducing energy-saving light bulbs, which use 80% less electricity on average, reducing energy consumption and therefore the amount of pollution by power stations. Inefficient coal power stations in Kazakhstan create three times as much CO2 when producing electricity as UK counterparts. Because electricity is so cheap, many schools and homes use cheaper, but inefficient, traditional bulbs instead.

· Efficient stove projects. In Mexico more efficient stoves have been introduced because they are cheaper and burn less fuel. They also make the kitchen safer as they produce less smoke, and cut CO2 by 1.5 tonnes a year, a home.

· Renewable energy projects,such as wind farms in India.

Isabelle Chevallot