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.”
(12/18/2008) Recovery of forests following the collapse of human populations in the Americas after the arrival of Europeans may have driven the period of global cooling from 1500-1750 known as the Little Ice Age, report researchers speaking at the annual meeting of the American Geophysical Union in San Francisco. By some estimates, diseases introduced by Europeans may have killed more than 90 percent of population on the New World within a century of first contact. The rapid depopulation led to large-scale abandonment, and subsequent reforestation, of agricultural lands in the Americas. Analyzing charcoal found in soils and lake sediments at sites across the Americas, Richard Nevle and Dennis Bird found evidence to suggest that this forest regeneration sequestered enough carbon to trigger global cooling.
(09/03/2008) In 2003 William Ruddiman put forth a controversial theory: 7,000 years ago the rise of agriculture spawned large-scale climatic changes. According to Ruddiman, the felling of forests for fields throughout Europe and Asia caused a rise in carbon dioxide, while the flooded fields for rice released methane gas. This combination of large amounts of carbon dioxide and methane entering the atmosphere caused the globe to warm, preventing the planet from entering another ice age.
(02/20/2008) Analysis of soil charcoal in South America confirms that from a historical perspective, fire is rare in the Amazon rainforest, but when it does occur, it appears linked to human activities. The research, published in Philosophical Transactions of the Royal Society B, is based on dating of soil carbon, which provides a good indication of when fires occurred in Amazonia, according to lead author Mark Bush, head of the Department of Biology at Florida Institute of Technology.