Addressing scientists on Dec 17 at a meeting of the American Geophysical Union, Vavrus and colleagues John Kutzbach and Gwenaëlle Philippon provided detailed evidence in support of a controversial idea first put forward by climatologist William F. Ruddiman of the University of Virginia. That idea, debated for the past several years by climate scientists, holds that the introduction of large-scale rice agriculture in Asia, coupled with extensive deforestation in Europe began to alter world climate by pumping significant amounts of greenhouse gases — methane from terraced rice paddies and carbon dioxide from burning forests — into the atmosphere. In turn, a warmer atmosphere heated the oceans making them much less efficient storehouses of carbon dioxide and reinforcing global warming.
That one-two punch, say Kutzbach and Vavrus, was enough to set human-induced climate change in motion.
"No one disputes the large rate of increase in greenhouse gases with the Industrial Revolution," Kutzbach notes. "The large-scale burning of coal for industry has swamped everything else" in the record.
But looking farther back in time, using climatic archives such as 850,000-year-old ice core records from Antarctica, scientists are teasing out evidence of past greenhouse gases in the form of fossil air trapped in the ice. That ancient air, say Vavrus and Kutzbach, contains the unmistakable signature of increased levels of atmospheric methane and carbon dioxide beginning thousands of years before the industrial age.
"Between 5,000 and 8,000 years ago, both methane and carbon dioxide started an upward trend, unlike during previous interglacial periods," explains Kutzbach. Indeed, Ruddiman has shown that during the latter stages of six previous interglacials, greenhouse gases trended downward, not upward. Thus, the accumulation of greenhouse gases over the past few thousands of years, the Wisconsin-Virginia team argue, is very likely forestalling the onset of a new glacial cycle, such as have occurred at regular 100,000-year intervals during the last million years. Each glacial period has been paced by regular and predictable changes in the orbit of the Earth known as Milankovitch cycles, a mechanism thought to kick start glacial cycles.
http://www.sciencedaily.com/releases/20 ... 190433.htm
Bill Ruddiman's 2007 paper, The early anthropogenic hypothesis: Challenges and responses doesn't appear to be available online but his 2003 paper, THE ANTHROPOGENIC GREENHOUSE ERA BEGAN THOUSANDS OF YEARS AGO is and it's well worth reading.
Bill Ruddiman was also also on Radio 4 a couple of years ago, I haven't listened to this yet:
BBC - Radio 4 - Frontiers 31/05/2006 -- Anthropogenic Climate Change
In this week's episode of Frontiers, Peter Evans meets climatologist Professor Bill Ruddiman whose views about climate change have divided scientific opinion.
http://www.bbc.co.uk/radio4/science/fro ... 0531.shtml
RealAudio of this show here: http://www.bbc.co.uk/radio4/science/ram ... 060531.ram
