The Great Warming: Climate Change and the Rise and Fall of Civilizations, by Brian Fagan, Bloomsbury, $26.95

You can hear it under the breath of grudging half-converts now sighing about inconvenient truths. In response to reports from the Inter­govern­mental Panel on Climate Change and the National Academy of Sciences which confirm that humans are causing global climate to change, they concede this fact and yet mutter: so what? Most people recognize an unsettling future when they see it, but some still wonder if climate change will be that bad. A vocal minority even trumpets the potential benefits of longer corn-growing seasons in the temperate Great Plains. So long as subsidized ethanol keeps prices up, expect a rush on Canadian real estate.

Brian Fagan, an emeritus professor of anthropology at the University of California, Santa Barbara, offers a unique contribution to this discussion. Dismissing the “chatterers and doomsday sayers,” Fagan notes that “almost none of these self-proclaimed prophets bother to look back at climate change in earlier centuries and millennia, except for politically charged discussions as to whether the world was warmer a thousand years ago than it is today.” The Great Warming escapes this tired debate. The globe is warming, Fagan assures us, and the primary cause of that warming is human activity. That settled, Fagan turns our attention back a full millennium to demonstrate historical climate data’s hidden potential as a source for highly plausible doomsday scenarios.

The Great Warming explores the confluence of climate and culture during a period of global warming beginning around 800 A.D. and ending around 1300 A.D., dubbed the Medieval Warm Period. Fagan’s investigation highlights a little-discussed danger that we will have to face as we confront a warmed world: famine. Calling it “the silent elephant in the climatic room” of all global warming debates, Fagan finds famine brought on by sustained droughts almost everywhere in this past world, from the American West to Northern China to the Peruvian highlands. Sometimes famine caused (only!) prolonged suffering and mass death, while in a few cases, as with the Maya, it led to the downfall of great civilizations. Modern civilizations throughout the world today, with much higher population densities and limited mobility, would be alarmingly susceptible to generation-long droughts. Even more than rising sea levels or catastrophic storms, Fagan cautions, we should be wary of drought and famine: forces that would lay low even the confident.

Despite this warning—amplified in a brief, closing chapter—Fagan’s passion lies beyond preaching doomsday. Readers should not underestimate this book, writing it off as another addition to a burgeoning genre: the travel guide to a torrid world. Fagan’s project is much bigger. He re-creates past societies in a lively and engaging manner, aided by his expert synthesis of obscure climatological data, much of which has only become available within the last 30 years. The reader witnesses: Europeans in the High Middle Ages, buoyed by good harvests and warmer temperatures; the Norse taking advantage of retreating glaciers to make their great northern sea voyages and their eventual commerce with people from as far away as the Bering Strait; the Mongols, conquering a great empire as they flee a harshening desert; and even Polynesian islanders making the most of changes in wind patterns to reach the impossibly small and distant Rapa Nui. Climate change contributed to these instances of cultural efflorescence and expansion.

In his ability to bring nature into our global, historical narratives, Fagan rivals Alfred Crosby, William H. McNeill, and Jared Diamond, scholars who revealed to large audiences the explanatory power of microscopic biota or gross geography. Fagan promises to do the same for long-term climate dynamics. He proves that the regional volatility associated with climate change—best exemplified by the swings of El Niño/Southern Oscillation, which Fagan places second only to the seasons as a determinant of climatic variation—shaped societies. And he does this without succumbing to a reductive determinism.

Take, for example, Fagan’s engaging story of the near invasion of Europe by fiercely disciplined Mongol horsemen. Fagan argues that the Mongols, a nomadic people who lived on the Eurasian steppe, had long moved with the rhythms of the “desert pump”—the desert expanding with heat and drought, then contracting as cooler rains made way for resurgent vegetation. Based on the best records available—tree-ring sequences read from preserved Siberian pines—higher temperatures and drought on the Mongol’s home steppes coincided with the string of great victories that established Genghis Khan’s magnificent empire. Still thirsting for more fertile land, the Mongols made it to Europe in 1241 and defeated Henry the Bearded in Silesia. Two factors—one human and one climatic—intervened at this moment. Genghis’s successor as Great Khan, his son Ögötai, died. Batu Khan, who sought the office of Great Khan himself, abandoned his plans to invade Europe and returned to the steppes to press his claim to the throne. When he got there, he found vastly improved grazing conditions brought on by cooler, wetter weather. The new productivity of the land having muted Batu’s incentive to drive west, Europe remained barely touched by the Mongol Empire.

On the opposite side of the world, Maya lords on the path to empire bound themselves to the rains and their ability to harvest them. In obeisance to their god-kings, Mayas constructed great Water Mountains: reservoirs that served the growing cities of the southern lowlands. The lords played out their divine claims in temple rituals—until the sustained and unprecedented droughts that arrived with the Medieval Warm Period undermined their political and religious legitimacy. The civilization, already stressed by war amongst local lords, was not prepared for multiyear droughts that could outlast even their advanced water technologies. With the simultaneous failure of the Water Mountains and the political structure, the great Maya cities collapsed in the early 10th century.

In contrast, the Chimu, in what is now Peru, survived their droughts. Like the Maya, the Chimu had built a centralized society depen­dent on massive waterworks. But, unlike the Maya, the Chimu knew the sting of great droughts and had already adapted by building redundant irrigation systems to withstand the flooding that preceded Pacific-spanning La Niña droughts, by planting a wide variety of crops on intensively farmed fields, and by hedging their bets with a trade in anchovies from the coast. As Fagan puts it, “Chimor’s cities survived the Medieval Warm Period because its lords closely supervised a subsistence culture that revolved around insuring against drought, floods, and deprivation.”

For those parts of the world that offer no tale of expansion or destruction, Fagan explains stability. The people of the Sahel—those responsible for the Sahara gold trade—barely felt the warm centuries. The gold trade was grueling, and camel drivers depended on constant reconnaissance to successfully navigate the expanding desert sea. But what was new about that? No drama animates Fagan’s chapters on the Sahara gold traders or the similarly mobile peoples who lived near Owens Lake in modern-day California. Yet drama seldom ends well: when it comes to climate change, we should prefer boring stories to exciting ones. The society that presents a boring story for future historians is one that remains flexible, and, in so doing, simply remains.

Ultimately, writing climate change into history exposes the complex relationship of the historical enterprise to the past, present, and future. The Great Warming depends on information that scientists have only begun to mine from tree rings, ice and deep-sea cores, and coral records. Yet, despite Fagan’s complaint that past climate data has been tied up for too long in debates on anthropogenic global warming, such data often owes its existence to those debates. Present concerns motivated climate research, from which we now draw insight into history. Insight into the foreign past can be disorienting but also productive. In the case of The Great Warming, our gazing dizzily into the past for a few hundred pages has done us a great service. Fagan cannot prove that drought and famine will be key problems in our warming world. He is looking at the past and at vastly different societies from our own. We would be fools, though, to ignore his warnings.