than 1,000 years. Has Pfister found any period in the past as warm as the 1980s and '90s? CHRISTIAN PFISTER: We had other periods which was also very warm, but it was equivalent to the 20th century prior to 1988. I would say this was a period of warm climate, but I would say it was a period of warm natural climate. With the period between 1988 and 1997, there's absolutely no equivalent as far as I can look back. NARRATOR: But this technique, too, is limited. Only a few areas, like Europe, have long-term records which survive. To get a handle on the global climate, scientists have been forced to go beyond human accounts of the weather. Five hundred miles from the North Pole, a frozen lake will be home to a team of scientists for the next few weeks. It's part of an international effort to try and reconstruct the climates of the past. RAYMOND S. BRADLEY, University of Massachusetts: We are here in the Canadian high Arctic, on Ellsmere Island, about 500 miles from the North Pole. And it might seem strange that we would come to this kind of environment to study the problem of global warming. But what we need to do is try to reconstruct the longer-term history of climate, and we do that by using so-called natural archives of climate, things that in some way retain in their structure a measure of how the climate has varied in the past. NARRATOR: Bradley and