Old wood offers new lessons for climate debate

Scientists found, based on the growth of rings in the trunks of trees, that temperatures about 800-1000 years ago were about equal to the current warming trend Keystone Archive

Global warming may not be that unusual. Swiss scientists have found evidence that things were just as odd during medieval times.

This content was published on March 23, 2002 - 10:24

A new study - which involved careful scrutiny of ancient tree rings from 14 sites on three continents - has found that, 1000 years ago, the northern hemisphere experienced a period of climactic warming similar to that observed today.

Coming amid the heated debate over global warming, the Swiss study is likely to fuel conjecture about whether scientific fears about late 20th and early 21st century atmospheric changes are justified.

A broad body of research suggests an unprecedented and continuing warming spell that began last century is caused by the greenhouse effect - the entrapment of gasses inside the earth's atmosphere.

But researchers from the Swiss Federal Research Institute, along with US-based scientists, have analysed the old tree rings to gain another perspective on climate change.

Hot Dark Ages

The scientists found, based on the growth of rings in the trunks of trees, that temperatures about 800-1000 years ago were about equal to the current warming trend.

But crucially for the current debate, during the so-called "medieval warm period" - as climatologists describe it - greenhouse warming was not a factor.

Study co-author Edward Cook, from the Lamont-Doherty Earth Observatory in Palisades, New York, said the research is not a refutation of the greenhouse effect.

Rather it shows the Earth to be "capable of rapid changes and long periods of above average warmth on its own without greenhouse warming", according to Cook.

"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.

Model stops short

Current wisdom - articulated by the Intergovernmental Panel on Climate Change - predicts that global warming will continue over the next century with temperatures rising by between 2.5 and 10 degrees.

Cook said the climate change panel's calculation is based on a model that compares pre-industrial-age climate with 20th century climate, without considering the medieval warm period, something that would alter the model.

"The medieval warm period is in some sense comparable up to 1990," Cook said.

He warned, however, that the real challenge remains factoring out natural climate fluctuations from man made triggers.

The report was published on Friday in the journal "Science".

Along with the US-based Cook, Zurich scientists Jan Esper and Professor Fritz Schweingruber worked on the tree-ring research.

swissinfo with agencies

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