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.
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.
References
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).