Is Global Warming Nature's Work?

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

The study, appearing Friday in the journal Science, analyzed ancient tree rings from 14 sites on three continents in the northern hemisphere and concluded that temperatures in an era known as the Medieval Warm Period some 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 greenhouse 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, New York. Cook is co-author of the study with Jan 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.

CBS News Correspondent Jim Axelrod reports the scientists who did this study are adamant their findings do not undermine the greenhouse theory. They are simply saying the dramatic warming we've seen in the last half century can be explained, at least partially, by the earth's natural cycles.

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 between 2.5 and 10 degrees. Based on this prediction, there have been international proposals for systematic reductions in the burning of fossil fuels. The proposal has been resisted by the United States, particularly by the Bush administration.

Cook said data used in the climate change panel's calculation is based on a model that compared the preindustral age climate with the climate of the 20th century. The model did not include a Medieval Warm Period. Including data from that era could change the calculations, Cook said.

"The Medieval Warm Period is in some sense comparable up to 1990 in the 20th century," said Cook. "But that does not say that the 20th century hasn't been perturbed by greenhouse gases. The real challenge is to factor out the natural variability from" manmade causes of global warming.

Cook said the panel's temperature warming prediction could be correct. Based on the new tree-ring data, however, he said the warming could be in the lower part of the temperature range forecast by the group.

Keith Briffa and Timothy Osborn, climate scientists at the University of East Anglia in Britain, said the study by Cook and his colleagues "provides evidence for greater climate swings in the last 1,000 years than has yet been generally accepted."

In a commentary in Science, Briffa and Osborn said a need exists for more such independent studies to refine predictions for global warming in this century.