are warming the globe. Fred Singer, an atmospheric physicist, is one of a small band of scientists who have become known as greenhouse skeptics. He argues that the 100-year record is not what it appears to be. FRED SINGER, George Mason University: The data are ambiguous. For example, the data show that the climate warmed between 1900 and 1940, long before humanity used much energy. But then the climate cooled between 1940 and 1975. NARRATOR: Only in the last few decades do rising global temperatures seem to coincide with the greenhouse gases we put into the atmosphere. Skeptics like Singer have also argued that long before the industrial revolution, the climate warmed naturally. For example, records show that in the Middle Ages, parts of the world, like Greenland, were warmer than today. The Vikings built settlements there and farmed the land. FRED SINGER: The Vikings were able to grow crops, and life was good in Europe. Cathedrals were being built. There was plenty of food. Climate change is a natural phenomenon. Climate keeps changing all the time. It's either warming or cooling. NARRATOR: But records of one place, like Greenland, during one time, the Middle Ages, may not prove anything. Only by knowing the average global temperature 1,000 years ago can scientists know whether the Earth back then was warmer or colder than today. JONATHAN OVERPECK: What we're looking for is a period in the past - or periods - that are as warm or warmer than today because, of