a huge climate change of up to 8 degrees Fahrenheit is to be expected. If this feedback is weak, as some skeptics argue, then a rise of only a degree or so will occur. RICHARD S. LINDZEN, Mass. Institute of Technology: If you didn't amplify what CO2 did with having water vapor and clouds go along with it, you'd only at most get a degree or so. INTERVIEWER: With a doubling? RICHARD S. LINDZEN: With a doubling. And tripling would not do much more. And people have pretty much accepted this would not be a major change in the world as we know it. NARRATOR: The computer models tell of possible futures, some scary, some less so. Which is true? Currently, there's no way to know. JAMES TREFIL, George Mason University: Computer models of climate are the most complicated, ingenious computer systems I've ever seen. I mean, they really are good. But in the end, you have to just face the fact that they are not going to be able to give you precise, certain answers. They're going to give you, "We think this is what's going to happen, and here's the limits of error." And that's the uncertainty, and you have to choose. NARRATOR: If the scientists are uncertain about the precise global temperature increases, they know even less about the specific effects the warming will produce. Of greatest concern is sea-level rise caused by melting glaciers and thermal expansion of warmer water. Coastal areas from Venice to California