There is now no doubt that the world is getting warmer. Data from both the Southern and No

rthern Hemispheres(地球的半球)shows a(n)【C1】______trend in average temperatures over the【C2】______hundred years. At the moment most scientists are not prepared to stick their necks out and【C3】______a cause to this trend. It is possible that it represents a natural climatic change caused by a【C4】______increase in the sun's radiation output. But these same scientists are【C5】______aware that recent temperature changes are at the limit of known natural【C6】______: it seems increasingly likely that the cause is a(n)【C7】______greenhouse effect.

The basic theory of the greenhouse effect is quite simple. The earth's atmosphere consists【C8】______of oxygen and nitrogen,【C9】______there are small concentrations of various "greenhouse" gases-【C10】______carbon dioxide, water vapor, and methane(沼气)-- which play a very important role in【C11】______the planet's "heat balance". As their name suggests these gases have a similar effect on glass in a greenhouse: They let heat from the sun【C12】______, and keep it in.

The atmosphere is more or less transparent to the【C13】______solar radiation which warms the earth(ground or oceans)on which it falls.【C14】______it has been heated, the earth is warmer than space and gives【C15】______energy in the form. of in visible long-wave infrared radiation. This【C16】______for nighttime cooling. Greenhouse gases【C17】______some of this long-wave radiation, and reradiate it in the lower atmosphere.

The gases【C18】______act rather like a blanket by preventing some infrared radiation from leaving the earth-atmosphere system.【C19】______certain limits, the more greenhouse gases present, the more infrared radiation will become【C20】______, and the higher the surface temperature of the earth.

【C1】

A．current

B．downward

C．upward

D．disturbing