The Third Horseman

by William Rosen

  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.

  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.

  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.

  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.

  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.

  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.

  Erik Thorvaldsson, better known to history as Erik the Red, wasn’t the first Norseman to discover the eight hundred thousand square miles of tundra and permafrost located between the Atlantic and Arctic, but he was the first to settle there. Sometime before 950, Erik’s father left the family home in Norway, one step ahead of the family of the man he had killed in a violent brawl, to settle in Iceland, which the Medieval Warm Period had turned into a warm enough place for decent if not great farming. Norse colonists established themselves in Iceland by 900, and were able to produce barley (at least until the twelfth century) and hay for dairy cattle. In around 982, Erik, as prone to violence as his father, was sentenced to a three-year exile for his own series of murders—the sons of one man and “a few other men”—and he took his banishment as an opportunity for one of history’s best-known real estate promotions. He established, during his three-year exile, a relatively prosperous camp along what was, as a result of the warming trend, an ice-free coastline. By the 870s, not only had the amount of pack ice in the North Atlantic fallen dramatically, but the soil of the islands of the far north was composed of less permafrost than virtually any time in the last one hundred twenty-five thousand years. When Erik returned to Iceland, he was able to promote his new settlement, which he named Greenland partly because of the enormous grassy meadows that he found there but mostly because his “people would be attracted to go there if it had a favorable name.”

  It was singularly appropriate that the Greenland colony—which would last until the fifteenth century, with upward of four thousand permanent residents, who built, among other things, a cathedral and two monasteries—was such a well-remembered beneficiary of the climatic change, since most of the weather of northern Europe, including the four centuries of the MWP, is determined by an exceedingly complicated set of ocean currents around Greenland and Iceland. The Irminger Current (also known as the East Greenland Current, which is a part of North Atlantic Current, and a subsidiary of the Gulf Stream) runs just south of Iceland, and carries very cold water from the Arctic south, and out of the northern sea lanes. When it is flowing along the path it took during the Medieval Warm Period, the cold water is pushed down an underwater cliff on the floor of the Denmark Strait—a very high cliff, more than four times higher than Niagara Falls, and with more than four hundred times the volume. Submerging this quantity of water several hundred meters below the surface keeps pack ice, which forms between January and April, at least one hundred kilometers away from Iceland, and the coast of Greenland, which is where it was kept during both the Medieval Warm and today.

  The disappearance of that ice not only drew Erik the Red to Greenland (and his son, Leif, to North America, and—probably—L’Anse aux Meadows) but, as a consequence of the MWP, led most of Europe into its first sustained population increase since the fall of the Roman Empire. And, eventually, to the wedding of Edward and Isabella in Boulogne in 1308.

  The connections between four centuries of historically good weather, and four days of historically luxurious celebration, are primarily economic. For virtually all human civilizations before the Industrial Revolution the largest contributor to national wealth was arable land: land on which crops could be grown and livestock fed. Every aspect of life depended on land, and the rural population who worked it. Agriculture didn’t just feed the guests at the royal wedding; it paid for the cathedral where the vows were exchanged, and even the clothes on the bride’s back.

  The agricultural laborers who collectively supported Europe’s armies, roads, cities, and most of its commerce were, in turn, dependent on the continent’s supply of sunny days. The addition, on average, of even ten or twenty days of sun each growing season—which is what a frost-free May two years out of three would produce, courtesy of the Medieval Warm Period—meant more food: enough food to allow a few more children to survive infancy, and a few more adults to survive for more productive years. Like compound interest, this meant dramatic change over time: a population explosion. Records are scanty before the year 1000, but for the next two centuries, Europe became home to a great many more Europeans: England’s population grew from 1.5 million to more than 5 million; France from fewer than 6 million to between 17 and 21 million; and Italy, whose population had declined by more than a third after the fall of Rome, rebounded to nearly double, from 5 million to more than 9 million. Farther east, the phenomenon was even more dramatic: the population of that portion of Europe that makes up modern Germany and Poland nearly tripled.

  More food increases fertility. Increased fertility means more mouths; and more mouths demand more food. The longer growing seasons of the Medieval Warm Period improved agricultural productivity, but not so much that it could keep up with the resulting population explosion. Only new land could do that. The Viking response was to colonize previously undesirable—and unpopulated—places like Greenland, but this was no sort of option for either continental Europe or islands like Britain or Ireland. There, unfortunately, the land that wasn’t under cultivation usually wasn’t for a very good reason: it was covered with trees.

  Western and central Europe, at the time the Roman Empire began its retreat, around the end of the fifth century, was 80 percent forest; by 1300, it was less than 30 percent, which means that, over seven centuri
es, at least 100 million acres were deforested. France’s forests alone were reduced from 74 million acres to 32 million. Not all of them were turned into farmland, or even pasture; Europe’s trees were valuable on their own, for building, heat, and—as armor manufacture became more and more established—fuel for smelting iron.*

  Agriculture and armor manufacture weren’t the only reasons for forest-clearing material improvement. Peasants climbed a hundred feet in the air to carve the branches from oaks and aspens, were crippled by deadfalls that refused to topple as predicted, and died in the fires they set to destroy the remaining stumps, as much to reclaim the tree-rich sanctuaries of pagan worship for a more Christian world as to claim them for the plow. Wild landscapes were in a state of sin; cultivated land was literally saved by the “prayer book and the ax.”

  The process was well along by the seventh century, as abandoned-and-reclaimed properties were established in the zones between existing settlements just in time for the great population explosion that began around 800. The resulting change in the topography of Europe was huge: The northern European plain, and England, from the Midlands to the Channel, were transformed into open stretches of field broken up by the occasional village. All that was left of the great forests was an occasional stand of trees, most notably in the west and southeast of England, Brittany, and Normandy. The eastern Franks called the new areas brabants, from which the formerly heavily forested portion of what is today Belgium got its name. (The forest itself was known as the Silva Carbonnaria, or charcoal-burners forest.)

  Converting millions of acres of forest into farmland, especially the sort used to grow cereals like wheat, rye, barley, and oats, actually produced a temporary increase in fertility: when you burn trees (the term of art is assarting: a collective endeavor in which trees are cut by a group, who then divide the “new” land like-as-like) in order to plant wheat, the ash left behind actually increases the productivity of the soil. Throughout Europe, sermonizers could, and did, cite Psalm 65: “The grasslands of the wilderness overflow; the hills are clothed with gladness.” Over the long term, however, it meant impoverishment, as more and more marginal land was producing a larger and larger percentage of the continent’s food.