NCPA - National Center for Policy Analysis

Climate Records Contradict Global Warming Claims

August 8, 2003

It is well-established that the Earth's climate went through a period of warming during Medieval times, followed by a Little Ice Age that persisted into the 19th century.

Yet promoters of the theory that humans are causing global warming claim that temperatures were relatively stable for many centuries -- until greenhouse gases from burning fossil fuels caused the 20th century to be the warmest in a thousand years. This view was promoted by the Third Assessment Report (2001) of the United Nations Intergovernmental Panel on Climate Change (IPCC).

Climate scientist David R. Legates says that the data supporting this claim were inappropriately manipulated:

  • The historical climate reconstruction in the report primarily used tree ring records to infer air temperature trends for past centuries.
  • Then, the surface temperature record of the 20th century was attached to the end of the proxy record -- although instrumental air temperature readings are not directly comparable to proxy records.
  • Putting the two different sets of data together created a "hockey stick" appearing to show a dramatic rise in temperatures during the 1990s (see figure).
  • Astrophysicists Willie Soon, Sallie Baliunas and colleagues reexamined a large number of proxy records and composite analyses, concluding that "The Medieval Warm Period and Little Ice Age are widespread climatic anomalies."
  • Two independent efforts, led by Jan Esper and Keith Briffa, using carefully selected tree-ring chronologies found the Medieval warming was comparable to the late 20th century, and was followed by a long and variable Little Ice Age.

Esper found swings in mean average temperatures between the Little Ice Age and the Medieval Warm Period about four times greater than the estimate in the IPCC report, and shows no dramatic change after about 1950.

Source: David R. Legates (NCPA adjunct fellow), "Revising 1,000 Years of Climate History," Brief Analysis No. 450, August 8, 2003, National Center for Policy Analysis.

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