Published by EH.NET (July 2006)

William F. Ruddiman, Plows, Plagues and Petroleum: How Humans Took Control of Climate. Princeton: Princeton University Press, 2005. xiv + 202 pp. $25 (hardcover), ISBN: 0-691-12164-8.

Reviewed for EH.NET by Robert Whaples, Department of Economics, Wake Forest University.

William Ruddiman’s provocative but plausible conclusion is that the economic behavior of humans began to profoundly influence global climate roughly 8000 years ago.

A recently retired Professor of Environmental Sciences at the University of Virginia and author of a leading textbook on climate history, Ruddiman begins with an overview of earth’s history, noting that the planet’s climate has drifted toward cooler conditions for approximately the past 55 million years. The dominant hypothesis is that volcanic carbon dioxide emissions have subsided while removal of CO2 from the atmosphere has accelerated due to interactions between monsoon rains and newly created ground-up rock debris in the Himalayas. About 2.75 million years ago — perhaps triggered by the closing of the Panama Isthmus and the diversion of Atlantic currents — cooling reached a critical threshold and massive ice sheets first appeared at high northern latitudes. Since about 900,000 years ago, the earth has gone through cycles during which glaciers have covered nearly a quarter of the Earth’s total land surface in ice ages lasting almost 100,000 years, which are divided by brief interglacials. These cycles seem to be explained by the eccentricity, axial tilt, and precession of the Earth’s orbit and cycles in the solar radiation and have been linked to cycles in the major greenhouse gases — methane and carbon dioxide. However, during the current interglacial (which began about 10,000 years ago), concentrations of methane and carbon dioxide have begun to deviate from the trend they followed in previous warm spells.

Ruddiman’s central argument is that this interglacial has differed from its antecedents because of human behavior. Based on previous cycles, one would have expected methane concentrations to have peaked about 11,000 years ago slightly above 700 parts per billion and then declined to about 450 parts per billion today. Methane levels followed the normal cycle at first, but about 5000 years ago they began to rebound and are nearly as high as they were before the decline began — about 275 parts per billion above trend. After ruling out natural causes, Ruddiman concludes that this deviation was probably caused by human activity — primarily the cultivation of rice in artificial wetlands in Asia. The argument seems quite plausible, but is by no means conclusive. Skeptics will require much more sophisticated quantitative estimates of methane emissions, tied to estimates of areas under cultivation in each period and cultivation practices. Perhaps some economic historian will take up this challenge.

Like methane, carbon dioxide has deviated from the cycle it has followed in previous interglacials and the deviation isn’t recent — beginning long before industrialization swept the globe. Ruddiman convincingly demonstrates that carbon dioxide levels began to rise above their normal trend about 8000 years ago. Rather than falling by 20 parts per million, they rose by 20 parts per million in the pre-modern era. Again, he argues that our ancient ancestors were responsible — primarily because they deforested vast swaths of land for farming. Ruddiman uses estimates of population, forest cleared per person and carbon emitted per each square kilometer cleared to approximate the total impact and concludes that the magnitude is reasonably close to the extra carbon dioxide accumulated during the period. Again there is some slippage between the estimates of human emissions and deviations from trend — but Ruddiman openly invites others to rework and refine his calculations, which seem quite plausible, though not conclusive.

The next step is to consider what the world would look like today if our ancient ancestors hadn’t cleared forests and cultivated rice. Plugging counterfactual greenhouse gas levels into a standard climate model, Ruddiman (with S.J. Vavrus and J.E. Kutzbach, “A Test of the Overdue Glaciation Hypothesis,” Quaternary Science Review, 2005) concludes that the world would be currently entering the next ice age. Temperatures would be much colder in areas of Canada centering on Hudson Bay and Baffin Island would be in a state of “glacial inception,” with the Labrador Plateau not far behind. In fact, Ruddiman concludes, we came very close to this threshold in the middle of the last millennium during the “Little Ice Age.” He argues that plagues — especially the Bubonic Plague of the 1300s and the Old World diseases that devastated the New World after the arrival of Columbus — reduced human populations in agricultural areas allowing enough afforestation that greenhouse gas levels dipped enough to pull temperatures down. This argument may be correct, but the evidence he presents is not that compelling to me. Figure 12.1 shows two series of carbon dioxide estimates from Antarctic ice cores. Unfortunately, the two series do not always correlate well. While the Taylor Dome series shows a sharp fall coinciding with the Bubonic Plague, the Law Dome series does not. The Law Dome series falls very sharply at about the time that most of the Western Hemisphere’s population died off, but the Taylor Dome series ends right before this critical period. Moreover, it isn’t clear that the Western Hemisphere pandemic led to massive afforestation — the bulk of the population declines came in the Andes and Mexico but it isn’t clear that forests grew rapidly in these areas.

Finally, Ruddiman considers the third P — petroleum — concluding that the current spike in carbon dioxide levels cannot last indefinitely because emissions will eventually decline to levels lower than the ocean’s absorptive capacity. In the end, temperatures will probably begin to drop and “the peak centuries of the greenhouse interval would pass too quickly for a large fraction of the south polar ice to melt” (p. 168). After several millennia, the Earth may well return to its long-overdue glaciation.

Ruddiman’s book has already begun to spark an important debate — a debate which economic historians should be eager to follow and to join.

Robert Whaples is Director of EH.NET. His recent publications include “Collapse? The ‘Dismal’ Science Doesn’t Think So: Economists’ Views of the Future,” Independent Review, 11 (2), Fall 2006.