by Brian Fagan
Author’s Note
Place names are spelled according to the most common usage.
The names of archaeological sites and historic places are spelled as they appear most commonly in the sources I used to write this book. Some obscure locations are omitted from the maps for clarity; interested readers should consult the specialist literature.
The notes tend to emphasize sources with extensive bibliographies, to allow the reader to enter the more specialized literature if desired. This being a historical narrative, sidebars at intervals in the text provide further information on such phenomena as the Intertropical Convergence Zone and major climatological methods.
All radiocarbon dates have been calibrated and the A.D./B.C. convention is used.
Temperature curves have been smoothed statistically for clarity.
CHAPTER 1
A Time of Warming
The occurrence in medieval York of the bug Heterogaster urticae, whose typical habitat today is stinging nettles in sunny locations in the south of England, discovered by . . . archaeological investigation to have been present there in the Middle Ages . . . presumably indicates higher temperatures than today’s.
—Hubert Lamb, Climate History and the Modern
World (1982)1
SOUTHERN ENGLAND, FALL, A.D. 1200. The chilly mist hangs low over the treetops. A pervasive drizzle drifts across the plowed strips, misting the weathered faces of the two men sowing wheat from canvas seed bags slung around their necks. Snub-nosed and tousle-haired, they are barefoot, clothed in dirty, belted tunics and straw hats, swinging effortlessly to and fro, casting seed across the shallow furrows. Behind them, an ox-drawn harrow, a square wooden frame with wooden spikes pointing into the earth, covers the newly planted seed. As one strip is sown, the men move on to the next, for time is short. They must plant before heavy autumn rains can wash the seed from the earth.
The routine of planting, learned in childhood, is as unchanging as the passage of the seasons. Older men remember cold, dreary days when even a sheepskin cloak could not keep out the pervasive chill. They also recall years when the sun blazed down from a cloudless sky, the heat shimmering above the fields. These were times when the village gambled that it would rain and planted anyhow. Sometimes, the bet paid off. All too often, it did not. When it didn’t, there would be hunger the following year.
English medieval farmers sow grain, then harrow it into the ground (top). Women reap and bundle the grain at harvest (bottom). (Reconstructions based on excavations at Wharram Percy, northeastern England.
Their seed bags empty, the two men stretch and sling new ones from their shoulders. They’re tired after days of backbreaking labor, harvesting summer crops, then plowing and planting for winter wheat. The work never ceases in a farming world where everyone lives on the edge, where the unspoken threat of hunger is ever present.
The village is coming off a good summer harvest after weeks of fine weather, so there is plenty to eat. Good fortune continues. The winter is mild and not too wet. January and February bring frost, even a little snow; but there are no late cold snaps, and spring is early, with warm temperatures and just enough gentle rain. As the days lengthen, the villagers weed the growing crop. By late July, the grain is ripe and the harvest begins. The fields bake in the hot sun, a deep blue sky with fluffy clouds overhead. The men bend to the harvest. They reap the ripe wheat with short iron sickles, gathering bundles of stalks in their hands and cutting them, only pausing to sharpen their blades. Behind them, the women have tucked their skirts into their belts to free their legs. Brightly colored cloths cover their hair. They bind and stack the sheaves in the field, but soon the grain will be carried indoors, stored on the stalk for threshing and winnowing under cover when the weather turns bad. Children play among the sheaves and gather grain among the stubble. The workers pause at midday to stretch their stiff backs, to drink some ale, as birds jostle and swoop overhead. Soon the harvest will resume, continuing until dark as the village races against time to gather in the crop while it is dry.
To judge by today’s subsistence farmers, those of A.D. 1200 would let nothing go to waste, even in a good harvest year like this one. You have only to glance at the deep wrinkles etched into adult faces to get the point. Even men and women in their twenties look old, their countenances withered by brutal hard work and occasional hunger or malnutrition. Yet these people lived in a world that was warmer than it had been for some centuries, in what climatologists call the Medieval Warm Period.
A THOUSAND YEARS ago, everything in Europe depended on agriculture. From Britain and Ireland to central Europe, 80 to 90 percent of the population struggled to coax a living—and, if they were lucky, a food surplus—from the soil. Europe was a continent of subsistence farmers, who lived from harvest to harvest, whose fate depended on the vagaries of temperature and rainfall.
There were many fewer people then. The population of London exceeded 30,000 for the first time in 1170, a vast metropolis by the standards of the day. Other English population centers were much smaller. Norwich in East Anglia, for example, had but 7,000 to 10,000 inhabitants. The combined population of France, Germany, Switzerland, Austria, and the Low Countries was about 36 million in 1200, compared with over 250 million today. Nearly all these people lived in hamlets and villages, or perhaps small towns, for cities were only just becoming a significant element in European life. And everyone, even the greatest lord, depended on a countryside farmed without machines, hybrid seed, or fertilizers. Horses and oxen, even wives, hauled the plow and the harrow. The harvest was gathered by hand, carried on people’s backs, perhaps transported by oxcart or river barge to market.
The rural landscape was a mosaic of forest and woodland, river valley and wetland, modified constantly by human activity. Many people lived in small, dispersed settlements, surrounded by haphazard fields. But increasingly, they dwelled in larger, more centralized villages, where the nearby arable land was divided into large, open fields in turn subdivided into small strips of about a half acre (0.2 hectare) each. Each tenant had possession of several groups of strips, often called furlongs, but not all of that land was under crops at once. Every farmer knew that arable land had to be grazed and manured by animals, then rested to regain its fertility and minimize plant diseases. The best-drained, lightest soils supported cereal crops. Animals grazed not only on stubble, but in the woods and in open pasture on heavier, more clayey soils. Like modern-day subsistence farmers in Africa, medieval European peasants knew the properties of different grazing grasses, the subtle indicators of renewed soil fertility, the seasons of wild plant foods. Their only protection against sudden frosts, storms, or drought was a diverse food supply based on far more than cereal crops.
Coaxing a living from Europe’s medieval soils was never easy, but it was done, and sometimes with considerable success, especially during runs of warm, drier summers. Farmers in England and France grew mainly wheat, barley, and oats. As a gross generalization, about a third of the land was planted in wheat, half in barley, the rest in other crops, including peas. Even in good years, the yields were small by today’s standards. A good wheat harvest would yield between 8 and 12.5 bushels (2.8 to 4.0 hectoliters) per acre (0.4 hectare). Compare this with today’s figure of over 47 bushels (16.5 hectoliters) an acre. When you realize that 2.3 bushels (0.8 hectoliters) of that yield went back into the soil as seed for the next crop, the yield was small indeed, leaving very little chance of a food surplus in any but the best years. The figures for barley, used for beer, were somewhat higher (23.5 bushels/8.3 hectoliters), but the amount of seed planted was greater. In good years, grain yields of slightly under four times seed grain sown were the norm. One survived by diversification.2
Everyone grew vegetables. Protein-rich peas and beans were planted as field crops in early spring and harvested in fall; the legumes were allowed to dry on the plant, the stalks plowed back into the soil as fertilizer. Vegetables and herbs of all kinds supplemented what was basically a meatl
ess diet based on bread and gruel.
Most farmers had a few head of livestock—a milk cow or two, some pigs, sheep, goats, and chickens, and, if they were lucky, a horse or some oxen, or at least access to them for plowing. Animals provided meat and milk, also hides and wool. Sheep shearing was an important occasion in spring, undertaken on a carefully chosen fine day when a warm wind from the west brought promises of summer. The breeze sends wood smoke cascading out of windows and doors, opened by the women to let in fresh air. Outside, the village flock crowds in a large wicker pen, sheep jostling against one another. A smell of wool fills the air. The men, dressed in leather jerkins, grab the sheep one by one and shear them with simple iron clippers, deftly flipping the docile beasts on their backs to complete the task. The shorn, bewildered beasts shake themselves as young boys drive them to a nearby corral. Hovering children gather up the wool and lay it on wooden racks to dry in the bright sunlight.
Most of the year, animals grazed and foraged on their own—this was especially true of pigs, which feasted off acorns and beechnuts in the fall. But winter feeding was another matter, the challenge being to keep breeding stock alive. Surplus males and dried-up cows no longer yielding milk were either sold or slaughtered in the autumn to free up hay for the most valuable animals. The hay harvest was all-important. Mowing began in June and continued into July, depending on the weather, for the hay had to be absolutely dry, lest it rot after harvest and become so hot that it would catch fire. On fine days, men with long-bladed iron scythes worked their way across a meadow, leaving the crop to dry in rows across the field. They would return and turn the crop over a couple of times to dry better before stacking it in such a way that the outer layer formed a thatch to keep off the rain. The hay harvest was a major event in the year but so dependent on dry conditions that a wet year could lead to stock losses the next winter, perhaps the loss of every beast. Once again, everything depended on the climate.
Even in a bad year, the farmer still had to pay taxes and church tithes, which ate into food supplies. A man with a wife and two children could survive at a basic level with 5 acres (2 hectares). But everyone, even young children, had to grow vegetables and forage for wild foods such as mushrooms, nuts, and berries. Five acres left precious little margin for poor harvests caused by frost or storms. A succession of years meant famine, famine-related diseases, at best malnutrition, and certainly some dying, especially in the cold and miserable months of late winter, when food supplies were always low and Lent with its fasting was just over the horizon.
Each year, as summer ripened into fall, every community reaped its harvest and gave thanks to God for his bounty, for life was never easy. The endless cycle of the seasons defined human existence. So did the routines of planting, growth, and harvest; the verities of birth, life, and death; and what everyone believed were the arbitrary whims of the Lord.
In an era long before long-range weather forecasting, everyone, whether king or noble, warlord, merchant, or farmer, was at the mercy of cycles of heavier rainfall and drought, savage gales and perfect summer days. They were unwitting partners in an intricate climatic gavotte between the atmosphere and the oceans. But, especially between A.D. 800 and 1300, the dance slowed slightly into a measured waltz, where summer warmth and more settled conditions tended—and one stresses “tended”—to be the norm. The gyrations of climate change slowed momentarily. Europe changed profoundly during these five centuries from 800 to 1300, the Medieval Warm Period.
IN THE LARGER scheme of things, the twenty generations or so of medieval warming are but the blink of an eye. The relatively minor temperature changes of these centuries pale alongside those that occurred at the end of the most recent ice age. About twelve thousand years ago, the world entered a period of sustained global warmth, known to geologists as the Holocene (after the Greek words holos, “whole,” and kainos, “new,” the word thus meaning “entirely recent”), which continues to this day. Generations of scientists, working with inadequate data, conjured up images of over ten millennia of a basically modern climate that had changed relatively little since the warm-up after the Ice Age. But a revolution in paleoclimatology, the study of ancient climate, has transformed our knowledge of the Holocene in recent years.
Today’s climatologists drill into sea and lake beds, take deep cores from Greenland and Antarctic ice sheets, and pore over tree-ring sequences taken from the trunks of ancient trees. Their research has revealed a Holocene climate constantly on the move. We can now discern not only millennium-long cold and warm oscillations, but also much shorter cycles, especially over the past two thousand years. The shifts from slightly wetter to slightly drier, from warmer to cooler and back again, never cease. Some endure a century or a decade; others, like El Niño events, last no more than a year or so. Few major climatic events lay within the span of generational memory, and were thus quickly forgotten in times when life expectancy everywhere was little more than thirty years. The new climatology has shown us that the climatic timepiece may accelerate and slow down, falter and change direction suddenly, even remain steady for long periods of time, but it never stops.
No one knows exactly what drives the climatic pendulum. Most likely, small changes in the earth’s obliquity trigger climate changes. So do cycles of sunspot activity. For instance, a dearth of sunspots during the seventeenth century marked a period of markedly cooler climate during the height of the so-called Little Ice Age. There are other climatic forcing agents, too, among them volcanic activity in Iceland, Southeast Asia, and elsewhere. A massive eruption of Mount Tambora on Java’s Sumbawa Island in 1815 blew off 4,250 feet (1,300 meters) of the summit of the volcano. Huge clouds of volcanic ash rose into the atmosphere, masking the sun and causing Europe’s celebrated “year without a summer” in 1816.3 In recent years, however, most climatologists have come to believe that complex, yet still little understood, interactions between the atmosphere and the ocean play a major role in climatic shifts. The climatologist George Philander calls it a dance between very different partners, one fast-moving, and the other clumsier. He writes: “Whereas the atmosphere is quick and agile and responds nimbly to hints from the ocean, the ocean is ponderous and cumbersome.”4 We dance along with these partners, opportunistically, sometimes decisively, and very often with reluctance.
Studying Ancient Climate Change
Archaeologists, historians, and paleoclimatologists use a wide variety of methods to study ancient climate change. Here are the principal ones:
Direct Methods
INSTRUMENT RECORDS
Instrument records are the most accurate and direct way of studying climate change. Unfortunately, such archives go back only some 150 years in Europe and North America and for much shorter periods elsewhere.
HISTORICAL DOCUMENTS
Archives provide invaluable snapshots of ancient climate, from such documents as diaries, logs, and official reports that mention contemporary events such as floods or droughts. The oldest are the reports of the flowering of cherry trees in Japan and Korea, which date back a thousand years. In Europe and the Mediterranean region, records for many areas go back to about 1500.
Indirect Methods (Proxies)
ICE CORES
Deep cores drilled into ice sheets such as those in Greenland, Antarctica, the Andes, and Tibet provide continuous records of temperature changes derived from measurements of oxygen- and hydrogen-stable isotopic ratios in the water molecules that make up the ice. Such changes in ratios can be connected to temperature shifts. One ice core from Antarctica takes the record back over 420,000 years. High-resolution sequences for the past two thousand years come from Greenland, the Andes, and elsewhere.
DEEP-SEA AND LAKE CORES
Marine sediments recovered from deep-sea cores contain temperature-sensitive foraminifera and marine diatoms and can go back tens of thousands of years. In some locations, like the Cariaco basin off Venezuela, and California’s Santa Barbara Channel, rapid accumulation rates have provided relatively prec
ise records of medieval warming and of later cooling. Lake cores yield seasonal layers that record changes in water balance, and hence information on ancient droughts.
CORAL RECORDS
Corals living near the sea surface produce annual density bands of calcium carbonate. By measuring the changing ratios of O-18 to O-16, researchers can detect temperature changes, the ratio decreasing with increasing warmth. Coral records tend to be incomplete. Few go back more than two or three centuries.
TREE RINGS (DENDROCHRONOLOGY)
Dendrochronology is based on the study of the annual growth rings of trees, the thickness of the rings providing a proxy for rainfall shifts. Originally developed in the American Southwest, tree-ring researches now provide important proxy data in many parts of the world. Records from Europe are notably comprehensive, as are those from parts of North America. In recent years, efforts have been made to collect more samples from Asia and the southern hemisphere, which promise to throw important light both on the Medieval Warm Period and on ancient El Niño events. Tree-ring records go back almost to the Ice Age in Europe, but are generally most common for the past one thousand to two thousand years.
These are the major paleoclimatological proxies. Others include cave deposits such as stalagmites, which record the changing isotopic composition of cave groundwater and temperature through time, and temperature information derived from boreholes.
Climatic Forcings
Forcings are powerful and unusual factors like volcanic eruptions that can cause climate change. In the context of the Medieval Warm Period, these are natural changes such as solar irradiance caused by small tilts in the earth’s orbit and by major volcanic eruptions that affect global energy balance. Large volcanic events add large amounts of ash and sulfur gases to the atmosphere, diminishing the amount of solar radiation reaching the earth and thereby cooling it. The effects are limited to a few years. Since 1860, the major climatic forcing has been humanly caused, in large part by the use of fossil fuels.
