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.
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