William Ruddiman has become well known for his theory that human-induced climate change started long before the Industrial Age. In 2003 he first brought forth the theory that the Neolithic Revolution-when some humans turned from hunter-gathering to large-scale farming-caused a shift in the global climate 7,000 years ago.
Since publishing his theory Ruddiman, professor emeritus of environmental science at the University of Virginia, has been busy answering critics. He has explored how early rice paddies could have released large amounts of methane into the atmosphere and developed a model that shows how the burning of the world’s then pristine forests could have lead to a positive feedback mechanism, since the carbon released by the forests could have raised ocean temperatures.
 Contemporary deforestation by burning in Laos. Photo by: Rhett A. Butler. |
Now Ruddiman in a paper published in Quaternary Science Reviews is answering another criticism of his theory that says human populations were too small to cause as much deforestation for food production as Ruddiman has suggested. According to Ruddiman, human populations may have been small, but they were hugely destructive when it came to the world’s forests. He says that contemporary civilization uses 90 percent less land per person for growing food than the farmers of 7,000 years ago.
“They used more land for farming because they had little incentive to maximize yield from less land, and because there was plenty of forest to burn,” said Ruddiman. “They may have inadvertently altered the climate.”
Due to the large amount of forested land, Ruddiman argues, that farmers used a rotation method for agriculture: once early farmers saw yields decline in one area they would simply burn more forest and begin planting in new fields. This could have led to a situation where five times more land was deforested than was actually farmed at any given time.
“It was only as our populations grew larger over thousands of years, and needed more food, that we improved farming technologies enough to begin using less land for more yield,” Ruddiman said. “We suggest in this paper that climate modelers might consider how land use has changed over time, and how this may have affected the climate.”
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