March 26, 2002
Tree Rings Show a Period of Widespread Warming in Medieval Age
By KENNETH CHANG
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.
In warm weather, trees thrive and grow a thick ring of wood in their trunks for that year. In cold years, growth slows and the tree ring is thin.
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 war ming 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 exper t at Lamont-Doherty on reconstructing climate from tree rings.
Greenhouse gases like carbon dioxide from factories and cars trap heat in the atmosphere. Globally, temperatures have risen about 1 degree Fahrenheit over the last century.
Although the climate has repeatedly swung between warm and cold over the Earth's 4.5-billion-year history, the usual interpretation is that the climate has been quite stable since the end of the last ice age 10,000 years ago.
"This record suggests that the amplitude of natural variability is larger than the other records have suggested in the past," Dr. Cook said.
The data may help scientists refine their climate models to provide better prediction s of future warming.
An earlier reconstruction of temperatures over the past millennium by Dr. Michael E. Mann, a professor of environmental sciences at the University of Virginia, found a much smaller temperature rise in the Medieval Warm Period, but the different numbers do not necessarily contradict each other.
Dr. Mann's data covered the entire Northern Hemisphere - in addition to tree rings, he included other indicators of past temperature like coral reefs and ice cores from Greenland. Temperatures in the tropics vary much less than those in the higher latitudes, he said.
Because the authors of the Science paper averaged their data over 40 years to smooth out year-to-year fluctuations, their temperature curve does not reflect the most recent warming.
"They've kind of smoothed out of the record," Dr. Mann said. "It doesn't support the conclusion that the medieval warmth was comparable to the latter 20th century warmth."
Rather, the peak temperatures in the Medieval Warm Period are similar to those seen in the first half of the 20th century, and that warming, most scientists agree, was induced naturally, by a brightening of the sun.
