NCPA - National Center for Policy Analysis

Tree Rings Establish Temperature Fluctuations Over The Centuries

March 26, 2002

A new study of old tree rings shows that 1,000 years ago -- long before power plants and sport utility vehicles -- temperatures across North American and Asia rose in a period of unusual warmth.

  • Temperatures were known to be warm in Europe between 900 and 1100 -- during what is known as the Medieval Warm Period.
  • Collecting wood samples in 14 locations that cover a swath of the globe from New Orleans 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, scientists say the data demonstrate that temperatures naturally rise and fall over the centuries.
  • The peak temperatures in the Medieval Warm Period are similar to those seen in the first half of the 20th century -- and that warming, many scientists contend, was induced naturally, by a brightening of the sun.

The scientist add that their data do not argue against the view that artificial emissions -- so-called greenhouse gases -- have set off global warming in recent decades.

Source: Kenneth Chang, "Tree Rings Show a Period of Widespread Warming in Medieval Age," New York Times, March 26, 2002.


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