THE GREAT WARMING

What happened when the world grew warmer from 800 to 1200 CE.

Drawing on data gathered during the past 30 years by climatologists using such modern tools as deep-sea cores, ice borings, computer modeling, tree and coral rings, Fagan (Fish on Friday: Feasting, Fasting, and the Discovery of the New World, 2006, etc.) offers a tentative history of the “Medieval Warm Period,” when rising surface temperatures produced sudden, unpredictable climate swings throughout the world. Although much remains unknown, there is good evidence that there were winners and losers in this period of global warming. It was a time of abundant harvests and the cultural achievements of the High Middle Ages in Europe, while other areas from the Americas to China and Eastern Africa experienced long periods of drought and famine. “Farmers went hungry, civilizations collapsed, and cities imploded,” writes Fagan. Prolonged drought stalks these pages, a silent killer the author considers a harbinger of what could happen during our own time of global warming. Medieval droughts lasted for decades in California and the American Southwest, he notes. Even the lower Hudson River Valley experienced arid conditions that, if they occurred today, would endanger urban water supplies. Much of the book describes how the Medieval Warm Period affected trade, warfare and other aspects of life. In Central America, drought repeatedly disrupted the lives of the Mayans, who relied on unpredictable water sources. Elsewhere, many rural societies coped by building canals for irrigation, borrowing food from neighbors in times of need, maintaining kinship ties with distant communities and moving there when droughts came. Today’s more densely populated planet, notes the author, with 250 million people living on agriculturally marginal lands, is far more vulnerable to long periods of drought, especially the developing world and such populous areas as Arizona, California and southwestern Asia.

An alarm bell ringing out from a distant time.