Plows, Plagues, and Petroleum: How Humans Took Control of Climate (review)

It is a common trope among historians of environment and technology that supposedly modern developments often prove much older than we think and that many contemporary transformations find their antecedents deep in the past. Nevertheless, most would regard global warming as something truly new under the sun—an unprecedented impact of industrial technology. Yet if William Ruddiman is right, even anthropogenic climate change has been with us for millennia.

In Plows, Plagues, and Petroleum Ruddiman advances two novel arguments about human-induced climate change. First, the author makes the case that a combination of carbon dioxide from forest clearance and methane from wet-rice agriculture began to alter Earth's atmosphere at least 5,000 years ago, preempting the onset of a new Ice Age. Second, Ruddiman contends that the Black Death of the fourteenth to fifteenth centuries and then the death of Native Americans from Old World disease in the sixteenth century led to such massive forest regrowth that CO₂ drawn from the atmosphere set off the "Little Ice Age." Thus humans "took control" of climate long before the burning of coal created a new greenhouse effect starting in the late 1800s.

Throughout, Ruddiman's book remains well-written and remarkably accessible for such a technical field as climatology. His ideas are wellordered and clearly presented with the aid of useful charts and graphs illustrating his main points about changes in the atmosphere. Ruddiman has also done an adequate if unremarkable job reading up on such topics as the history of agriculture, deforestation, and disease. Altogether, this is a rare work on climate history that professors can safely assign to upper-level undergraduates—and that alone makes the book worthwhile.

Contentious though they are, Ruddiman's arguments are never mere speculation. The book does an excellent job explaining the basic atmospheric science behind glacial cycles, and it presents convincing quantitative evidence of past atmospheric changes, pointing out the likely role of anthropogenic deforestation and agriculture in the late Holocene. The author's credentials as an established climate scientist lend further weight to the theory.

Nevertheless, Ruddiman's arguments are speculative. The author's main contention about ancient human impact on atmospheric carbon dioxide and methane rests on reasonable but still unproven analogies with past interglacials. Likewise, the author's argument that an early modern drop in atmospheric CO₂ triggered the Little Ice Age is intriguing, but given the already-established roles of solar and volcanic forcing, it may be one explanation too many. More important, by considering only effects on greenhouse-gas levels, the author simplifies the climate impact of deforestation and agriculture in temperate zones. Early clearance and farming also raised surface albedo and dust in the atmosphere, with cooling effects that might have offset greenhouse-gas emissions. Nor does Ruddiman consider another theory recently making the rounds among climate scientists: that early Holocene forest growth was itself largely the result of the manmade extinction of large Pleistocene browsers and grazers like the mammoth!

These complications point to a more fundamental issue in the book: the notion that at some point in the past, human technology "took control of climate." What Ruddiman really demonstrates is that far from achieving "control," human land use has long had profound and complex unintended consequences for Earth's atmosphere. For historians of technology, this will probably prove the most interesting lesson from Plows, Plagues, and Petroleum and the one most likely to stir up discussion in classes and seminars.