The Great Warming

by Brian Fagan

  The droughts of the warmer centuries lasted for long periods of time in western North America and the Andes, and, if the Guliya and Huguangyan cores are any guide, so did those in East Asia. The droughts were not continuous, but cyclical, which would have had dangerous shock effects in the loess lands where the northern borderlands lay. When a sudden wet year followed a long drought cycle, floods would have inundated the arid fields and disused irrigation works in short order. The centuries of the Medieval Warm Period were climatically extremely volatile in this region of dramatic rainfall shifts, perhaps even more so than almost anywhere else on earth. The vagaries of drought and flood must have rippled through the realms of politics and war, for both farmer and nomad lived at the subsistence level and at the mercy of the climate, whatever the deeds of great lords and warring armies.

  THE EXTREME CYCLES of medieval climate also affected the complex relationships between the nomadic peoples of eastern Eurasia and those who dwelled on the settled lands. The most powerful of these tribal groups were the Khitan, herders and horse people whose origins went back deep into the past.15 Like those of other nomadic peoples, their lives were governed in part by rainfall on the steppe, by the gyrations of the desert pump. In periods of drought, they pressed southward into better-watered and more settled lands. After the 840s, and as the T’ang hold on power weakened during drier cycles, the Khitan defeated their tribal neighbors, then turned their attention to the powerful states to the south. At first the Khitan contented themselves with raids and temporary incursions into more settled lands, after which they withdrew northward again. To what extent these movements were the consequence of drought on the steppe, we do not know, but, judging from centuries of nomad history, many of the most serious incursions certainly occurred during dry years when grazing was in short supply.

  The collapse of T’ang power and increasing rivalry between different warlords operating in the borderlands led the Khitan to unite. With the accession of A-pao-chi as Great Khan in A.D. 906–07, the Khitan embarked on ambitious campaigns of conquest. Within twenty years, they had become masters of the nomadic peoples of Mongolia and Manchuria. Theirs was a well-organized kingdom, with cities for Chinese from the border regions, a diversity of industries and areas of settled farming, and a dual form of organization that accommodated both the Chinese and nomadic ways of life. The pattern of nomadic life was changing, as farmer and herder became increasingly interdependent, a useful form of insurance in climatically volatile times.

  A-pao-chi died in 926, to be followed by the Liao, Hsia, and Chin states during a period of constant warfare and seething rivalries. But behind all these political and military events and a pastiche of rulers lay the harsh economic realities of subsistence agriculture on the settled lands. We know from Chin records that the state produced about 90 million shih (a shih is about 125.5 pints [59.4 liters]) of millet and rice annually. A tenth of that went to the government as land tax. The average grain consumption of an individual was about 6 shih annually, so a year of good rainfall produced just enough grain to feed the population adequately. But an average year left no surplus to build up reserves for distribution during droughts. The food supply was never secure in the north, as it was in the southern Sung kingdom in the rich environment of the Yangtze Valley, which produced as many as two rice harvests a year.16

  Chin rulers were well aware of the precarious food situation and attempted to increase the acreage of land under cultivation by fostering irrigation works. They also attempted to increase crop yields by terracing hillsides. But both these measures had unforeseen consequences, especially terracing, which led to inexorable deforestation and rapid soil erosion. The latter had a particularly serious effect in the Huang He basin.

  Agricultural production was precarious, even during good years, so the drought cycles of the warm centuries must have had a serious impact on political events in the north. Contemporary records do not dwell on droughts and other natural disasters—hardly surprising, for the peasantry were illiterate, anonymous, and almost a “background noise” to the goals of warlords, emperors, and ambitious officials. But the vagaries of the summer monsoon made it imperative that northern states import rice from their southern neighbors, the Sung. The need had been there for centuries. Overland transport was slow and unreliable, so the logical way to transport grain was by water. Coastal routes were unreliable and dangerous because of pirates and storms; inland waterways linking the Yangtze and Huang He basin were the best solution, although punishingly expensive.

  Efforts to construct a waterway had begun as early as 486 B.C. The Sui rulers of the late sixth and early seventh centuries A.D. linked earlier sections, joining the rich agricultural regions of the lower Yangtze with their western capital at Luayang. A patchwork of lakes and canals became the Grand Canal, Da Yun He, the longest artificial waterway in the world, far longer than Suez or Panama. By the tenth century, the system boasted locks, feeder lakes, and lateral canals. At its peak during the fifteenth and sixteenth centuries, the canal system extended over 1,553 miles (2,500 kilometers), ran through 24 locks and under about 60 bridges, and carried about 441,000 tons (400,000 tonnes) of grain annually.

  For centuries, northern China and its loess lands were dependent on critical food supplies from the south. Not even the most organized preindustrial state and effective administration could overcome the prolonged droughts and sudden floods that regularly devastated food supplies and those who produced them. The north was profoundly vulnerable a thousand years ago. The Huang He basin is even more vulnerable to catastrophe today.

  CHAPTER 13

  The Silent Elephant

  I have seen a herd of elephants traveling through dense native forest . . . pacing along as if they had an appointment at the end of the world.

  —Isak Dinesen, Out of Africa1

  AN ARID LANDSCAPE IMPRINTS ITSELF on your mind. I remember as if they were yesterday November days of over forty years ago in central Africa. Every morning, after weeks of intense heat, a brazen sun rose from a dusty horizon, not a cloud in the heavens. The temperature climbed as the shadows shortened. A dusty blue parabola of cloudless sky reflected the heat radiating from the parched earth. Occasional gusts of wind propelled williwaws across the grassland. Humans and beasts alike sought the mocking illusion of cool shade under trees or roof eaves. Cattle stood motionless, heads drooping, waiting patiently for the cool of evening. I watched farmers gaze stoically at the maize withering in the cracked fields, planted a month before when a heavy shower brought a promise of more rain. As the sun descended, you deluded yourself that the temperature was falling. But it was still 87 degrees F (30.5 degrees C) at midnight. Hunger was not far over the horizon.

  By all accounts, the droughts of the next century will be infinitely worse than this one.

  When I started researching this book, I was expecting to find widespread evidence of increasing temperatures a thousand years ago, of startling changes in agricultural practices, and of ocean voyaging and prosperity in environments basking in unaccustomed warmth. The earlier chapters indeed found me exploring a bustling Europe with bountiful harvests. I followed Norse voyagers across the North Atlantic as they developed fleeting contacts with the Inuit of the far north. So far so good—but when I traveled to the Eurasian steppes, to the West African Sahel, and to the Americas, I encountered major and prolonged droughts that changed history.

  I stress the word “prolonged.” The dry spells of a thousand years ago spanned not years, but generations. The medieval droughts in California’s Sierra Nevada lasted for decades, far longer than those of modern times. A long drought cycle lasting half a century triggered major adjustments in Ancestral Pueblo life in the Southwest, as we have seen. Drought settled over Nebraska and the Plains.

  The Southwest has always been arid, but is not the only part of North America to suffer drought. Pollen cores from the Piedmont Marsh of the lower Hudson River Valley on the East Coast chronicle drought conditions between A.D. 800 and 1300, time
s when the estuary became saltier. If similar drought conditions were to prevail in the same area today, the water supplies of millions of people would be endangered, among them the citizens of nearby Poughkeepsie, New York, which draws its water supplies from the Hudson, as do other suburban towns in the area.2

  Far to the south, in Central America, great Maya cities tottered under medieval drought while Andean civilizations wilted in the face of an evaporating Lake Titicaca and faltering runoff in coastal river valleys. Looking at the global picture, it is tempting to rename the Medieval Warm Period the Medieval Drought Period.

  ______________

  THE REVOLUTION IN climatology began in earnest about thirty years ago, when techniques for deducing the climatic record from proxies, such as deep-sea cores, ice borings, coral, and tree rings, entered the scientific mainstream. Satellite observations and computer modeling joined the meteorological armory during an explosion of research into El Niños and the Southern Oscillation. Since the 1980s, humanly caused global warming has engaged the attention of climatologists in the face of well-documented, virtually continuous warming since 1860. Suddenly, the climate changes of the past two thousand years have assumed great importance in the public arena as anthropogenic warming has become a scientific reality and a major political issue. For this we must thank not only Al Gore and his documentary on global warming, but also a growing public consciousness that rising temperatures, more extreme climatic events, and higher sea levels are facts of life for humankind’s immediate future.3

  Almost immediately, the Medieval Warm Period assumed great importance to the debate over warming in many people’s minds.

  “We’ve been through this before,” both supporters and debunkers of global warming cried. Claims and counterclaims rocketed across auditoria from Tokyo to Scandinavia, as scientists, journalists, and activists argued over whether the Medieval Warm Period was warmer than the rapidly heating world of today. As new tree-ring sequences and other evidence have became available, so the debate about Warm Period temperatures has intensified, with no end in sight.

  The Medieval Warm Period is still a shadowy entity, but we know a great deal more about it than we did in Hubert Lamb’s day. A growing number of sources tell us that there was never long-lasting medieval warmth, but that between 1000 and 1200, temperatures were a few degrees warmer in some parts of the world, notably parts of China, Europe, and western North America.

  Today’s preoccupation with medieval warmth is entirely understandable in a time of uncontrolled anthropogenic warming, with all the harrowing threats of melting Greenland ice sheets, rising sea levels, and increased storminess.

  What would happen if the melting Greenland ice sheet partially shut down the Gulf Stream? Would Europe be plunged into a near–Ice Age, as indeed happened some twelve thousand years ago during the climatic episode known as the Younger Dryas, named after a polar flower?

  What would happen to the Low Countries and to some Pacific atolls if sea levels rose as much as a foot (0.3 meters) or more by century’s end as a result of partially melted ice sheets?

  These are perfectly legitimate concerns, which will require concerted political will to solve in coming generations. But our preoccupation with heat and rising sea levels ignores an even greater threat: drought. Why this surprising neglect? Undoubtedly the devastation of the Southeast Asian tsunami in 2004 and Hurricane Katrina the following year reinforced fears about extreme weather events and flooding in particular. But these two events, coming in two of the warmest years since the Ice Age, seem to have delivered a message that warmer centuries mean more rain, not less. Then there’s another reality: most, though not all, of the people likely to be affected by severe drought in the future live in the developing world, and we in the United States are still much preoccupied with the flooding brought by Katrina.

  I COULD HEAR the Zambezi River riffling in the rocky shallows, in the distant background the unceasing roar of Mosi-oa-Tunya, “The Smoke That Thunders,” Victoria Falls. Dense bush pressed on the clearing, trees arching overhead, dry leaves rustling softly in the afternoon heat. I was completely alone—or so I thought. Then I heard the sound of trampling and crashing branches: I realized with horror that I had walked into the midst of a small herd of elephants. The great beasts were invisible but close by, seemingly unaware of my presence. I tiptoed back the way I had come until I emerged from the trees. As I reached the Zambezi, I looked back. A huge bull elephant flapped his ears at me, feet firmly set in the shallows. He watched closely, unmoving, as I beat a careful retreat.

  Elephants can tread delicately when they wish and can easily become invisible until it is too late to avoid them.

  When the novelist George Orwell, of 1984 fame, was a police officer in Burma in the 1930s, he was confronted with a berserk elephant in a bazaar. At a distance, “peacefully eating, the elephant looked no more dangerous than a cow.” But the beast had killed a man, and “a mad elephant had to be killed like a mad dog.”4 Orwell was struck by the violent contrasts in his by now seemingly placid prey. And so it is with drought. As my research progressed away from Europe, I realized that drought was the hidden villain of the Medieval Warm Period. Prolonged aridity was the silent elephant in the climatic room, and the unpredictable swings of the Southern Oscillation were what brought the beast through the door.

  A surge in ENSO research over the past twenty years has revealed that El Niños, and their sister La Niñas, are not merely local phenomena, but, next to the passage of the seasons, among the most powerful factors in global climate change. Major ENSO events bring heavy rainfall and floods to the Peruvian coast and torrential precipitation to California; they reduce the frequency of tropical storms and hurricanes in the Atlantic. They also bring severe drought to Southeast Asia and Australia, to Central America and northeast Brazil, and to parts of tropical Africa. The less conspicuous, and often longer-lasting, La Niña can be just as destructive, especially in its ability to nurture drought over large tracts of the world—as was the case during the Medieval Warm Period, when the cool, dry sister of El Niño persisted for years on end.

  WHETHER THE MEDIEVAL Warm Period was warmer than today, and why, is still a matter of much debate. Our current warming has not lasted nearly as long as the period studied in this book. It is, however, a steady and well-documented trend, with no downward curve in sight. And unlike the situation a millennium ago, humans are numerous enough, and our outputs profuse enough, to push the trend further and faster. What is not debatable is that if we reenact the climate history of a millennium ago—let alone see the earth get even warmer—we will see how vulnerable humans are to the forces of their environments.

  But if you look at the warm centuries with a global perspective, the wide incidence of drought is truly striking and offers a sobering message about tomorrow’s world. Prolonged aridity was widespread in medieval times and killed enormous numbers of people. Evidence is mounting that drought is the silent and insidious killer associated with global warming. The casualty figures are mind numbing. About 11 million people between Kenya, Somalia, Ethiopia, and Eritrea were in serious danger of starvation as a result of multiyear droughts in 2006. The International Institute of Tropical Agriculture in Nigeria estimates that by 2010 around 300 million people in sub-Saharan Africa, nearly a third of the population, will suffer from malnutrition because of intensifying drought.5 (Relatively few people die of hunger during a drought. They perish from epidemics of dysentery and other diseases spread by poor living conditions. For instance, 1.6 million children a year die today because of a lack of access to good sanitation and clean drinking water.)

  The long-term future is even more alarming. A study by Britain’s authoritative Hadley Centre for Climate Change documents a 25 percent increase in global drought during the 1990s, which produced well-documented population losses.6 The Hadley’s computer models of future aridity resulting from the impacts of greenhouse gas emissions are truly frightening. At present, extreme drought affects 3 percent o
f the earth’s surface. The figure could rise as high as 30 percent if warming continues, with 40 percent suffering from severe droughts, up from the current figure of 8 percent. Fifty percent of the world’s land would experience moderate drought, up from the present 25 percent. Then the center ran the model without factoring in the impact of greenhouse gases, which they assumed were the temperature change villains. The results implied that future changes in drought without anthropogenic warming would be very small indeed.

  In human terms, the United Nations Environment Program reports that 450 million people in twenty-nine countries currently suffer from water shortages.7 By 2025, an estimated 2.8 billion of us will live in areas with increasingly scarce water resources. Twenty percent of the world’s population currently lacks access to safe, clean drinking water. Contaminated water supplies are a worse killer than AIDS in tropical Africa. If the projected drought conditions transpire, future casualties will rise dramatically. The greatest impact of intensifying drought would be on people already living in arid and semiarid lands—about a billion of us in more than 110 countries around the world. And those who would be hit hardest are subsistence farmers, especially in tropical Africa. Seventy percent of all employment in Africa is in small-scale farming, and completely dependent on rainfall.

  The number of food emergencies in Africa each year has already almost tripled since the 1980s, with one in three people across sub-Saharan Africa being malnourished. The Nigerian institute’s projection for 2010 is just the beginning. Future drought-related catastrophes will make these preliminaries seem trivial and could affect more than half of tropical Africa’s population.

  Peru offers another frightening example. The Cordillera Blanca, the largest glacier chain in the tropics, is melting fast because of rising temperatures. The Quelccaya ice cap in southern Peru, a crown jewel of climatic data, is retreating about 197 feet (60 meters) a year, three times faster than in the 1960s. Taken as a whole, the Peruvian Andes have lost at least 22 percent of their glacier area since 1970. Two thirds of Peru’s 27 million people live on the coast, where only 2 percent of the country’s water supply is to be found. A thousand years ago, with many fewer people to feed, the lords of Chimor could adapt their irrigation strategies to prolonged droughts. Their modern successors, living in crowded cities, shantytowns, and an increasingly congested rural landscape hemmed in by desert, cannot do so.