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     Trees could be more important to the earth's climatethan previously thought, according to

a new study thatreveals forests help to block out the sun.

     Scientists in the UK and Germanyhave discovered that trees release achemical that thickens

clouds abovethem, which reflects more sunlight andcools the earth. The research suggeststhat

chopping down forests could speed

up global warming, and that protecting existing trees couldbe one of the best ways to

deal with the problem.

     Dominick Spracklen, of the Institute for Climate andAtmospheric Science at Leeds

University, said, "We thinkthis could have quite a great effect. You can think of forestsas

climate air conditioners. "

     The scientists looked at chemicals called terpenes(萜烯) that are released from

northern forests across northernregions such as Canada, Scandinavia and Russia. The

chemicals give forests a special smell, but their function haspuzzled experts for years.

Some believe the trees releasethem to communicate, while others say they could offer

protection from air pollution.

     The team found the terpenes react in the air and formaerosols(气雾剂)．They help to

turn water vapour(蒸气)in the atmosphere into clouds.

     Spracklen said the team's computer models showedthat the aerosols doubled the

thickness of clouds about1,000 meters above the forests, and would reflect an extra5%

sunlight back into space.

     He said, "It might not sound a lot, but that is quite astrong cooling effect. The climate

is such a finely balancedsystem that we think this effect is large enough to reduce

temperatures over quite large areas. It gives us anotherreason to preserve forests. "

     Becausetreesrelease . moreterpenesinwarmerweather, the discovery suggests that

forests could act as anegative feedback (反馈) on climate, which slows the rise in

temperature.

1. To slow global warming, scientists advise _

A. preserving the existing forests

B. growing more trees in the world

C. releasing aerosols above the forests

D. releasing terpenes in the air  

2. What is the function of terpenes according to Spracklen?

A. They absorb sunlight and turn it into energy.

B. They help thicken the clouds that reflect sunlight.

C. They make the temperature rise in cold northernarea s.

D. They help trees grow better.

3. Trees have a great effect on the earth's climate by  ______

A. reducing air pollution

B. reducing harmful chemicals in the air

C. producing a large amount of oxygen

D. reducing the earth'-s temperature