THE ASSOCIATED PRESS

WASHINGTON

An unusually warm period a millennium ago may have been part of a natural planetary cycle, researchers say in a study of tree rings that scrutinizes the link between human activity and climate change.

The study, appearing today in the journal Science, analysed ancient tree rings from 14 sites on three continents in the Northern Hemisphere. It concluded that temperatures in an era known as the Medieval Warm Period about 800 to 1,000 years ago closely matched the warming trend of the 20th century.

In recent years, many climate scientists have said an unprecedented warming spell that began last century and continues is caused by the greenhouse effect.

The green house effect is blamed on an increase in the atmosphere of gases, principally carbon dioxide from the burning of fossil fuels, which trap heat just as do glass panes in a greenhouse.

The tree ring study gives another perspective on Earth's natural cycles, said Edward Cook of the Lamont-Doherty Earth Observatory in Palisades, N.Y. Cook is co-author of the study, with Ian Esper and Fritz Schweingruber of the Swiss Federal Research Institute.

Cook said the study shows the Earth to be "capable of rapid changes and long periods of above-average warmth on its own without greenhouse warming."We don't use this as a refutation of greenhouse warming," said Cook."But it does show that there are processes within the Earth's natural climate system that produce large changes that might be viewed as comparable to what we have seen in the 20th century."

Cook said the study found that, based on the growth of rings in the trunks of trees that lived hundreds of years ago, the temperatures during the Medieval Warm Period were about equal to the warming trend that started in the 20th century.

"Greenhouse gases were not a factor back in the Medieval Warm Period," said Cook.

The Intergovernmental Panel on climate Change, an international group, has predicted that the current warming trend will continue deep into the 21st century, with average temperatures rising by 1.5 to 5.5 degrees C.

QUOTABLE Green House gases were not a factor back in the Medieval Period

STUDY CO-AUTHOR EDWARD COOK