Global Climate Change Realities

Ruddiman Hypothesis

Trained as a marine geologist, University of Virginia emeritus professor William Ruddiman for the past fifteen years has worked on a hypothesis that posits that pre-industrial age humans raised greenhouse gas levels in the atmosphere. Looking back seven thousand years into the Holocene—the current 11,500-year-old geological epoch—Ruddiman has proposed that early agriculture emitted enough methane and carbon dioxide to offset what would have been a global cold cycle. Ruddiman says that in contrast to the familiar view that human-caused greenhouse gases began with the industrial revolution, “the baseline of human effects on climate started earlier and that the total effect is larger.” Ruddiman’s work and “the Ruddiman hypothesis” provide a classic illustration of the working out of a scientific theory, with detractors, new allies, and new technologies and facts that bear on the original idea.

University of Virginia climatologist William Ruddiman has spent a good bit of his career studying the Pleistocene cycle of ice ages that began about 2.6 million years ago. Periods of large-scale glaciation and deglaciation are governed by the Milankovitch cycle, in which shifts of the Earth’s orbit and its inclination toward the sun change how much sunlight reaches the northern hemipsphere to warm the surface. Based on solely these orbital cycles, global average temperatures of our current interglacial period—the Holcene—should be dropping, with the result that glaciers should now be growing in northern Canada and Siberia. That is not happening. Why?

For Further Reading

Fuller, Dorian, Jacob van Etten, Katie Manning, et al., “The Contribution of Rice Agriculture and Livestock Pastoralism to Prehistoric Methane Levels: An Archaeological Assessment,” The Holocene 21, no. 5 (2011): 743-59.

Kaplan, Jed, Kristin M. Krumhardt, Erle C. Ellis, et al., “Holocene Carbon Emissions as a Result of Anthropogenic Land Cover Change,” The Holocene 21, no. 5 (2011): 775-91.

Marcott, Shaun, Jeremy D. Shakun, Peter U. Clark, Alan C. Mix, “A Reconstruction of Regional and Global Temperature for the Past 11,300 Years,” Science 338 (2013): 1198-1201.

Mitchell, Logan, Ed Brook, James E. Lee, et al. “Constraints on the Late Holocene Anthropogenic Contribution to the Atmospheric Methane Budget,” Science 342 (2013): 964-66.

Ruddiman, William, “How Did Humans First Alter Global Climate?” Scientific American 292 (2005): 46-53.

Ruddiman, William, “The Anthropocene,” Annual Review of Earth and Planetary Sciences 41 (2013): 45-68.

Ruddiman, William, Steve Vavrus, John Kutzbach, and Feng He, “Does Pre-industrial Warming Double the Anthropogenic Total?” The Anthropocene Review 1, no. 2 (2014): 147-53.