四级冲刺练习阅读（173）

　　Forgotten Feedback Disrupts the Greenhouse

　　An increase in the release of carbon dioxide from soils could speed up the greenhouse effect, according to a new analysis by Gundolf Kohlmaier of Frankfurt University. He warned that the response of forests and their soils to an increase in temperature could introduce unexpected feedbacks in the progress of the greenhouse effect that are not currently included in models of climate. Kohlmaier presented his findings to a conference on Climate and Development, held in Hamburg last month.

　　The concentration in the atmosphere of greenhouse gases, principally carbon dioxide, will probably be double that of pre-industrial times by 2030. Most models of climate predict that the increase will result in an atmosphere that is approximately 4C warmer than today and up to 10 C warmer in polar regions. But the current generation of models ignores two factors. The first is the response to warming and the changing chemistry of the air over the oceans. Oceans and the organisms in them currently absorb around half of the carbon dioxide released into the air. The second is the response of plants and soils on land.

　　Both oceans and plants on land are major sources and sinks for carbon. Their response to the changing atmosphere above them could set in chain either devastating positive feedbacks or stabilizing negative feedbacks. It is not certain which way the feedbacks will work..

　　The destruction of forests in the past two centuries, mostly in temperate lands, has contributed almost as much to the greenhouse effect as the burning of fossil fuels. But large uncertainties about the rate of destruction of tropical rain forests and the speed of re-growth mean that no one is sure how much carbon flows between the forests and the atmosphere. Recent estimates, taking account of the rapid planting of trees in many developed countries, put the release at perhaps 1 billion tonnes a year one-fifth of the release from the burning of fossil fuel.

　　Tropical forests release about a quarter of their carbon as trees are destroyed-for instance, during the burning of forests that happens in the Amazon rainforest each year. Deforestation will continue to exacerbate the greenhouse effect, warns Kohlmaier. But it could also have a more possible consequence, weakening one of the planets most effective mechanisms for damping the greenhouse effect.

　　Optimists believe that living organisms will find means to stabilize unwanted changes to their environment. They believe that the extra carbon dioxide in the atmosphere will often fertilize plants, allowing them to grow faster, and thus absorb still more carbon dioxide from the atmosphere.

　　Some data from tree rings, from as far apart as Canada and Tasmania, suggest that this may already be happening. The fertilization effect could also be reflected in a recent increase in the difference in levels of carbon dioxide in the atmosphere between summer and winter. Forests are effectively breathing harder by taking up more carbon dioxide during the summer.

　　But Kohlmaier warns that destruction of the naturally occurring biota by deforestation. . . may alter or stop this stabilizing negative feedback . A solution could be to plant trees. More trees would both absorb more carbon dioxide and increase the potential for this negative feedback to stabilize temperatures.

　　Kohlmaier estimates that if all the forests of the Amazon basin were destroyed, they would release 73 billion tonnes of carbon, but if allowance were made for possible extra absorption of carbon dioxide from the air as a result of the fertilization effect, this could amount to a loss of 133 billion tonnes. The loss of all the worlds tropical forests would result in 300 billion tonnes more carbon dioxide being released into the air.

　　Not all researchers are convinced by the likely power of the fertilization effect. They point out that trees need other things besides carbon to grow notably nutrients such as nitrogen and water. Some models of climate predict that certain deserts will spread as the world warms, reducing the domain of the tropical forests.

　　But the greatest uncertainty is over the vast northern forests covering 6 million square kilometres of North America, Europe and Siberia. These regions will warm the most in the coming decades. There are two reasons for this. First, the stable atmosphere that is typical over polar regions will trap heat close to the ground. Secondly, the melting of ice that results from a warmer climate will make the plants surface less reflective, allowing it to absorb more of the Suns heat.

　　The trees in the boreal forest could grow much faster absorbing up to 50 per cent more carbon, according to some estimates and they could spread north by up to 1000 kilometres across the currently treeless tundra , whose soils could thaw out. A recent study concluded that the boreal forests might increase by two-thirds between now and 2030.

　　Nature could use the boreal forest to fight back against the greenhouse, say the optimists. Barrie Pittock, from the Australian governments division for atmospheric research, argues that an increase of 1 per cent in the amount of carbon in live vegetation round the world could offset the current release of carbon dioxide from burning fossil fuels.

　　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.