四级冲刺练习阅读173

　　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 todayand up to 10C 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 rainforestsand the speed of re-growthmean 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 yearone-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.