NCPA - National Center for Policy Analysis


October 12, 2004

A new study casts doubts on the accuracy of the global warming timeline, which has previously shown that the current warming trend has been unmatched by any other periods in the past 1000 years.

In the online publication Science Express, Hans von Storch and his co-authors examined the climate timeline over the last millennium and concluded that the methodology that has been used may not reflect actual, more drastic temperature changes over time.

  • Scientists built an overall record of climate variability by examining tree rings and other hints of past conditions; the record was then combined with the more measurable climate history over 100 years.
  • They used statistical formulas which accounted for greater uncertainty in earlier climate years, allowing for a broader range of climate variation.
  • The result over 1000 years has been a climate timeline for temperatures in the Northern Hemisphere that looks something like a hockey stick -- a relatively flat trend line until about 1900, when the line suddenly spikes upward.

However, in order to test the accuracy of this method, scientists reversed the procedure of constructing the timeline by essentially destructing it. This method of working backwards produced a similar over the past 100 years, but sharply underestimated the large warming and cooling trends in earlier years.

While the new study indicates that the earth's climate is every more sensitive than originally thought to external influences, it also sheds more skepticism on the belief that sharp warming trends are very recent and a result of man-made activities.

Source: Andrew C. Revkin, "New Research Questions Uniqueness of Recent Warming," New York Times, October 5, 2004; and Hans von Storch, et al., "Reconstructing Past Climate from Noisy Data," Science Express, September 30, 2004.

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