By WILLIAM GRIMES
If you don’t think climate change produces winners as well as losers, consider this: In the 12th and 13th centuries England exported wine to France. Vineyards also flourished in improbable regions like southern Norway and eastern Prussia. A centuries-long spell of mild, predictable weather blessed Western Europe with abundant crops, healthy populations and budget surpluses sufficient to finance projects like Chartres Cathedral.
This is the credit side of a global balance sheet carefully itemized by Brian Fagan in “The Great Warming,” his fascinating account of shifting climatic conditions and their consequences from about A.D. 800 to 1300, often referred to as the Medieval Warm Period. The debit side is appalling: widespread drought, catastrophic rainfall, toppled dynasties, ruined civilizations. Abandoned Maya temples in the Yucatan and the desolation of Angkor Wat, supreme achievement of the Khmer empire, bear witness to climatic change against which royal power and priestly magic proved impotent.
Mr. Fagan, an anthropologist who has written on climate change in “The Long Summer” and “The Little Ice Age,” proceeds methodically, working his way across the globe and reading the evidence provided by tree rings, deep-sea cores, coral samples, computer weather models and satellite photos. The picture that emerges remains blurry — scientists still understand little about such weather-changers as El Niño and La Niña — but it has sharpened considerably over the past 40 years, enough for Mr. Fagan to present a coherent account of profound changes in human societies from the American Southwest to the Huang He River basin in China.
Longer summers and milder winters in Europe, especially stable from 1100 to 1300, allowed Norse explorers to range as far as Greenland and Labrador. At the same time a population boom in the rest of Europe led to radical deforestation, as trees were cleared to create farmland. By the end of the Medieval Warm Period half the forests that covered four-fifths of Western and Central Europe in A.D. 500 had disappeared.
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