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Right Now | Epochs of Ice

The Science of History

March-April 2016

Illustration by Sam Falconer


Illustration by Sam Falconer

In his 1998 book Consilience, Pellegrino University Professor emeritus Edward O. Wilson dreamed of a time when the boundaries between sciences and humanities would fade away, leaving an intellectually unified store of human knowledge. His vision was prescient. Today, it’s possible to use isotope analysis to pinpoint the origin of a single thread of silk that traveled the Silk Road. DNA tests of ancient skeletons can reveal how people lived, ate, and died—and two years ago helped confirm the role of the Plague of Justinian in the collapse of the Roman Empire. Ice-core samples drawn from polar caps or glaciers are telling historians more than they ever believed possible about the worlds their research subjects lived in. These findings share once-inaccessible insights into the human past—and into humanity’s fragile climate future.

Until recently, ice-core research was limited to Arctic regions, where abundant snowfall produces a thick frozen record dating back thousands of years. Now, laser-based technology developed at the University of Maine’s Climate Change Institute is sensitive enough to produce 50,000 measurements per meter—enabling researchers from UM and from Harvard’s Initiative for the Science of the Human Past (SoHP) to analyze samples from the tightly packed Colle Gnifetti glacier in the Swiss Alps. Goelet professor of history Michael McCormick, who chairs SoHP, recalls a prominent Swiss climate scientist telling him, “There are no good Swiss ice cores.” In fact, there were—but the technology to read them didn’t exist.

Colle Gnifetti marks the first time historians have been able to look directly at weather patterns around the Mediterranean that date back nearly 2,000 years—much farther than traditional resources, like thermometer readings or tree rings. The Mediterranean “is where history happened, and we no longer have to guess the conditions [people] lived in,” McCormick says. “We can see it there in the ice record.” And with unprecedented precision: “If you ask what the weather was like on Christmas of 1257, I can tell you,” says history of science lecturer Alexander More, a postdoctoral fellow with SoHP. Unlike other tools used to approximate weather, ice records offer proxies for many types of climate events—temperature, precipitation, air circulation, even burning biomass—that can be tested against one another or combined to analyze the climate context from a moment in history.

The Harvard-UM team compared data from the Colle Gnifetti ice core to historical weather records from 1200 to 1400—an era of explosive economic growth in Europe but also of the Black Death and the beginning of the Little Ice Age, a period of cooling that followed the relative warmth of the Middle Ages. They found that historical reports of bad harvests showed a striking correspondence with drops in temperature, approximated by the frequency of oxygen isotopes in the ice record. A similar, though weaker, relationship appeared between historical reports of precipitation and drops in temperature recorded in the ice core.

Both findings advance historical narratives about the relationship of the Little Ice Age, famine, and the Black Death, which killed as much as half of Europe’s population. The historical and scientific records—“independent lines of argument deriving from independent evidence deriving from different areas of reality,” McCormick explains—increasingly appear to tell a similar story: the cooling period of the late Middle Ages contributed to greater rainfall and flooding—a breeding ground for disease vectors—and famine, which weakened the population and aggravated morbidity.

The data, which More stresses are still preliminary, also challenge accepted truths about European history. Estimates about when the Little Ice Age began run from around 1250 to 1650. Parts of the ice record suggest temperatures may have begun dropping earlier still, closer to 1200, but other parts don’t appear to agree with the historical record at all, prompting both groups of scholars to take a closer look at their results. Laboratory science, McCormick points out, “is no more sure than any other kind of knowledge, and has to be submitted to the same kinds of challenges and testing as our most nuanced interpretation of a medieval love letter.”

More also cautions against allowing current ideas about climate change to influence inquiry into the past. “We have to keep an always skeptical mind, and not be so excited by our findings that we dismiss clear signs of problems,” he says. Even so, a confluence of evidence, strengthened by the Colle Gnifetti data, suggests climate change played a pivotal role in the events of the late Middle Ages. Less directly, climate-induced resource scarcity appears also to have stimulated European colonial ambitions. If that’s true, the implications for the present appear unnervingly clear. The scale of climate change in the last two decades, McCormick says, “appears to be beyond anything we’ve seen in the last 2,000 years—and we’ve seen some scary things in the last 2,000 years.

“Twenty years ago, we couldn’t know the climate in the Roman Empire was warming,” he adds. “There were some slight hints that it got warm in the Middle Ages.…Now we can see that climate change occurred much more swiftly than I had thought possible.”

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