bit of land that we have available to us right now. [www.pbs.org: What other land could be submerged?] NARRATOR: But greenhouse skeptics think such scenarios are alarmist. Modest warming, they argue, may even be good. FRED SINGER: We have to ask what are the impacts of a warmer climate? What is the impact on agriculture? The answer is, it's positive. It's good. What's the impact on forests of greater levels of CO2 and greater temperatures? It's good. What is the impact on water supplies? It's neutral. What is the impact on recreation? It's mixed. You get, on the one hand, perhaps less skiing. On the other hand, you get more sunshine and maybe better beach weather. Let's face it, people like warmer climates. There's a good reason why much of the U.S. population is moving into sun belt, and not just people who are retiring. NARRATOR: But most climate scientists are less optimistic. STEPHEN H. SCHNEIDER, Stanford University: We're talking about humans modifying the climate so that, if we're lucky, we only get another degree in the next century. And it could be several degrees per century, is our best guess. Compare that to the degree per millennium of history, and now ask one more factor. How are the species of trees, for example, and birds and so forth- how are they going to migrate? In history, they just migrated. Now they have to cross factories, farms, freeways and urban settlements. So if you have the combination of fragmented habitats