The Great Warming

by Brian Fagan

  Locations mentioned in chapters 6 and 7. Some minor places are omitted for clarity.

  Owens Lake on the eastern flanks of the Sierra Nevada Mountains in eastern California provides telling evidence of the epochal droughts that descended on the West between A.D. 900 and 1250. The lake once covered over 115 square miles (300 square kilometers) at the mouth of the Owens River and held water continuously for at least 800,000 years. (Owens Lake was more than 250 feet [75 meters] deep until the Los Angeles Department of Water and Power diverted the streams that fed it in 1913 and it became a large salt flat.) The mountain runoff that flowed down the river varied dramatically from year to year in centuries-long, decadal, and even annual cycles of unusually wet and dry years. In drier periods, cottonwoods and Jeffrey pines grew in the moist soils of the receding lake bed. When wetter years brought rising water levels, the trees drowned. For years, the dead trunks and branches would stand above the water, but they would eventually disintegrate, leaving only the stumps rooted into the now covered lake bed.

  The geographer Scott Stine has spent much of his career studying these once flooded tree stumps, which were exposed by receding lake waters in drought years. By radiocarbon-dating the outermost tree layers and then counting the tree rings from the stumps, he has reconstructed a precise chronology of droughts and wetter periods during the Medieval Warm Period that are startlingly consistent over a wide area of the American West.2

  Stine’s research began during a major drought during the 1980s, when dry conditions and heavy water demand in Los Angeles caused Mono Lake, the northernmost catchment of the Los Angeles aqueduct, to drop by more than 49 feet (15 meters). He collected samples from numerous stumps, radiocarbon-dated them, and found that there were two generations of trees and shrubs that grew in the lake during the Medieval Warm Period. The first generation perished when the lake rose some 62 feet (19 meters) around A.D. 1100. The rise came during a brief very wet cycle when the rainfall was higher than in any year in modern times, and the fourth highest of the past four thousand years. But plentiful rainfall gave way in about 1250 to an intense drought phase, which lasted for over a century. The lake fell precipitously and a second generation of trees rose in the newly exposed bed.

  The Mono Lake tree stumps chronicled a Medieval Warm Period marked by extreme rainfall swings within a century or less. Intrigued, Stine now turned his attention to Lake Walker to the northeast, a body of water nourished by two Sierra rivers. The western of the two flows through a narrow canyon studded with large submerged pine stumps. Since the canyon is very narrow and lateral movement of the river is restricted, it seems certain that these trees flourished at a time of greatly reduced flow, for pine roots cannot tolerate more than brief periods of inundation. The tree stumps documented a very low level around 1025, when the water stood more than 131 feet (40 meters) below today’s shoreline; this was followed by a brief wet cycle, then another drought, the chronology the same as that from Mono.

  Owens Lake has also provided evidence of severe medieval droughts at the same time. Hunter-gatherers wandering the dry lake bed between A.D. 650 and 1350 left distinctive stone projectile points behind them at a time when the lake was severely desiccated. Radiocarbon dates from a nearby, and contemporary, root of a once flourishing shrub narrowed the occupation down to the time of the first major drought recorded at Lake Walker. Mono, Walker, and Owens lakes all record the same medieval dry cycles. The first began before A.D. 910 and lasted until about 1100. The second commenced prior to 1210 and ended in about 1350. How severe was the drought? Stine used a modern baseline, that of the six-year California drought that began in 1987, when Sierra Nevada runoff was only 65 percent of normal. Despite prolonged dry conditions, the lakes never fell as low then as they had in earlier times. To account for the desiccation experienced by Owens Lake, for example, inflow to the lake must have dropped to between 45 percent and 50 percent of modern amounts.

  Stine’s droughts were so severe that one can trace them over large areas of the West. They turn up in tree rings from the White Mountains of eastern California, where long-lived bristlecone pines are sensitive to temperature or rainfall changes. One record produced evidence that the period 1089 to 1129 was the wettest cycle of the past thousand years. In the southern Sierra, a thousand-year record from foxtail pines and junipers revealed the same two savage droughts, with the four warmest periods of the past millennium occurring between the tenth and fourteenth centuries. The warmest stretch of all was between A.D. 1118 and 1167.3 Evidence of the droughts extends as far north as east-central Oregon,

  Evidence of the droughts extends as far north as east-central Oregon, into the Rockies, and into the adjacent Great Plains. Upright tree trunks stand in 78 to 98 feet (24 to 30 meters) of water in Jenny Lake in Grand Teton National Park. The divers who inspected them even found a raptor nest in the branches of one of the submerged trees. Outer wood from one stump has been radiocarbon-dated to about A.D. 1350, virtually contemporary with the dead stumps in Sierra lakes, as if water levels were also reduced in this vicinity. The size of pocket gophers in northern Yellowstone National Park was the smallest in three thousand years as small rodent populations fell rapidly in the face of aridity.

  These major droughts occurred because the winter jet stream over the northeastern Pacific, with its associated storm tracks, stayed well north of California and the Great Basin. A classic instance of the same phenomenon came during the 1976–77 winter rainy season, which was the driest over much of California since records began. Alaska, on the other hand, enjoyed the wettest ever known. The same jet stream pattern persisted over the American West during much of medieval times, while Alaska was exceptionally wet. We know this because geologists have recovered newly exposed tree stumps under retreating glaciers in Prince William Sound and dated them to between A.D. 900 and 1300— the Medieval Warm Period. The ice sheets had advanced during the cycle of wet years, as they do today during wet winters.

  Stine’s observations, and those of others, have received strong validation from a grid of no less than 602 tree-ring sequences dating back as far as two thousand years.4 For the first time, climatologists have been able to compile a grid of drought data from the entire region, which uses two indexes, the Palmer Drought Severity Index, a well-established way of measuring fluctuations in wetness and aridity, and a Drought Area Index, which counts the number of locations on a grid that exceed an arbitrarily established threshold of the Drought Severity Index. These calculations enabled the researchers to put current twenty-first-century droughts in the West into longer-term perspective. The four driest periods centered on A.D. 935, 1034, 1150, and 1253, all of them within a four-hundred-year interval of overall aridity, which coincides with the Medieval Warm Period. After 1300, there was an abrupt change to persistently wetter conditions, which lasted for six hundred years, then gave way to today’s drought conditions in the West. None of today’s droughts, which last as long as four years, approach the intensity and duration of the Medieval Warm Period droughts. The latter were so severe that there is talk of a “megadrought epoch” a thousand years ago.

  It’s the prolonged nature of these dry periods that distinguishes the aridity of the Medieval Warm Period from today. How did these droughts occur, and, above all, how did they persist for so long? The researchers believe that unusually warm conditions contributed to the occurrence of more frequent, persistent droughts, which were caused in part by great evaporation and reduced moisture levels in the soil. Climatic fluctuations in the Pacific have a considerable effect on rainfall in the West, especially the effects of El Niños and their opposites, cool and dry La Niñas (the El Niño–Southern Oscillation [ENSO]), and a phenomenon known as the Pacific Decadal Oscillation (see sidebar), which can cause drought in the West.5 Furthermore, increased northern hemisphere temperatures during the twentieth century and unusual warming of the western Pacific and Indian oceans have contributed to drought formation over middle latitudes in the northern hemisphere. The same effects pres
umably occurred during the Medieval Warm Period. In other words, large-scale warming at a more global level contributed to the megadrought epoch.

  There are other factors, too. Increased upwelling of cold water in the eastern Pacific is apparently linked to heating of the tropical atmosphere. The enhanced upwelling in turn promotes the development of La Niña–like cool, dry sea surface temperatures over the eastern Pacific— and drought in western North America. As we shall see in chapter 10, such conditions were indeed persistent during the warm centuries. During the period 1150 to 1200, volcanic activity was reduced globally and sunspot activity was high, both of which contributed to a cooler, La Niña–like state across the eastern tropical Pacific, a condition that brought drought to wide areas of the Americas.

  The Pacific Decadal Oscillation

  The Pacific Decadal Oscillation (PDO) is a long-term fluctuation of the Pacific Ocean. During cool phases, an area of lower than normal sea surface heights and temperatures is present in the eastern equatorial Pacific. (As the ocean warms, it expands and the surface becomes higher.) An equivalent wedge of warmer, higher than normal sea surface heights and temperatures connects the north, west, and southern Pacific. During warm phases, the eastern Pacific warms up and the western parts of the ocean cool. These changes in cold and warm water alter the path of the jet stream, which flows farther north during cool phases, thereby reducing rainfall in the west. PDO phases wax and wane about every twenty to thirty years. El Niños and La Niñas superimpose themselves atop these long-term fluctuations. It appears that we are currently entering a cool phase, which means less rainfall over much of western North America for two or three decades. *

  *See http://sealevel.jpl.nasa.gov/science/pdo.html.

  THE MEDIEVAL DROUGHTS of the Sierra were among the most severe of the past four thousand to seven thousand years, far more savage than those we complain about today. What, then, were the impacts on the human populations of the day, when at most a few hundred thousand people lived in the Great Basin and along the Oregon and California coasts? The harshest conditions occurred in the arid interior, especially in the Great Basin and Mojave Desert, where human populations were always tiny, even near the swamps and lakes that supported larger numbers.

  The Great Basin covers over 360,000 square miles (1 million square kilometers) of the desert West between the Rocky Mountains and the Sierra Nevada—parts of California, Oregon, Utah, and Idaho, and nearly all of Nevada. This is a world of great environmental diversity, of high mountains and intervening valleys, where the topography varies dramatically and environmental zones are stacked vertically. The arid southwestern portions of the basin support the sparsest vegetation, while the mountains are somewhat wetter and have more complex climatic regimens. Fortunately for hunters and gatherers, there are some significant lakes and wetlands in valley bottoms, which were once magnets for the small numbers of people who lived in the basin. Most of this vast area receives almost no rainfall, and the amounts vary greatly from one year to the next. In ancient times, this meant that the plant food supply could be as much as six times more plentiful in a wet year than in a dry one, although the word “wet” is really a misnomer in such an arid world. To survive here required unlimited patience, opportunism, and a very broad diet. To eat one food at the expense of all others was to invite disaster. The secrets of survival were constant mobility and an intimate knowledge of dozens of edible plants.6

  Climatic conditions were never particularly stable in the Great Basin, for major changes in local environments could occur even within the brief compass of a single year. Food supplies were patchy, often confined to relatively productive locations, such as small wetlands, separated from other such places by much more barren landscapes. While a few bands dwelled in exceptionally rich lakeside or marshy environments, people in drier areas lived in tiny family groups. They were constantly on the move, subsisting off seasonal foods at widely separated locations. Their social life revolved around intelligence gathering, around “mapping” the changing availability of different foods throughout the year. Every western hunting band spent a great deal of time acquiring information, from fellow kin in other bands, from visitors, from people bartering glasslike tool-making stone from far away.

  People living in such arid environments had few options, with the result that their basic way of life survived virtually unchanged over thousands of years. At Hogup Cave in central Utah, over 13 feet (4 meters) of sporadic occupation chronicle more than eight thousand years of occasional visits and a conservative lifeway that changed little over the centuries.7 Whenever they stayed at the cave, local bands relied heavily on pickleweed, a low-growing succulent that thrives on the edges of salt pans and dried-up lake beds. They gathered all kinds of edible plants in fiber baskets (these are preserved in the cave deposits), then dried or parched the seeds in tightly woven basketry trays, tossing them with some hot embers. Then they hulled and ground the seeds with hand stones and milling slabs. Hogup residents also hunted thirty-two species of small animals and thirty-four of birds with snares, nets, and spears. By consuming a wide variety of foods, they minimized risk of starvation, the cave being one stop on a seasonal round that covered a large territory.

  Another seasonal location remained in use for thousands of years, a rock shelter near the former Lake Bonneville (known to archaeologists as Bonneville Estates) in the high desert of eastern Nevada, a virtual inland sea during the Ice Age 18,000 years ago but now long dry. Local bands visited the rock shelter at intervals for over 12,500 years. By 6000 B.C., the visitors ate a wide range of seeds such as buckwheat and wild rye, also piñon nuts. This basic diet continued in use for thousands of years.8

  Survival in such a landscape depended on diversification, mobility, and intelligence about food and water, a strategy that worked successfully through drier and wetter cycles and through the Medieval Warm Period. Great Basin peoples like the Paiute and Shoshone depended heavily on piñon nuts, a major winter food harvested in late summer and early fall. Nineteenth-century accounts tell us that the people used long sticks to gather green pinecones from the trees; they roasted the cones to release the seeds before parching and grinding. Piñons stored in grass- or skin-lined bags would keep as long as four or five years. Since abundant nut harvests occurred in three- to seven-year cycles, the caches helped tide people over from one season to the next.9

  Piñons were long a staple in Great Basin life, but the trees are vulnerable to droughts, which bring outbreaks of bark beetles. The insects bore holes in the trees to lay their eggs. Soon the larvae emerge and kill the pines. The U.S. Forest Service estimates that the droughts of 2001 to 2005 have killed an estimated eighty million piñon trees in Arizona and New Mexico alone, leaving huge swaths of browned forest across the landscape. We have no means of knowing what damage the much severer droughts of a thousand years ago did to piñon forests, but nut harvests must have plummeted for generations during medieval times. The only way one could survive drought under this circumstances was by consuming a wide variety of plant foods, which served as a cushion if the staple failed.10

  Even in wetter times, population densities were never high. Just like the people of Africa’s Sahel, family bands fanned out over the desert in wetter years when there were plants to be found and standing water to drink. In times of drought, as the desert pump pushed them outward, they would retreat to the few places where there were reliable water supplies. Diets varied greatly from place to place. Some groups living by wetlands, like certain groups in Nevada, may have obtained as much as 50 percent of their diet from fish. In contrast, the people living around Owens Lake in California ate almost entirely plant foods. Mobility and a varied diet worked even through the worst of times. The highly mobile Shoshone people of the California desert are thought to have lived in their homeland for between five thousand and six thousand years, a remarkable example of just how flexible hunter-gatherer life can be.

  CONDITIONS WERE EVEN severer in the Mojave Desert of the south, which is
famous for its high summer temperatures, and was an even more demanding and drier environment for ancient humans than the rest of the Great Basin. During the 1890s, the anthropologist David Prescott Barrows studied the foraging practices of the Cahuilla Indians, who flourished in the Mojave for thousands of years. He found the same strategies for coping with dry conditions as elsewhere in the desert West.12 Even in wetter times, everyone lived in places with the coolest temperatures and where water was most plentiful. The Cahuilla diet was broadly based. Each band exploited several hundred plants for food, manufacture, or medicine; the Cahuilla harvested six varieties of acorns in fall and relied on mesquite trees, 1.2 acres (0.5 hectares) of which could yield as much as a hundred bushels of beans. Edible cacti, piñon nuts from trees on higher ground, and fan palm dates—the list of plants goes on and on. Eighty percent of all their foodstuffs came from within 5 miles (8 kilometers) of their settlements. But, for all the variety of plants, the unpredictability of the environment left the people in a continual state of uncertainty, as it had their ancestors all over the desert West for thousands of years. Fortunately, the large cushion of edible plants and constant mobility made even severe droughts survivable.