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.

Across vast swaths of the globe, however, severe, persistent droughts lasted not just for years but for generations. The Sierras of modern-day California experienced the severest droughts of the past 4,000 to 7,000 years. Acorn trees died, and along with them peoples largely dependent on acorns for food. Although data remain sketchy, it seems probable that extended droughts dried up pastureland on the Central Asian steppe, propelling the armies of Genghis Khan westward.

In the southern Yucatan arid conditions proved too much for the elaborate reservoirs, called “water mountains,” that the Maya used to irrigate their fields. Mr. Fagan permits himself an ominous aside: “The analogies to modern-day California, with its aqueducts for water-hungry Los Angeles, or to cities such as Tucson, Ariz., with its shrinking aquifers and falling water table, are irresistible.”

Mr. Fagan is as interested in human adaptation as he is in weather. While California’s acorn eaters suffered, peoples in the Southwestern deserts expanded their diet to include new edible plants. In the Sahara caravan organizers simply adjusted their routes according to changing rainfall patterns.

“The camel and its load-carrying saddle proved an effective weapon against heat and drought even in the worst years, when extreme aridity affected cattle people living far south of the desert,” Mr. Fagan writes.

Northern China got the worst of both worlds during the Medieval Warm Period: violent climatic swings that resulted in lengthy dry spells or torrential rainfall. Meanwhile, in the South Pacific, faltering trade winds allowed Polynesian voyagers to head east, eventually reaching Rapa Nui (Easter Island) around 1200.

Mr. Fagan has a somewhat rigid, formulaic way of presenting his material. Well aware that the general reader can handle only limited amounts of ice-core data, he tries to generate period atmosphere by including present-tense “you are there” episodes. “The hushed crowd in the plaza gazes upward to the temple at the summit of the pyramid,” one section begins. A little drama certainly helps, but he overworks this device. The book is overpopulated with sweating plowmen and fishermen peering into the mist.

The causes of the Medieval Warm Period remain unclear, and there is debate over what the actual temperatures were. Mr. Fagan draws one unambiguous conclusion from the evidence, however, in a final chapter on the present-day implications of the great warming of a thousand years ago. Drought is the great enemy, “the silent and insidious killer associated with global warming,” he writes.

Population density has placed enormous pressure on increasingly scarce water resources. As a result modern droughts, brought on by El Niño events, have taken an enormous toll in lives and wreaked measureless economic devastation. Prepare for worse.

“Judging from the arid cycles of a thousand years ago, the droughts of a warmer future will become more prolonged and harsher,” Mr. Fagan writes. “Even without greenhouse gases, the effects of prolonged droughts would be far more catastrophic today than they were even a century ago.”

For a spark of hope Mr. Fagan offers the example of Chimor, a kingdom in coastal Peru tormented by El Niño flooding and severe droughts throughout the Medieval Warm Period. The Chimu people thrived nonetheless by diversifying their food supply and protecting their scarce water resources. In a historically arid region with uncertain food supplies, they successfully tapped their centuries of experience with irrigation, soil conservation and water management. Look no further for a global-warming role model.

Link to article: http://www.nytimes.com/2008/03/21/books/21book.html?ref=environment