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

Against the suppositionthat forest fires in Alaska, Canada and Siberia warm the climate, scientistshave discovered that cooling may occur in areas where burnt trees allow moresnow to mirror more sunlight into space.

    Thisfinding suggests that taking steps to prevent northern forest fires to limitthe 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 frequentlyover the next several centuries as a result of a longer fire season. Sunlighttaken in by the earth tends to cause warming, while heat mirrored back intospace tends to cause cooling.

    Thisis the first study to analyze all aspects of how northern fires influenceclimate. Earlier studies by other scientists have suggested that fire innorthern regions speed up climate warming because greenhouse gases from burningtrees and plants are released into the atmosphere and thus trap heat.

   Scientists found that right after the fire, large amounts of greenhouse gasesentered the atmosphere and caused warming. Ozone（臭氧） levelsincreased, and ash from the fire fell on far-off sea ice, darkening the surfaceand 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 thefire, because fewer trees covered the ground. Snow on the ground mirrored moresunlight back into space, leading to cooling.

    “Weneed to find out all possible ways to reduce the growth of greenhouse gases inthe atmosphere.” Scientists tracked the change in the amount of radiationentering and leaving the climate system as a result of the fire, and found ameasurement closely related to the global air temperature. Typically, fire innorthern regions occurs in the same area every 80 to 150 years. Scientists,however, found that when fire occurs more frequently, more radiation is lostfrom the earth and cooling results. Specifically, they determined when firereturns 20 years earlier than predicted, 0.5 watts per square meter of areaburned are soaked up by the earth from greenhouse gases, but 0.9 watts persquare meter will be sent back into space. The net effect is cooling. Watts areused to measure the rate at which energy is gained or lost from the earth.

46. According to thenew findings, taking steps to prevent northern forest fires may __________.

A. result in a warmingclimate     

B. cause the forestfires to occur more frequently

C. lead to a longerfire season             

D. protect the forestsand the environment there

47. The following areall the immediate effects after a forest fire EXCEPT __________.

A. large amounts ofgreenhouse gases enter the atmosphere

B. the levels of ozonewhich is a type of oxygen increase

C. snow on the groundmirrors more sunlight back into space

D. ashes from the firefall on the ice surface and take in more radiation from the sun

48. Earlier studiesabout northern forest fires __________.

A. analyze all aspectsof how northern fires influence climate

B. indicate that forestfires will pollute the atmosphere

C. suggest that peopleshould take measures to protect environment

D. suggest that thefires will speed up climate warming

49.The underlinedphrase “soak up” in the last paragraph most probably means __________.

A.released         B.absorbed       C.created      D. distributed

50. From the passage wecan draw a conclusion that forest fires in Alaska, Canada and Siberia may__________.

A. warm the climate asthe supposition goes

B. allow more snow toreflect more sunlight into space and thus cool the climate

C. destroy large areasof forests and pollute the far-off sea ice

D. help to gain moreenergy rather than release more energy

试题答案

【答案】

 A

 C

 D

 B

 B

【解析】