Against the supposition that forest fires in Alaska，Canada and Siberia warm the

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Against the supposition that forest fires in Alaska，Canada and Siberia warm the climate，scientists have discovered that cooling may occur in areas where burnt trees allow more snow to mirror more sunlight into space.
This finding suggests that taking steps to prevent northern forest fires to limit the release of greenhouse gases may warm the climate in northern regions.Usually large fires destroyed forests in these areas over the past decade.Scientists predict that with climate warming，fires may occur more frequently over the next several centuries as a result of a longer fire season.Sunlight taken in by the earth tends to cause warming，while heat mirrored back into space tends to cause cooling.
This is the first study to analyze all aspects of how northern fires influence climate.Earlier studies by other scientists had suggested that fires in northern regions sped up climate warming because greenhouse gases from burning trees and plants were released into the atmosphere and thus trapped heat.
Scientists found that right after the fire，large amounts of greenhouse gases entered the atmosphere and caused warming.Ozone (臭氧) levels increased，and ashes from the fire fell on far­off sea ice，darkening the surface and causing more radiation from the sun to be taken in.The following spring，however，the land within the area of the fire was brighter than before the fire，because fewer trees covered the ground.Snow on the ground mirrored more sunlight back into space，leading to cooling.
“We need to find out all possible ways to reduce the growth of greenhouse gases in the atmosphere，” scientists said.They tracked the change in the amount of radiation entering and leaving the climate system as a result of the fire，and found a measurement closely related to the global air temperature.Typically，fire in northern regions occurs in the same area every 80 to 150 years.Scientists，however，found that when fire occurs more frequently，more radiation is lost from the earth and cooling results.Specifically，they determined when fire returns 20 years earlier than predicted,0.5 watts per square meter of area burned are soaked up by the earth from greenhouse gases，but 0.9 watts per square meter will be sent back into space.The net effect is cooling.Watts are used to measure the rate at which energy is gained or lost from the earth.
小题1:According to the new findings，taking steps to prevent northern forest fires may________.

A．result in a warming climate

B．cause more forest fires

C．lead to a longer fire season

D．protect the forests there

小题2:The following are the immediate effects after a forest fire EXCEPT________.

A．large amounts of greenhouse gases enter the atmosphere

B．the levels of ozone which is a type of oxygen increase

C．snow on the ground mirrors more sunlight back into space

D．ashes from the fire fall on the ice and darken the surface

小题3:Earlier studies about northern forest fires________.

A．had analyzed all aspects of how northern fires influenced climate

B．had indicated that forest fires would pollute the atmosphere

C．had suggested that people should take measures to protect environment

D．had suggested that the fires would speed up climate warming

小题4:We can infer that forest fires in Alaska，Canada and Siberia may________.

A．warm the climate as the supposition goes

B．cool the climate by reflecting more sunlight into atmosphere

C．make more space for the growth of young trees

D．help to gain more energy rather than release more energy

答案

小题1:A
小题2:C
小题3:D
小题4:B

解析

小题1:　A
解析　细节理解题。文章第二段第一句说，为了限制温室气体的排放而采取措施预防北方森林大火，反而会导致北方地区气候变暖。因此A项正确。B项和C项都是由气候变暖引起的；D项在文中并未说明。
小题2:C
解析　细节理解题。由文章第四段前两句可知，火灾发生后，大量的温室气体进入大气层，臭氧增多，灰烬落到远方的冰山上，把冰面弄黑。这都是发生火灾后会立即出现的情况。由第四段最后两句可知，地面上的雪折射更多的太阳光返回大气层，在第二年的春天才会发生，故选C项。
小题3:　D
解析　细节理解题。由文章第三段第二句可知D项正确。
小题4:　B
解析　推理判断题。由文章第一段和第四段的具体分析可知，北方的森林大火能使地面上的雪向大气折射回更多的阳光，以此达到降低气温的效果。