Ancient tree rings reveal past climate.

Fifty millennia ago, volcanic ash and mud buried a forest of conifers along a Pacific shoreline in what is now southern Chile. In 1960, an earthquake loosened these sediments, and erosion then exposed the long-entombed trees. Now, by examining the tree rings of the remaining stumps, an international team of scientists has reconstructed the earliest year-to-year record yet of climate variation.

The stumps of the tree species Fitzroya cupressoides are roughly 50,000 years old, says lead scientist Fidel A. Roig of the Laboratory of Dendrochronology at IANIGLA-CONICET, an earth-sciences research center in Mendoza, Argentina. Roig notes that there's a virtual forest of these stumps, which are still woody and well preserved.

Data from these trees "provides a year-by-year indication of general climate variability for a period before there was anything even approaching that sort of resolution," says research team member Keith R. Briffa of the University of East Anglia in Norwich, England.

Scientists have looked at past climate patterns--some going back hundreds of thousands of years--by studying layers in ocean sediments and ice cores. But older layers often become too compressed to reveal year-to-year differences, explains team member Hakan Grudd of Stockholm University.

However, using annual growth-ring patterns in trees, some researchers have inferred temperatures dating back about 10,000 years, or to the end of the last ice age. More ancient records have been difficult to re-create because the trees needed for such studies have either rotted away or been destroyed by glaciers, according to the researchers.

In the new analysis, which appears in the March 29 NATURE, Roig and his coworkers took cross sections of 28 of the ancient stumps and measured the width of each tree ring. By averaging the data, they produced a growth record of the 1,229 years before the trees were buried, the researchers say. They then developed a similar chronology for modern F. cupressoides trees growing nearby.

The team discovered strikingly similar growth-ring patterns in both chronologies, indicating that climate patterns 50,000 years ago resemble those of roughly the past 1,000 years, says Briffa. For instance, the researchers found patterns in the ancient trees that match year-to-year changes in the modern trees due to El Nino, the periodic spike in tropical Pacific Ocean temperatures that dramatically affects the weather.

If the same factors that affected climate 50 millennia ago remain operative today, then it's likely that those factors will determine the climate in the coming millennia, says Gordon C. Jacoby, a dendrochronologist at the Lamont-Doherty Earth Observatory of Columbia University in Palisades, N.Y. This could be helpful information for researchers trying to model future climate conditions, he adds.

Connie A. Woodhouse, a paleoclimatologist at the National Geophysical Data Center in Boulder, Colo., cautions that the new data provide only a "snapshot" of an ancient climate. She says she hopes researchers will uncover more trees that can bridge the gap between old and new climate records.