But there is another side to the argument. As the climatic zone inhabited by the boreal forests extends north, it will also be squeezed from the south as the temperate lands extend. Canadian researchers say that within 50 years, the Yukon in northern Canada could have the climate of present-day Alberta, the heart of the grain belt.
Also, the spread of a forest into new territory is slow, lagging well behind the pace of climatic change expected in the coming decades. The pace of destruction of trees in climates that have too warm or too dry a climate will be much more swift. Forests released 4 billion tonnes of carbon into the atmosphere during the hot dry summer that afflicted much of the world in 1983.
George Woodwell, the director of the Woods Hole Research Center in Massachusetts, sees such destruction as but a sample of the destruction that appears to be in store as climate changes . Woodwell believes that the release of carbon from trees that died under the stress of global warming will swamp any effect of fertilization by carbon dioxide.
Kohlmaier has another worry. This relates to soils, and especially the humus in soils, which he says is very sensitive to changes in temperature. He has used a model of the exchange of carbon between soils, vegetation and the air to estimate the likely effect of changes in temperature on the release of carbon dioxide from soils. In his paper to the conference in Hamburg, he reported that a temperature increase of 2 C would release between 224 and 383 billion tonnes of carbon. According to these calculations, an increase of 4 C in temperatures might increase that release to some 600 billion tonnes more than a centurys release of carbon at the current rate of burning of fossil fuels.
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