The Great Warming

by Brian Fagan

  Computer Modeling

  Sophisticated computer models simulate the behavior of the world’s climatic system, using increasingly large quantities of raw data derived from buoys, instrument records, proxies, and satellites. Computer models are used both to understand the natural variability of global climate and to measure the effects of different forcings. They provide the basis for assessing the effects of anthropogenic global warming and both short- and long-term weather forecasts.

  We’ve learned, too, that the gyrations of the climatic dance have a startlingly direct effect on human societies, like the exceptionally heavy rains caused by a massive El Niño that destroyed generations’ worth of irrigation canals in riverbeds along Peru’s north coast in the sixth century A.D., or the major drought cycles in the American Southwest that triggered population movements by Ancestral Pueblo families over a wide area a thousand years ago. At the same time as the Pueblo of the Southwest were moving away from their homes in the face of drought, Europe’s medieval farmers were basking in more predictable weather conditions and ample, but usually not excessive, rainfall. The effects of the slightly warmer and drier conditions appeared in all kinds of subtle ways—in better harvests, in population growth and accelerating deforestation, in an explosion of trade and deep-sea fishing, and in a veritable orgy of cathedral building. This is not to say, of course, that greater warmth caused all these changes; far from it. What’s exciting is that we can now begin to link seemingly minor climatic shifts to all kinds of historical events in ways that were unimaginable even a generation ago. With a few notable exceptions, like the Swiss historian Karl Pfister, who has spent years studying the dates of wine harvests, most historians have tended to ignore climatic shifts, largely because as nonscientists they were unversed in the new climatological data. Today, we can see that climate change was one of many significant factors involved in shaping medieval history, especially the lives of ordinary people living in small villages, growing crops, or fishing in the North Sea.

  IN ABOUT 1120, the monk and historian William of Malmesbury traveled through the Vale of Gloucester in England’s West Country and admired the fertile summer landscape. “Here you may behold highways and publick roads full of Fruit-trees, not planted, but growing naturally,” he wrote. “No county in England has so many or so good vineyards as this, either for fertility or for sweetness of grape. The wine has no unpleasant tartness or eagerness; and is little inferior to the French in sweetness.”5 William observed that the grapes were planted in the open, trained up on poles and not protected against cold winds by strategically placed walls. At the time, climatic conditions were ideal. Vines need freedom from spring frosts, especially at, or after, flowering, as well as sufficient sunshine and warmth in summer, not too much rain, and enough autumn sunshine and warmth to raise the grapes’ sugar content. Numerous vineyards flourished in England at the time, considerably farther north than the northernmost vineyards in France and Germany during the 1960s. During the twelfth and thirteenth centuries, England’s climate was so temperate that her merchants exported large quantities of wine to France much to the consternation of French growers, who complained loudly. Not that England was alone in its winemaking. Between 1128 and 1437, wine was produced in eastern Prussia at 55 degrees north, also in southern Norway. The Black Forest had vineyards up to 2,560 feet (780 meters) above sea level. Today, the highest vineyards in Germany are at 1,800 feet (560 meters). At the time, summer temperatures were 1.8 to 2.6 degrees F (1.0 to 1.4 degrees C) above those of half a century ago in central Europe, fractionally lower in England.6

  We first learned about these warm centuries from the work of the British meteorologist and climatic historian Hubert Lamb, one of the little-known heroes of climatology. He studied the minutiae of climate change over the past two thousand years during the 1950s and 1960s, at a time when most historians denied that temperature and rainfall played any part in shaping historical events. Lamb was a brilliant weather detective, who had few modern-day proxy records like tree rings or ice cores to work with. Instead, he relied on scattered geological clues and on a broad array of historical records, which he fitted into a complex jigsaw puzzle, while working back in time from two hundred years or more of instrument observations throughout Europe. Among his astonishing achievements were detailed accounts of major storms in the English Channel and North Sea. For instance, Lamb reconstructed four fierce storm surges in about 1200, 1200–19, 1287, and 1382 that killed at least one hundred thousand people along the Dutch and German coasts. His recounting of the massive Atlantic depressions that overwhelmed the Spanish Armada in 1588 is a masterpiece of climatological detective work.7 Climatologists still cite Lamb’s work with respect, as they do that of the French historian Emmanuel Le Roy Ladurie, who wrote one of the first climatically driven accounts of European history, based in large part on the dates of wine harvests over several centuries— early in warm years, much later in cool, wet ones.8

  Locations mentioned in chapters 1 and 2. Some minor places are omitted for clarity.

  Much of Lamb’s early work was well-reasoned extrapolation. For instance, he used fifty-year averages of summer wetness and winter mildness indices, derived from records dating back to as early as 1432, to reconstruct the climate in medieval times and even earlier. He identified four centuries of significantly warmer climate after 800 that he named the Medieval Warm Period (sometimes called the Medieval Climatic Anomaly). He never thought of this as a block of time when Europe basked in warm sunlight. Rather, it was a period of cyclical fluctuations with occasional very cold winters like that of 1010–11, which gripped even the eastern Mediterranean in intense cold.

  Few winters were so frigid over the next three centuries. However, the persistent warmer conditions melted ice caps, raised mountain tree lines, and caused significant sea level rises of 24 to 31 inches (60 to 80 centimeters) in the North Sea, sufficient to cause catastrophic flooding when high tides coincided with storm surges.9

  Even without storms, the sea level rise altered the configuration of low-lying coasts. For instance, the Fenland of eastern England is a once glacial landscape of marshes, swamps, and turgid streams. This was remote, inaccessible country. As recently as the early twentieth century, the Fens supported eel fishers and marsh dwellers who lived in a world apart from the farmers that surrounded them. The Fenland was both a rich food source and a strategic advantage for those who knew how to use it. The Saxon chieftain Hereward the Wake held out at the abbey of Ely, in the heart of the Fens, against William the Conqueror for five years after the Norman Conquest of 1066. He and his men hid among a maze of obscure willow-girt islands. When William captured Ely in 1071, Hereward simply melted away into hiding, to vanish from history.10

  The North Sea continued to rise after 1000. In Great Britain, a tidal inlet extended as far inland as Norwich. In William the Conqueror’s time, the town of Beccles, now far from the North Sea, was a thriving herring port. Before the Conquest, local fishers had supplied thirty thousand herring annually to the nearby abbey of St. Edmund. William doubled the assessment. Great storms in 1251 and 1287 inundated huge tracts of the Netherlands to form a huge inland water, the Zuider Zee, as thousands of acres of coastal Denmark and Germany also vanished under the ocean.11

  Hubert Lamb dated the culmination of warmth over wide areas to different times. There was significant warming in Greenland from about 900 to 1200. Europe experienced its warmest temperatures between 1100 and 1300, when dry summers and mild winters were the norm.

  AS HAPPENS WITH ideas in academic circles, the Medieval Warm Period became a fixture in the scholarly literature: five centuries when Europe basked in idyllic summers. Medieval warming became a dim background noise to the larger events of history, but few historians investigated the phenomenon more closely while climatic proxies were still in their infancy. Hubert Lamb himself never thought of the Medieval Warm Period as a finite segment of time, for he was well aware of the realities of European climate. A half century of climatic
detective work since Lamb’s research has shown that he was right. There was warming, especially between 1100 and 1200, but the climate was, as ever, infinitely variable. The Medieval Warm Period was not a discrete episode when climate was distinctly different from what came before; nor, indeed, was the Little Ice Age that followed it from about 1300 (the beginning date is uncertain) to 1860. Nonetheless, as William of Malmesbury’s praise of English wine attests, an average rise of even a degree or two can change the face of a landscape or dash a civilization.

  Reconstructing the climate of the Medieval Warm Period has assumed pressing importance in many scholars’ minds as part of the politically charged and once controversial debate over the reality of humanly caused global warming. Those who oppose the concept of anthropogenic global warming compare the temperature curves for the warm centuries to the persistent, almost straight-line warming since the height of the Industrial Revolution in 1860.

  The controversy erupted when three climatologists, Michael Mann, Raymond Bradley, and Malcolm Hughes, published a reconstruction of northern hemisphere temperatures for the past six hundred years, then for the past thousand years, using proxies such as tree rings, ice cores, and coral, as well as instrument records from the past 150 years.12 Their seesawlike graph with its stark record of rising temperatures since 1860 received widespread attention when it was published in the report of the Intergovernmental Panel on Climate Change in 2001.13 The Mann curve is popularly known as the Hockey Stick, on account of the long, almost straight-line warming it shows over the past 150 years. Compared with present-day warming, the temperatures of earlier centuries are almost flat, which is what infuriated those who deny humanly caused global warming. They would have one believe that the Medieval Warm Period was warmer than the present-day climate.

  How warm were these five centuries in the northern hemisphere and were they, in fact, hotter than today? We know from instrument data since 1861 that winter temperatures have warmed by about 1.4 degrees F (0.8 degrees C) and summer ones by about 0.8 degrees F (0.4 degrees C). For earlier centuries, we have to rely on proxies and the occasional historical record, like those used by Hubert Lamb. Numerous proxies take us back to 1600 and show that the seventeenth century was cool, with summer temperatures about 0.9 degrees F (0.5 degrees C) cooler than between 1961 and 1990. Earlier records are far less complete, but they document gradually declining temperatures back to 1000, with 1000 to 1100 being about 0.2 degrees F (0.1 degree C) above the millennial mean. Earlier than 1000, the record sputters, because we lack good proxy sequences.14 It appears that Hubert Lamb was correct, at least as far as Europe was concerned. The eleventh and twelfth centuries, and perhaps the preceding two centuries, were relatively warm and settled, but with temperatures slightly cooler than those of today. Few scientists doubt that today’s persistent warming is humanly caused and unique.

  Reconstructions of northern hemisphere temperatures, compiled from the work of six different research teams. These are combined with the global mean surface temperature record obtained from instruments (shown in dark). Each curve is somewhat different; each is subject to different uncertainties and limitations that increase as one goes back in time. But the reconstructions are generally consistent over the past 1,100 years, and especially for the past four centuries and the past 150 years with their warming. The temperature fluctuations of the Medieval Warm Period reflect constantly changing climate conditions. (From the National Research Council’s Surface Temperature Reconstructions of the Past 2,000 Years [Washington, D.C.: National Academies Press, 2006], fig. S-1. Details of the different temperature curves, which are irrelevant here, can be found in that publication.)

  But was the Medieval Warm Period a global phenomenon, marked by universal warming and generally benign climatic conditions? Unlike the Little Ice Age, which left a significant climatic imprint in lands as far apart as New Zealand, the Andes, and Greenland, the warmer medieval centuries were more elusive in their impact. Then, as now, all climate was local, even if it originated in much larger-scale interactions between atmosphere and ocean. Lengthy cycles of warmer conditions in Europe brought a measure of stability to food supplies and produced favorable conditions that fostered the development of larger, more powerful kingdoms. In contrast, the same centuries gave birth to episodes of sometimes catastrophic rainfall and intense droughts to people living in arid and semiarid lands: in western North America, in India, on the margins of deserts—for example, the Sahara—and on the Eurasian steppes, where water supplies varied widely. The eastern Pacific was cool and dry; the Arctic saw much less summer ice. The term “Medieval Warm Period” is something of a misnomer, but most people continue to use it on the grounds that everyone knows what centuries are involved and because, as we shall see, there is at least sketchy evidence for warmer temperatures from Tibet to the Andes, western Europe, and North America to tropical Africa. The Medieval Warm Period is some form of global phenomenon, though not quite the one Hubert Lamb originally envisaged a half century ago. But, beyond doubt, these warmer centuries brought enormous benefits to a Europe basking in summer warmth and good harvests, especially between 1100 and 1300, during the High Middle Ages. That warmer, more stable climate lasted but two hundred to three hundred years, yet this was long enough to transform history.

  A CACOPHONY ASSAULTS the ear on every side. People jostle around the stands, bargaining, gossiping, and handling produce. Brightly colored lettuces and carrots lie on piled market stands. Women sniff suspiciously at ripe apples. Farmers in tunics and stockings quaff wooden tankards of ale in the shade. Suddenly, silence falls. The crowds part as a procession of men-at-arms in bright regalia escort the lord of the nearby castle through the market. He rides a fine white horse, ornately caparisoned; wearing light armor and steel helmet, he looks neither left nor right. The silent townspeople touch their forelocks or go down on one knee. His lordship nods solemnly and passes on, his squires and attendants riding with him in close order. As the procession departs, the bustle resumes.

  Climatic trends across the world during the warm centuries. This is a very generalized table for guidance only.

  The lord might appear in all his splendor, escorted by uniformed retainers and soldiers; endemic warfare might consume the energies of kings and barons. But behind the façade of princely processions and splendid display lay a continent that lived with the constant threat of hunger. The margin between plenty and starvation was narrow indeed, defined by unexpected spring frosts, long weeks of heavy rain, or months of seemingly unending drought. Everyone living in the countryside suffered through periods of malnutrition. We know this from the telltale stress lines found on their bones, marks of suffering they took to their graves. Even in good years, many rural communities survived at the subsistence level, or close to it. All it took was a period of heavy rain, some floods, or an epidemic of cattle disease to bring hunger to the threshold. Even in the best of times, farming was unrelentingly hard labor. The life expectancy of a Winchester farmworker in 1245 was about twenty-four years—if he survived childhood diseases. (If one factors in the high infant mortality rate, life expectancy was even shorter.) Occupational conditions such as spinal deformations from lifting heavy bags or scything hay are common among the dead found in medieval cemeteries. Fisherfolk suffered from os-teoarthritis of the spine from moving boats and hauling laden herring nets. The human cost in constant, backbreaking toil and inadequate diet was enormous, even in good years.

  The warm centuries brought significant relief to Europe’s subsistence farmers. The growing season for cereals was as much as three weeks longer. Summer after summer, warm, settled weather would begin in June and extend through July and August into the hectic days of harvest. Even more important, the May frosts that had plagued growing crops for centuries were virtually unknown between 1100 and 1300. Warm summers and mild winters allowed people to take risks with planting marginal lands and at higher altitudes where, hitherto, colder temperatures would have precluded any form of cultivation. A gr
owing farming population moved northward and uphill.

  The figures speak for themselves. Small communities of farmers flourished at 1,049 feet (320 meters) above sea level on Dartmoor in southwestern England during the twelfth century. None farmed there in the twentieth century. Today, the Pennine Moors in northern England support no crops; but in 1300, the local shepherds complained about encroaching farmlands. Kelso Abbey, in southern Scotland, had well over 250 acres (about 100 hectares) under cultivation at an altitude of over 980 feet (300 meters) above sea level, well above today’s limits. Fourteen hundred sheep and sixteen shepherds’ households thrived on the abbey’s land. Farmers grew wheat as far north as Trondheim in Norway.15 Far to the south in the Swiss Alps, smallholders planted crops deep in mountain river valleys that had been covered with glaciers two centuries earlier. At lower altitudes, longer growing seasons reduced the risk of crop failure significantly, while the warm weeks of the summer growing period increased yields and allowed the accumulation of at least some food surpluses, which fed growing towns and cities. Herds grew; rural and urban populations increased. The demand for arable land skyrocketed as the calls of the church and the nobility on commoners for labor, taxes, and tithes increased steadily. Europe rang with the sound of iron axes striking down primordial oak forests and clearing new land.