Published online 10 December 2003 | Nature | doi:10.1038/news031208-7

News

Man has been changing climate for 8,000 years

Agriculture may have released huge amounts of greenhouse gases into atmosphere.

Clear-cutting and irrigation may have altered the planet's climate.Clear-cutting and irrigation may have altered the planet's climate.© GettyImages

Humans began altering the climate 8,000 years ago, long before the industrial revolution, claims a leading climate scientist1.

Massive clearance and irrigation for agriculture released huge amounts greenhouse gases into the atmosphere, says William Ruddiman of the University of Virginia in Charlottesville.

By the time the industrial revolution got under way, we had already raised the global temperature by an average of 0.8ºC and by as much as 2 ºC at high latitudes, he proposes - enough to deflect an impending ice age. Today's winters would be as much as 7 degrees cooler at high latitudes if it were not for the pre-industrial input of greenhouse gases, he says.

Concentrations of greenhouse gases in the atmosphere fluctuate cyclically: small changes in Earth's orbit affect the amount of solar radiation reaching our planet. Records of these cycles in ice cores dating back 400,000 years suggest that carbon dioxide and methane should have been declining steadily for at least the past 10,000 years.

Instead, carbon dioxide has been rising for 8,000 years and methane for 5,000. "I think it's a near dead certainty that these changes aren't natural," Ruddiman told this week's annual meeting of the American Geophysical Union in San Francisco.

The idea is likely to spark debate among climate scientists, but at least one sceptic is already changing his mind. "I hadn't fully appreciated the actual magnitude of the human disturbance," says Thomas Crowley, who works on global warming at Duke University in Durham, North Carolina. "I've been thinking more and more that Ruddiman is on to something."

Trend spotting

Increased solar radiation every 22,000 years has been linked to stronger monsoons, which in turn lead to more wetlands. Decay of wetland vegetation releases more methane into the atmosphere. Atmospheric methane reached its most recent peak 11,000 years ago and should since have been dropping along with solar radiation.

Ruddiman argues that the reversal of this natural trend 5,000 years ago was caused by the advent of irrigation of rice crops and tending of large herds of livestock in Asia.

“I hadn't fully appreciated the actual magnitude of the human disturbance”

Thomas Crowley
Duke University

A similar story could explain the unexpected change in the carbon dioxide cycle. Every 100,000 years, carbon dioxide has risen sharply and then declined steadily for at least 15,000 years. But following the last peak 10,000 years ago, levels dropped slowly for only 2,000 years, then began increasing again.

This change coincides with the beginning of major deforestation for agriculture in Eurasia 8,000 years ago, reckons Ruddiman. "Humans were doing things on a scale that can explain why the natural trends failed," he says. 

Duke University

  • References

    1. Ruddiman, W. F. The Anthropogenic Greenhouse Era Began Thousands of Years Ago. Climatic Change, 61, 261 - 293, doi:10.1023/B:CLIM.0000004577.17928.fa (2003). | Article | ISI |