natural recorders. Their annual rings not only give scientists a built-in date, they also contain a record of past temperature and precipitation. FRANCO BIONDI: Each one of these layers is an annual layer seen in cross-section. The annual layer is formed by these two portions together, the early wood and the late wood. And the width of the layer is our first indication of a climate pattern. So the narrow band that you see here indicates a year when the winter and spring were drier than normal. And then the larger layers that you see here indicate a period when winter and spring precipitation were more abundant than normal. NARRATOR: Trees are very common on land, but for the three quarters of the planet covered by ocean, scientists need other recorders. Like trees, corals can live for hundreds of years and grow at different rates, depending on the temperature at the sea surface. The Earth's temperature is also preserved in the chemistry of ice layers extracted from glaciers all around the world. Each natural recorder produces estimates of temperature and rainfall for a given place and time. Combining them all, scientists have been able to extend back the temperature record for the northern hemisphere about 1,000 years. According to this graph published in 1999, the 20th century's one degree warming stands out dramatically. [www.pbs.org: Visit the "stories in the ice"] But what about Greenland and its controversial medieval warm period? According to the combined data, there was no medieval warm