NCPA - National Center for Policy Analysis

Global Warming Started Long Before Industrial Revolution

December 10, 2003

People may have begun altering the world's climate as early as 8,000 years ago, says William Ruddiman, a climate scientist with the University of Virginia.

Ruddiman's analysis shows that between 8,000 and 150 years ago, humans put enough greenhouse gases into the atmosphere to account for a global temperature increase of 1.4 degrees Fahrenheit -- roughly the same warming effect that greenhouse gases have caused over the past century. If his data is correct, this doubles the estimated human impact on climate.

His evidence comes from ice cores drilled from Greenland and Antarctica that record climate history over the past 400,000 years. They show that levels of so-called greenhouse gases -- those thought to be responsible for increasing global temperatures -- began their rise along with human civilization:

  • The data show that after the end of the last ice age, about 8,000 years ago, levels of carbon dioxide mysteriously started rising at a time when they should have been dropping.
  • The same holds true for methane levels from about 5,000 years ago.
  • People started clearing forests for pastures and cropland -- a process that puts more carbon dioxide into the atmosphere -- about 8,000 years ago in Europe and Asia.
  • People also started raising livestock and flooding rice paddies -- processes that emit methane -- about 5,000 years ago in Asia.

Still, Ruddiman says, the work doesn't change the prevailing scientific opinion about greenhouse gases today -- that over the last 150 years, human activities have built up enough of them in the atmosphere to contribute noticeably to the observed global warming.

Source: Alexandra Witze, "Theory: Global warming older than we thought," Dallas Morning News, December 10, 2003 and Andrew Bridges, "Scientists Measure Human Impact on Climate," Associated Press, December 10, 2003; based upon Thomas J. Crowley, "When Did Global Warming Start?", Climatic Change, Vol. 61, Issue 3, December 2003.


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