NCPA - National Center for Policy Analysis

Temperature Index Doesn't Prove Global Warming Claims

November 12, 1998

Vice President Al Gore regularly announces that the last month, season, year and decade are the hottest on record -- proof that manmade global warming is a real and growing threat. But what Gore is talking about is not actual measured temperatures, but rather an "index," which critics say is biased to show an upward temperature trend.

The index was developed by the National Climatic Data Center (NCDC), the record-keeping division of the Commerce Department's National Oceanographic and Atmospheric Administration. It combines measurements of land surface temperatures, sea surface temperatures taken from ships, and temperatures taken from a network of buoys deployed in the 1980s.

But the index is flawed in several respects, say critics:

  • The NCDC agrees the methodology used to develop the index "was not documented in the open refereed literature," which is standard scientific practice.
  • It does not include satellite-based measurements -- which experts say are the most accurate and which the NCDC agrees show a slight cooling trend.
  • The sea surface temperatures were adjusted upward by 25 percent after 1982 in order to calibrate it with land surface temperatures.
  • And many of the land-based thermometers are subject to the "urban heat-island effect" which means their temperature records are biased upward because the surrounding asphalt, concrete and steel effectively absorb and retain heat.

Apparently, an unknown number of these thermometers are on rooftops, exposed to sunlight and reflected heat from tar roofs. When the National Weather Service moved its rooftop San Francisco station, it lowered temperature readings considerably, and the NWS recently ordered all rooftop sites to be move to "more representative locations."

Source: Paul J. Georgia, "Temperature Data Still Riddled With Errors," CEI On Point No. 16, October 7, 1998, Competitive Enterprise Institute, 1001 Connecticut Avenue, N.W., Suite 1250, Washington, D.C. 20036, (202) 331-1010.


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