NCPA - National Center for Policy Analysis


February 1, 2008

In 2007, the Intergovernmental Panel on Climate Change (IPCC) issued its Fourth Assessment Report.  The report included predictions of big increases in average world temperatures by 2100, resulting in an increasingly rapid loss of the world's glaciers and ice caps, a dramatic global sea level rise that would threaten low-lying coastal areas, the spread of tropical diseases, and severe drought and floods.

These dire predictions are not, however, the result of scientific forecasting; rather, they are the opinions of experts.  Expert opinion on climate change has often been wrong, say Kesten C. Green, of the Business and Economic Forecasting Unit, Monash University, and J. Scott Armstrong, of the Wharton School, University of Pennsylvania.

Climate scientists now use computer models, but there is no evidence that modeling improves the accuracy of predictions.  For example, according to the models, the Earth should be warmer than actual measurements show it to be.  Furthermore:

  • The General Circulation Models (GCMs) that are used failed to predict recent global average temperatures as accurately as fitting a simple curve to the historical data and extending it into the future.
  • The models forecast greater warming at higher altitudes in the tropics, whereas the data show the greatest warming has occurred at lower altitudes and at the poles.
  • Furthermore, individual models have produced widely different forecasts from the same initial conditions, and minor changes in assumptions can produce forecasts of global cooling.

Thus it is not surprising that international surveys of climate scientists from 27 countries in 1996 and 2003 found growing skepticism over the accuracy of climate models.  Of more than 1,060 respondents, only 35 percent agreed with the statement, "Climate models can accurately predict future climates," whereas 47 percent disagreed.

Source: Kesten C. Green and J. Scott Armstrong, "Global Warming: Experts' Opinions versus Scientific Forecasts," National Center for Policy Analysis, Policy Report No. 308, January 2008.

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