Study Shows Large Natural Warming

by William Yeatman on April 2, 2002

in Science

A period of high global temperatures near the beginning of the last millennium closely matches the warming witnessed, about one degree Fahrenheit, during the 20th century, according to a new study in the March 22 issue of Science.

The studys authors, Drs. Jan Esper and Fritz Schweingruber at the Swiss Federal Research Institute, and Edward Cook at the Lamont-Doherty Earth Observatory, state that “Much of the current debate on the Earths climate variability is driven by the observation of a modern, century-long temperature increase, culminating with the last decade of the 20th century as the warmest since 1856.”

Using tree ring data from 14 different sites on three different continents in the Northern Hemisphere, the researchers constructed a temperature record of the last 1000 years. What they found was that the “MWP [Medieval Warm Period] was likely to have been a large-scale phenomenon in the NH [Northern Hemisphere] extratropics that appears to have approached, during certain intervals, the magnitude of 20th-century warming, at least up to 1990.”

This finding contradicts an earlier study by Mann, et al. that appeared in Geophysical Research Letters in 1999. That study combined tree ring data and the instrumental temperature record and “shows an almost linear temperature decrease from the year 1000 to the late 19th century, followed by a dramatic and unprecedented temperature increase to the present time,” according to Esper, et al. That study served as the basis for claims in the Third Assessment Report of the United Nations Intergovernmental Panel on Climate Change that the current warming is greater than at any other time in the last 1000 years.

A commentary that accompanies the study notes that, “The warming of the 20th century is seen more clearly as a continuation of a trend that began at the start of the 19th century, not the early 20th.” It also notes that, “the curve of Esper et al. provides evidence for greater climate variability in the last 1000 years than has yet been generally accepted.”

“We dont use this as a refutation of greenhouse warming,” Edward Cook told CBS News. “But it does show that there are processes within the Earths natural climate system that produce large changes that might be viewed as comparable to what we have seen in the 20th century. Greenhouse gases were not a factor back in the Medieval Warm Period.”

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