New evidence has emerged that suggests human-induced climate change began not 200 years ago, but thousands of years ago with the onset of large-scale agriculture in Asia and extensive deforestation in Europe.
The prevalent view is that the invention of the steam engine and the advent of the coal-fueled industrial age 200 years ago marked the beginning of human influence on global climate.
But now, using powerful simulations on the world's most advanced computer climate models, scientists have determined that man started manipulating climate thousands of years ago.
"This challenges the paradigm that things began changing with the Industrial Revolution," said Stephen Vavrus, a climatologist at the University of Wisconsin-Madison's Center for Climatic Research and the Nelson Institute for Environmental Studies.
"If you think about even a small rate of increase over a long period of time, it becomes important," he added.
Vavrus and colleagues John Kutzbach and Gwenaelle Philippon have provided detailed evidence in support of this controversial idea, which was 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 (CO2) 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, was enough to set human-induced climate change in motion, said Kutzbach and Vavrus.
Looking 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.
According to Kutzbach and Vavrus, that ancient air 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," explained Kutzbach.
According to the same computer simulations, the cumulative effect of thousands of years of human influence on climate is preventing the world from entering a new glacial age, altering a clockwork rhythm of periodic cooling of the planet that extends back more than a 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," he added.