There are many ways to get from the west coast of Greenland to L'Anse aux Meadows, none of them easy. You can fly from Kangerlussuaq to Reykjavik in a little less than five hours, and from there to New York in just under six; from New York to St. John's, on the south­ east side of the island of Newfoundland, will take about three more. From there, you either drive or fly four hundred miles west-perhaps ten hours by road; an hour and fifteen minutes by twin-engine turboprop-to Deer Lake. Then, another three hundred miles by car on Canada's picturesque Route 480, along the Newfoundland coast, until you run out of road, and walk the last bit to a peat farm, a reconstructed forge, and half a dozen sod-roofed houses.

All of the available options are far easier than the direct route, first taken a thousand years ago: nine hundred stomach-heaving miles across the North Atlantic in a sixty-foot-long square-rigged wooden ship. But that ship, and others like it, are the reason L'Anse aux Mead­ ows is a UNESCO World Heritage Site: the first European settlement in the New World, and probably the most famous place ever colonized by the merchants and traders we know as the Vikings.

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It's nowhere near the largest. A Viking settlement on the banks of the Dnieper River, near Smolensk, has more than three thousand funeral mounds scattered across its forty acres. For centuries, the people who built them controlled the trade that moved along Europe's rivers all the way from the Baltic to Constantinople, and even as far as ninth­ century Baghdad, when it was Islam's--and probably the world's--richest and most sophisticated city. Along the Dnieper and Volga, the traders were generally known as Varangians or “rowers” and formed the personal guard of the Byzantine Emperor; sometimes as Rus, from which modern Russia takes its name. Closer to home, in Ireland, they were often known as Finngaills, or “fair foreigners.” The earliest occupants of Britain called them Danes—in the story of Beowulf, “east-Danes” or “spear-Danes.” Most frequently, they were known as Northmen, or Norsemen; after 793, when they raided the monastery at Lindisfarne, on the east coast of England, with “rapine and slaughter,” it was said that all over Europe, people prayed, “A furore norman norum libera nos, Domine”: From the fury of the Northmen, deliver us, Lord.

The Norse—the word “Viking” comes from an Old Norse word meaning “voyaging,” later refined to mean “raiding,” rather than “trading”—were merchants, warriors, farmers, and artisans. Despite a well-earned reputation for fearsomeness in battle, they appeared less savage to their contemporaries than to their modern mythmakers; in 1220, the chronicler John of Wallingford described them as, in thirteenth-century terms, a bit dandified:

They were—according to their country’s customs—in the habit of combing their hair every day, to bathe every Saturday, to change their clothes frequently and to draw attention to themselves by means of many such frivolous whims. In this way, they besieged the married women’s virtue and persuaded the daughters of even noble men to become their mistresses.

But first, last, and always, they were sailors. Their only real competition for the title of the greatest sailing culture in history came from the eleventh- and twelfth-century Polynesians who colonized Hawaii and Easter Island, and their greatest accomplishments are best understood in the same context: travel across vast distances with neither magnetic compasses nor maps, navigating by their knowledge of currents, swells, and the migrations of birds and fish.

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The fish weren’t just an aid to sailors but the most important reason they went to sea in the first place. Long before they were trading gold and amber along the shores of the Caspian, Black, and Mediterranean seas, Norse sailors honed their maritime skills in the most basic of human activities: gathering food, especially cod from the North Atlantic, which was, and is, the world’s greatest fishery.

By the beginning of the ninth century, they were ready to expand to the west, east, and especially south. Historians have been puzzling over the impetus for centuries; Edward Gibbon, in the forty-ninth book of his Decline and Fall of the Roman Empire, argued that the brutal conquest of the Saxons by Charlemagne in 804 not only opened the door to invasion of Europe from Scandinavia, but provoked it:

The subjugation of Germany withdrew the veil which had so long concealed the continent or islands of Scandinavia from the knowledge of Europe, and awakened the torpid courage of their barbarous natives.

More methodical, though less eloquent, historians have looked, instead, to increased numbers of gravesites in the relatively poor lands of ninth-century Scandinavia and Iceland—areas, by most estimates, able to support no more than one to two people per square kilometer— as a clue to just the sort of population pressure that might have inclined Norsemen to go a-viking. Or, perhaps the Norse were simply reacting to a later invasion by Europe’s Christian sovereigns, who were forcibly converting the pagan peoples on the continent’s periphery by the beginning of the tenth century.

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There is, though, a more powerful and plausible cause for the explosive spread of the Norse. The great achievements of the Viking Age were almost entirely enabled by the impersonal workings of climate. This shouldn’t come as a surprise. All human civilizations are hostage to weather, but none more so than sailors, who must confront both the violent nature of the ocean’s surface and the capricious atmosphere that imparts motion to their wind-powered vessels. When those mariners are surrounded by seas that produce icebergs and pack ice for up to six months of the year, even a few more weeks of warmer weather a year were literally life-changing.

Fluid dynamics is the branch of physics that studies liquids and gases in motion—among other things, weather, which gets its dynamism from the heat energy of the sun. That energy is received by every object in the solar system, but if the object in question lacks a fluid atmosphere, it has no weather, which is why a barren rock like Mercury, the closest planet to the sun, has none, and Jupiter, which receives a tiny fraction of the solar energy that hits Mercury, has hurricanes twenty-five thousand miles in diameter that last hundreds of years.

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Earth’s weather lacks Jupiter’s violence, but has its own complexities. Not because the source of heat—the sun—is so variable, but rather because the amount of heat energy absorbed by the Earth during its annual orbits is distributed unevenly. The consequences of that variability are such things as the ice ages—there have been at least four in the last billion years—when glaciers left huge chunks of the northern hemisphere covered with ice sometimes hundreds of feet thick, as well as eras when temperatures were 4 to 5 degrees warmer than today, causing sea levels to be at least twenty-five feet higher.

Weather and climate remain the product of complex interactions between ocean and atmosphere, a dance set to almost unimaginably complicated rhythms, made even more complicated because one partner— the atmosphere—is enormously quicker to respond to change than the other.

The boundary between atmosphere and water is where the dance partners meet, but their rhythms are created elsewhere: in the ocean’s depths, a three-dimensional maze of conveyor belts, powered by heat and salt. The top layer is warmed by the sun, whose rays penetrate a good forty meters, and not only contains most of the ocean’s marine life (and CO2) but stores more than ten times as much energy as the entire Earth’s atmosphere. The reason is specific heat: the amount of energy, measured in calories, needed to raise the temperature of a given mass of a particular substance by one degree Celsius. When the given mass is a gram, the specific heat is measured in small c calories; when it’s a kilogram, the measure is kilocalories.* Whether measured in grams or kilos, the specific heat for water is higher than for virtually any other common substance. It takes one calorie to heat a gram of water by a single degree, which is nearly twice as much as alcohol, five times as much as aluminum, and—most important—more than four times as much as air. And that’s just the top forty meters; because the total mass of the oceans is four hundred times that of the atmosphere, the amount of heat energy stored in the Earth’s oceans is some sixteen hundred times that of the atmosphere.

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The result of this enormous oceanic engine, dependent as it is on tiny changes in the proportions of heat and salt, is that a tiny blip in oceanic temperature can alter atmospheric temperatures for a thousand years. Which is what happened, sometime around the ninth century, when a few of those oceanic conveyor belts fell into a state of equilibrium for a moment infinitesimally short in geologic time, but a significant fraction of human history. The Medieval Warm Period— sometimes, more cheerfully, called the Medieval Climate Optimum (or, more honestly, the Medieval Climate Anomaly)—lasted only from the end of the ninth century to the beginning of the fourteenth; four centuries when the Northern Hemisphere experienced its warmest temperatures of the last eight thousand years.

The causes of the Medieval Warm Period are the subject of so many competing theories that it seems certain that they are going to remain murky for a while; but its existence is pretty much inarguable. The geological footprint left by moraines—the rocky debris carried by glaciers as they advance and recede—includes plant material that can not only be dated pretty precisely but carries evidence of small changes in annual temperature. Dendrochronologists—biologists who derive all sorts of information from the width and composition of tree rings—have spent decades studying dozens of different species of trees that add a ring each year, and long ago learned that, in temperate climates, the rings differ in width depending on the year’s climate. With a tree of a known date—a tree with a hundred rings was a hundred years old when cut down, and used, for example, in a building that is known to have been built, for example, in the year 1000—the temperature of any particular year can be calculated with a high degree of accuracy.

It’s more than just the ring’s width: the amount of the radioactive isotope Carbon-14 in tree rings measures the amount of solar activity in any particular year. The reasons are, like everything having to do with climate history, intricate: Carbon-14 is formed by cosmic-ray interactions with the nitrogen and oxygen in the Earth’s upper atmosphere, so, when there’s less solar activity, the amount produced by cosmic rays is relatively greater. Lower solar activity, more Carbon-14. And, sure enough, what are known as “cosmogenic anomalies” match up with what the chronicles report as warm eras in western Europe, not just during the MWP, but the early Iron Age from about 200 bce.

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There’s more. There’s ice. For more than forty years, geologists have been drilling out cylinders of ice in places like Greenland and Antarctica—places where the ice sheets haven’t melted in hundreds of thousands of years. Since the ice accumulates every year at a regular rate, a core—usually between about two and three inches in diameter, but up to two miles long—forms a calendar that records the composition, and the temperature, of the atmosphere over time. And, once again, the ice cores show an unmistakable warming period between the ninth and thirteenth centuries.

Its geographic extent is a little more problematic. Hubert Lamb, the English climatologist who first posited (and named) the Medieval Warm Period, was working from a limited data set; most of his historical sources—estate records, monastery documents, and the like—were European, and insufficient to demonstrate the global phenomenon he believed he had discovered. One result is that the Medieval Warm Period is regularly used as evidence for those who want to challenge the reality of man-made climate change—“during the Middle Ages, temperatures were even warmer than they are today.”

In reality, though, it turns out to be far easier to measure the temperature locally, whether in Scandinavia or China, than to solve the notoriously tricky puzzle of worldwide climate. Hubert Lamb was right, but the era he discovered and named was a Northern Hemisphere phenomenon, and particularly one that affected the civilizations along the north Atlantic between about 800 and 1200. The best estimates are that temperatures of northern Europe averaged a healthy 2oC higher than they do today; climate-change skeptics notwithstanding, there is still little evidence that worldwide temperatures were, on average, warmer than today.

Why the MWP’s effects were confined to the Northern Hemisphere— and especially to Europe—can be explained by a climatic seesaw known as the North Atlantic Oscillation, the prime determinant for the weather of northern and western Europe. The first end of the oscillation is a persistent zone of relatively low atmospheric pressure over Iceland; the second, a high-pressure zone over the Azores. The weather fronts that bring rain to Europe follow a track determined by the pressure gradient between the two. Thus, when the Azores High is, relatively high, and the Iceland Low relatively low, heat from the Atlantic is conveyed to Europe, making for warm summers and mild winters. As a result, the gradient during the MWP generally favored warmer weather in Europe, though not the entire world.

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That the North Atlantic Oscillation affected “only” a portion of the world’s climate doesn’t make it a trivial instrument of change. Its effects were as serious as it got for Europeans living in the era that began with the Viking expansion, and that ended just about the time that Edward II and Isabella of France were celebrating their marriage vows. To the eight out of ten people who farmed the land, sun and rain were what turned land into food. Sun and rain, in the proper proportions, were what supported human life. And there was a lot more of human life at risk in 1308 than had been the case in the year 800.

It’s not that European weather during the four centuries of the MWP was uniformly good. Both modern anthropology and historical documents testify to a depressingly long list of droughts, storms, freezes, and lost harvests during the four centuries of the MWP, possibly because of the very human habit of spending more time recording disasters than prosperity. But the weather between the ninth and fourteenth centuries was nonetheless markedly better—a little bit warmer, and a little bit more predictable—than any recorded period since the birth of civilization. An increase in temperature and reduction in variability doesn’t have to be enormous to initiate a very long, and very consequential, series of events.

The first, and most significant, effect of such predictably good weather was a huge expansion in the kind of land that could be made to produce food. During the MWP, cereals were harvested in European farms at altitudes of more than a thousand feet above sea level— unthinkable today—and vineyards started appearing in northern England. Throughout northwest Europe, land that hadn’t produced respectable amounts of food in millennia became productive. Including the lands of the Norsemen.

Excerpted from "The Third Horseman: Climate Change and the Great Famine of the 14th Century" by William Rosen. Reprinted by arrangement with Viking, a division of Penguin Group (USA) LLC. Copyright © 2014 by William Rosen.