四级冲刺练习阅读（173）_大学英语阅读

四级冲刺练习阅读（173）

　　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.