18 December 2008
Did human-induced climate change begin thousands of years ago?
by Kate Melville
Climate change dogma posits that the invention of the steam engine and the advent of the coal-fueled industrial age marked the beginning of human influence on global climate. But a radical new climate theory contends that the Earth would currently be experiencing an ice-age if it weren't for the fact that humans began planting crops and clearing forests thousands of years ago.
"This challenges the paradigm that things began changing with the Industrial Revolution," says Stephen Vavrus, a climatologist at the University of Wisconsin-Madison's Center for Climatic Research. "If you think about even a small rate of increase over a long period of time, it becomes important."
Presenting the research at a meeting of the American Geophysical Union, Vavrus and colleagues John Kutzbach and Gwena�lle Philippon provided detailed evidence in support of a controversial idea first put forward by climatologist William F. Ruddiman (University of Virginia). Ruddiman's 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 the researchers, 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 just now teasing out evidence of past greenhouse gases in the form of fossil air trapped in the ice. That ancient air, says Vavrus, 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, who asserts that 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 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.
"We're at a very favorable state right now for increased glaciation," said Kutzbach. "Nature is favoring it at this time in orbital cycles, and if humans weren't in the picture it would probably be happening today." The new research underscores the key role of greenhouse gases in influencing Earth's climate. Whereas decreasing greenhouse gases in the past helped initiate glaciations, the early agricultural and recent industrial increases in greenhouse gases may be forestalling them.
Source: University of Wisconsin-Madison