For Earth's sake, don't get rid of the tree line: Recent research has shown how important the forests are in stabilising the world's climate, says Bill Burroughs

LOOKING DOWN on a snow-covered landscape from an airplane, it is striking how the areas of woodland stand out starkly against a white background. What is not often realised is that this contrast is a visible sign of an important climatic phenomenon. For it is showing that the snow-covered forests of the northern hemisphere can absorb more sunlight during the winter months than the adjacent treeless snowy areas and are therefore warmer.

At the local level, it is easy to understand what is happening. Not only do the exposed surfaces of the trees absorb heat, but this heat also accelerates the melting of snow that settles on them. However, the influence of changes in tree cover on important features of the climate is a more complicated business.

The general principle of the warming effect has been understood by climatologists for many years. Aircraft measurements have shown that, on average, forest areas with stable snow-cover reflect almost half the sunlight falling on them, while open areas covered by snow reflect more than two-thirds of the Sun's rays.

So chopping down large areas of trees can alter the local climate. Indeed, it has been suggested that deforestation of Eurasia and northern America may have played a significant part in the general global cooling that has occurred during the past 5,000 years.

Until recently it was not known just how great a role forests played in the global climate. Now the results of a computer model - developed by Gordon Bonan and colleagues at the National Center for Atmospheric Research in Boulder, Colorado, and published in Nature magazine - have at last provided a clue to this puzzle.

Boreal forests consist of evergreen needle-leaf, deciduous needle-leaf and deciduous broadleaf trees and cover an area of more than 4 million square miles (11 million square kilometres). When the computer model considered the impact of removing all the forests north of 45 degrees north and replacing them with bare soil, the amount of cooling was startling. At 60 degrees north in April, the average temperature fall due to this change was 12C, but even in late summer, when there was no snow and the soil absorbed as much sunlight as the forest it replaced, the cooling was still 5C.

The impact of deforestation was felt throughout the year because the winter cooling was so great that it had an appreciable impact on ocean temperatures. So, given the thermal inertia of the oceans and the influence they have on global weather patterns, they spread the cooling effect right through the summer.

While the results of this extreme simulation must be taken with a pinch of salt, they do show that the boreal forests play an integral part in the Earth's climate. Furthermore, they raise a number of intriguing questions about how both human activities and natural fluctuations can influence the climate through changes in the extent of the boreal forests.

Clearly, further deforestation at high latitudes in Eurasia and North America will cool the climate. Indirectly, certain air pollution may have a proportionately greater impact. The emission of sulphur compounds, which create acid rain that damages forests, also produces sulphate aerosols. These particles will cause additional cooling as they reflect sunlight back into space.

As for natural climatic changes, the fact that the boreal forests are neatly delineated by the July isotherms of 13C and 18C - to the north of this zone, it is too cold for the trees to survive and to the south it is too warm - suggests that the forests play an interactive role in the climate. A possible sequence of events could be that a natural cooling could eventually drive the treeline southwards, creating more snow-covered tundra and hence lead to a further cooling and the forests moving yet farther south. Conversely, a warming trend would be amplified in the same way as the forests extended northwards.

Moreover, these results predict that the removal of the boreal forests could be more damaging than the destruction of tropical rainforests.

This means that we have more reason to worry about the felling of large tracts of Siberian forests than simply the possibility that it will destroy the remaining habitat of the Siberian tiger. We could be altering a particularly sensitive component of the global climate just when the whole system is experiencing unprecedented disturbance.

So when you next view a snowy landscape from the air, or simply see a photograph of snow-covered trees, remember that such images contain a lot of information about some powerful climatic forces at work.

(Photograph omitted)