Ruddiman, WilliamPlows, Plagues, and Petroleum: How Humans Took Control of Climate. Princeton, N.J.: Princeton University Press,2005. Pp. xiv + 202. Illus. Hardcover, $24.95.

Humans have been affecting climate for at least 8,000 years, although the effect has been much greater in the last 200 years. Global warming is caused by gases entrapped in the atmosphere, and the most important contributors are methane and carbon dioxide. The amount of methane in the atmosphere, measured by entrapped air in Antarctica ice, increased greatly about 5,000 years ago.  The deduced cause was the development of irrigation of rice fields—the rice stalks and weeds decayed, emitting methane.  The same ice cores show that carbon dioxide concentrations in the atmosphere increased about 8,000 years ago.  The most likely explanation is the cumulative effect of cutting down forests.  Carbon dioxide concentration in the atmosphere follows a natural cycle of about 100,000 years—the cause is not clear.  The cycle was disrupted about 8,000 years ago, and this disruption probably delayed or avoided another ice age.  The general increase in carbon dioxide has been marked by downward wiggles in AD 200-600, in 1300-1400, and in 1500-1750.  These dips correlate with major disease outbreaks, particularly bubonic plague, leading to severe drops in population and abandonment of farmland.  The industrial era has produced a very large increase in greenhouse gases—carbon dioxide and methane.  The earth’s climate responds slowly to changes, so the corresponding warming is still in progress, perhaps half done.  Also another industrial era emission is sulphur dioxide, creating aerosols in the atmosphere.  These aerosols reflect sunlight so have a cooling effect.  Ruddiman proposes several scenarios for how the increased carbon dioxide atmospheric concentration will play out.  His opinion, however, is that the most pressing problem facing humans is not global warming but resource depletion.  This review does not do justice to the careful science, carefully described.  Two chapters deal with the causes of ice ages—mainly variations in the earth’s orbit and variations in the earth’s tilt angle.  From another perspective, the book is a compelling elucidation of a scientific mystery—an exciting example of how a scientist reasons.  From a third perspective, the book describes a scientist’s experience in proposing a new theory and gaining gradual acceptance.