填空题 短文2（12.5分） Like street–corner prophets proclaiming that the end is near, scientists who study the earth’s atmosphere have been issuing predictions of impending doom for the past few years without offering any concrete proof. So far even the experts have had to admit that no solid evidence has emerged that this is anything but a natural phenomenon. And the uncertainty has given skeptics—especially Gingrichian politicians—plenty of ammunition to argue against taking the difficult, expensive steps required to stave off a largely hypothetical calamity. Until now, a draft report currently circulating on the Internet asserts that the global-temperature rise can now be blamed, at least in part, on human activity. Statements like this have been made before by individual researchers—who have been criticized for going too far beyond the scientific consensus. But this report comes from the International Panel on Climate Change(IPCC), a respected UN-sponsored body made up of more than 1,500 leading climate experts from 60 nations. This shift in scientific consensus is based not so much on new data as on improvements in the complex computer models that climatologists use to test their theories. Unlike chemists or molecular biologists, climate experts have no way to do lab experiments on their specialty. So they simulate them on supercomputers and look at what happens when human-generated gases —carbon dioxide from industry and auto exhaust, methane from agriculture, chlorofluoro-carbon from leaky refrigerators and spray cans— are pumped into the model’s virtual atmospheres. Until recently, the computer models weren’t working very well. When the scientists tried to simulate what they believe has been happening over the past century or so, the results didn’t mesh with reality; the models said the world should now be warmer than it actually is. The reason is that the computer models had been overlooking an important factor affecting global temperatures; sulfur dioxide that are produced along with CO2 when fossil fuels are burned in cars and power plants. Aerosols actually cool the planet by blocking sunlight and mask the effects of global warming. Once the scientist factored in aerosols, their models began looking more like the real world. The improved performance of the simulations was demonstrated in 1991, when they successfully predicted temperature changes in the aftermath of the massive Mount Pinatubo eruption in the Philippines. A number ofstudies since have added to the scientists’ confidence that they finally know what they’re talking about—and can predict what may happen if green house gases continue to be pumped into the atmosphere unchecked.

填空题 Why can’t scientists and experts offer any concrete proof of impending doom?

填空题 What does a current draft report on the Internet confirm?

填空题 How do climate experts do lab experiments on their specialty?

填空题 What important factor mentioned in paragraph four can also affects global temperatures?

填空题 The simulated computer models are proved reliable when 1.