March 26, 2002
Tree Rings Show a Period of Widespread Warming in Medieval Age
By KENNETH CHANG
A new study of old tree rings shows that 1,000 years ago, long before power plants and sport utility vehicles, temperatures across North America, Europe and Asia rose in a period of unusual warmth.
Temperatures were known to be warm in Europe between 900 and 1100, what is known as the Medieval Warm Period. Collecting wood samples in 14 locations that cover a swath of the globe from New Orleans north to the top of Alaska, researchers from Columbia University's Lamont-Doherty Earth Observatory and the Swiss Federal Research Institute found evidence that the warm temperatures extended to much of the Northern Hemisphere.
Writing in the current issue of the journal Science, the scientists say the data demonstrate that temperatures naturally rise and fall over the centuries. The scientists, however, add that their data do not argue against the view that artificial emissions — so-called greenhouse gases — have set off the global warming of recent decades.
"I never intended or meant to imply that that's the case," said Dr. Edward R. Cook, an author of the Science paper and an expert at Lamont-Doherty on reconstructing climate from tree rings.