四级冲刺练习阅读（173）_大学英语阅读

四级冲刺练习阅读（173）

　　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.