Why it matters

Origins of climate change

27th December 2008

Could the climate be so sensitive that man-made global warming began 10,000 years ago?

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The idea has been controversial from the start, when William Ruddiman of the University of Virginia first proposed that human-induced climate change could have begun more than 10,000 years ago.

Last week, Stephen Vavrus, a climatologist at the University of Wisconsin-Madison’s Center for Climatic Research, presented supporting data, gathered with Prof. Ruddiman, to the American Geophysical Union.

The theory posits that the Earth has swung between ice ages and warm temperate interglacial periods for the past million years, with each shift triggered by regular changes in the Earth’s orbit around the sun and fuelled by “positive feedback” effects – such as the loss of ice and snow, which reflect sunlight back to space, and the increase in darker-coloured, less reflective water, soaking up heat and further warming the planet (as is happening right now).

About 5,000 to 8,000 years ago, levels of the greenhouse gases carbon dioxide and methane started to rise more than the levels typical of other post-ice-age periods, based on ice-core records from Antarctica. According to Prof. Ruddiman, the change was caused by human activities, such as clearing forests in Eurasia, releasing carbon dioxide and cultivating rice paddies in Asia, releasing methane. Global warming was promoted by the resulting positive feedback effects – warming the oceans, for example, which lowered their carbon dioxide content (cold liquids, like champagne, hold more gas than warm ones). Even before the start of the Industrial Revolution, the scientists say, levels of methane and carbon dioxide were already accelerated.

Prof. Vavrus presented data from computer simulations done with Prof. Ruddiman. “As computers improve, we can make more sophisticated models that incorporate more and more of the feedback effects together.”

The warming effect from ancient man has prevented an ice age from occurring right now, they say. “If humans hadn’t intervened, it would be two degrees Celsius colder” on the planet over all, Prof. Ruddiman says. “There should be permanent ice sheets covering much of Canada and Eurasia.”

He adds that it doesn’t mean global warming is beneficial: “When my initial paper was published, the climate-change skeptics jumped all over it and said, ‘See, greenhouse gases are our friend,’ ” he says. “But then they realized that if they accepted my hypothesis, it means the climate system is as sensitive as mainstream scientists say it is.”

Published in The Green Report in The Globe and Mail

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• Tags:
• Climate Change
• History

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