Man Changed Climate For 8,000 Years

Measurements of ancient air bubbles trapped in Antarctic ice offered evidence that humans have been changing the global climate since thousands of years before the industrial revolution.

Beginning 8,000 years ago, atmospheric levels of carbon dioxide began to rise as humans started clearing forests, planting crops and raising livestock, an American scientist said in early December. Methane levels started increasing 3,000 years later.

The combined increases of the two greenhouse gases implicated in global warming were slow but steady and staved off what should have been a period of significant natural cooling, said Bill Ruddiman, emeritus professor at the University of Virginia.

The changes also disrupted regular patterns that dominated the 400,000 years of atmospheric history that scientists have teased from samples of ancient ice.

"You have 395,000 years of history, which sets some rules, and 5,000 years that break those rules," Ruddiman said.

Ruddiman briefed reporters on his theory at the fall meeting of the American Geophysical Union. Further details appeared in the December issue of the journal Climatic Change.

Previously, scientists widely assumed it was only with the onset of the factory age that human activity had any significant effect on the global climate. The prehistoric changes in carbon dioxide and methane levels have been noted before but were attributed to natural causes, Ruddiman said.

Atmospheric levels of carbon dioxide and methane naturally fluctuate, in part because of changes in the orbit of the Earth and the resulting variations in the amounts of sunlight.

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Khorsheed.com – Nov, Dec 2003