Studies indicate that a megadrought may explain why the Anasazi abandoned their cave dwellings in the thirteenth century. Photo © Tom Tillman.
Memories linger even now of the clouds, towering and black. Without warning they rolled across the once-cheerful Great Plains, turning the grim economic times of the 1930s into an era of despair. There was no refuge from the dust. Even wet towels laid carefully across window sills and draped over keyholes failed to deter the fugitive dirt. Somehow, it always got in. The dust traveled far, even to New York City. In Kansas, Oklahoma, Texas and Colorado, where the Dust Bowl was most severe, the roiling clouds were deadly. The young and old, even the formerly robust, succumbed to pneumonia. The luckier ones, the quitters, abandoned the dryland farms they had once believed were their promised lands and migrated westward, where the writer John Steinbeck saw them and envisioned a family named Joad.
It’s easy now to see both the innocence and arrogance of the environmental calamity. Several decades of wet weather had supported the widespread plowing of grasslands in a semi-arid climate. Then came drought, lasting the better part of the decade. In all, about a third of a million people left the Great Plains. It was, until Hurricane Katrina, the greatest population displacement in the United States caused by an environmental event.
The Dust Bowl, say climatologists, is unlikely to occur again. Farmers and government scientists learned much from the experience about how to farm the land—and where not to. But drought most certainly will return, perhaps even more harshly. And turning to the American Southwest, a region defined by the Colorado River and its tributaries, experts say new evidence reveals a clearer picture of extended and sometimes severe droughts in the past 1,100 years that very well may reappear—this time with an overlay of hotter temperatures caused by increased levels of greenhouse gases. What effect these human-caused emissions will have on precipitation is still uncertain. On the matter of temperature, however, nearly all the computer models reach one conclusion: It will get hotter, much hotter, in places like Tucson, Colorado Springs and Reno. And hotter—even if precipitation stays the same—means drier. In other words, the “average” of the future will resemble what in the past we called drought.
DROUGHT: THE NEW NORMAL
Some climatologists think an abrupt shift in climate has already begun in the Southwest. Among those making the argument is Richard Seager, a senior research scientist with Columbia University’s Lamont-Doherty Earth Observatory. In a white paper issued in March, he and other climatologists reported a “broad consensus amongst climate models that this region will dry significantly in the twenty-first century, and that the transition to a more arid climate should already be under way. If these models are correct,” Seager added, “the levels of aridity in the recent multi-year drought, or the Dust Bowl and the 1950s droughts, will, within the coming years to decades, become the new climatology of the American Southwest.”
Seager, in an e-mail interview, said he came to this conclusion only last year when a co-author, Ants Leetmaa of the National Oceanic and Atmospheric Administration (NOAA) laboratory in Princeton, asked if any droughts projected for the future reached the same severity as those of 1,000 years ago. Seager didn’t know. But he fed the necessary mathematical formulae related to increased evaporation into the nineteen computer models used by the Intergovernmental Panel on Climate Change. “Then I did go ‘wow,’” he says. “We were shocked by the level of near-unanimity of the models.” The models show substantially increased drying in the Southwest (and the subtropics in general) beginning early in the twenty-first century.
For a water manager responsible for delivering water to a city, there’s plenty to be nervous about, and many are. “There is such a weight of evidence that it’s sort of hard to ignore,” says Marc Waage, manager of the raw water supply for Denver Water, which serves 2.1 million people—and may serve 2.8 million in the future.
A National Research Council report, which incorporates the work of Seager and many other scientists, drew national attention when it was issued in February. The report pointed toward a collision of two trends: less water, resulting from increased warming; and continued rapid population growth. Led by Nevada and Arizona, the seven states of the Southwest recorded a 25 percent population growth from 1995 to 2005—with no end in sight to the influx of modern-day sodbusters looking for a better life in the exurbs. The fallout from the intersection of these trends will be “possible reductions in water supplies that cannot be fully averted through traditional means,” says the report. The scientists see a future rife with “increasingly costly, controversial and unavoidable trade-off choices.”
That time, arguably, has already arrived. The drought that began in 1998 or 2000—dates vary according to precise locales—seared the Southwest in 2002, a pivotal year that captured everyone’s attention. “For the first time in our history, our reservoir levels declined from April 15 to July 1,” says Chips Barry, manager of Denver Water. “That was unheard of, and it still is. That’s when you get your water, not use it. And that was very alarming to us.”
There was also alarm in rural areas. Forest fires were rampant. The Hayman, Coal Seam and Missionary Ridge fires blazed in Colorado, while in Arizona and Oregon, respectively, the Rodeo-Chediski and Biscuit fires burned huge swaths of national forests. Just as important was what did not burn. Staggered by the lack of moisture, many forests became inviting targets for bark beetles. Absent any recent, extended deep-freeze winters to kill the beetles, 80 and even 90 percent of the trees in some forests are now expected to die.
Dramatic changes were also vividly manifest in the Colorado River’s two big reservoirs, Lake Mead and Lake Powell. Water levels rapidly dropped to 32 percent of capacity at Lake Powell by late 2005, leaving white bathtub rings seventy feet above water level on the red sandstone cliffs. Hydrologists predicted that with another bad water year, the reservoir would become functionally empty. Instead, the 2005–2006 winter was above average. Since then, however, the drought has returned. Snowmelt from the Rocky Mountains is projected to be only 50 percent of average this year. Lake Mead, located near Las Vegas, is only 54 percent full and expected to drop this summer to the lowest levels since 1965. Half-full or half-empty, these reservoirs are a full-time concern in the Southwest. Given fifteen average years, say hydrologists, they will refill. But if droughts are exacerbated by warming, “average” may need to be recalibrated.
WHAT THE TREES SAY
The Colorado River defines the Southwest. It starts in Rocky Mountain National Park, northwest of Denver, and tumbles down to desert country, fortified along the way by a succession of tributaries that originate near ski towns like Aspen, Breckenridge and Crested Butte. Another river of nearly equal dimensions, the Green River, originates in Wyoming’s Wind River Range and joins the Colorado near Moab, in Utah. By then, nearly 90 percent of the river’s water has been collected, most of it from snowmelt. The river continues, slashing through Canyonlands National Park and the Grand Canyon before beginning a more leisurely flow into the desert near Las Vegas, rolling past California’s Imperial Valley, the nation’s salad bar, and then into Mexico on its way to the Sea of Cortez.
The Colorado is a hard-working river, providing enough water through a tangled array of canals, aqueducts and tunnels to serve nearly 35 million people. Among them are residents of Cheyenne, Denver and Albuquerque to the east, and Salt Lake City, Los Angeles and San Diego to the west. Tellingly, none of these cities is located within the river’s watershed. This is a muscular river, but with little volume given its large watershed. The Sacramento River in California carries nearly twice as much water; the Columbia in the Northwest, thirteen times as much. Yet every bit of water from the Colorado is thoroughly used, the river reduced to a trickle before it reaches the ocean. And that’s the point: When there’s no margin, drought really does matter.
Climates of the past can be documented in various ways, but one of the most important methods is by studying tree rings, a scientific discipline called dendrochronology. In the Southwest, dendrochronologists study ring-width patterns of ponderosa pine, Douglas-fir and piñon pine trees. These species typically live 300 to 500 years, but a few live to be more than 800 years old. The rings are cross-referenced against each other, and also against precipitation records of the past 125 years. Less direct investigations, using dead trees, have allowed climates to be approximated to as far back as 200 A.D.
What these tree rings say is that the Southwest was far more arid in the past. Compacts reached in 1922 and 1948 that apportion the water in the Colorado River among the seven basin states assumed that the river carried more than 16 million acre-feet of water. That assumption was based on the climate of the early twentieth century, a time when wheat farmers in Kansas and Oklahoma thought they had it made. In fact, it was an unusually wet period for the last century, but even more so given the new climatic record for the last millennium. Tree-ring reconstructions of stream flow point to a volume of water in the river anywhere from 10 to 14.7 million acre-feet.
The tree-ring research suggests not only a different long-term average for the Colorado River, but more severe and longer-lasting droughts. For example, says Connie Woodhouse, a dendrochronologist and paleoclimatologist with the University of Arizona, among the driest years revealed by tree rings were three in the 1840s. Other tree rings tell of droughts singularly less severe, but longer—lasting ten or eleven years, sometimes even decades. A period from 800 to 1300 A.D. was generally more arid and punctuated by what paleoclimatologists call megadroughts. Some lasted thirty years. Archaeologists think that one of the final megadroughts, from about 1270 to 1300, may have partly caused the Ancestral Pueblo (also called the Anasazi) to vacate their cliff-dwelling communities at Mesa Verde in Colorado and Chaco Canyon in Arizona.
“There are some lessons to be learned from the past, one of them being climate variability,” says Woodhouse. There is variability from year to year, and decade to decade—including the possibility of droughts lasting for decades. “There is no indication that will change in the future,” she says.
Admittedly, the Anasazi had none of the hydraulic sophistication of the twenty-first century. They collected water in dams built near their cliff-side homes, but these were itty-bitty things. By contrast, the Colorado River’s two giant dams—Hoover, which forms Lake Mead, and Glen Canyon, backing Lake Powell—can each store up to two years of the annual flow of the river. Add in the various upstream reservoirs in Colorado, Wyoming and other states, and sixteen times the river’s annual flow can be stored.
But these reservoirs are also vulnerable. Lake Powell loses six feet of water per year to evaporation, or nearly one-tenth of the river’s flow. Even reservoirs located nearly two miles above sea level in the Rockies can lose up to a foot to evaporation each year.
Water managers in the West have begun to incorporate this revised view of natural variability into their plans. At the instigation of former Interior Secretary Gale Norton, the seven basin states began to put together an operating plan for the Colorado River based on shortages. In turn, entities such as the Metropolitan Water District of Southern California, which provides more than half the 4 million acre-feet of water annually consumed in the region, are beginning to incorporate the potential for long-term drought into their twenty-year plans. “To me, the real message is the length of the droughts we have seen in the past—not just how bad it is on an annual basis, but how many back-to-back years,” says Roger Patterson, the district’s assistant general manager. San Diego, he notes, has had its driest back-to-back years since record-keeping began in about 1880. Southern California, with 18 million residents already between Ventura and San Diego, is gaining 220,000 new residents every year.
FEWER DROPS TO DRINK
On top of this enhanced appreciation for what is “natural” in the Southwest comes the increased probability of the unnatural: the effect of greenhouse gases. Computer models to 2001 showed fuzzy pictures. More recent modeling shows a sharper resolution—a hot one. The Intergovernmental Panel on Climate Change report issued this year singles out the American Southwest, along with southern Europe, for increased heat. The mercury is already rising. Temperatures across the Colorado River basin during the current drought, for example, have been at least 1.5 degrees Fahrenheit warmer than those during the drought of the mid-1950s.
Ironically, given its image as desert landscape, the Southwest depends on snow for about 75 percent of its water. Most of it comes from the mountains of Colorado, Utah and Wyoming, some 40 percent of the “wet” coming from land that is the highest 20 percent in elevation. The cold temperatures at higher elevations mean the precipitation arrives mostly as snow, and continuing cold ensures that it lingers as such—in some places far into the summer—with little evaporation. This mountain snowpack is akin to a high-elevation reservoir.
But rising temperatures may substantially change this model. Even if precipitation stays the same, less of it may come as snow, and more of it will fall as rain. The California Department of Water Resources, for example, projects 25 percent less snowmelt from the Sierra Nevada. What snow remains is likely to melt more rapidly. In some places, it is already ten days earlier as compared to runoff when Harry Truman was president. The high mountain reservoir is being diminished. Precipitation that falls as rain, compared with snow, is more likely to evaporate.
Unlike temperature, projections for precipitation diverge widely, and none are declared with much confidence. Maybe the Pacific Northwest will get wetter, perhaps 10 percent so, and maybe the Southwest gets 10 percent less precipitation. Or maybe not. Taken as a group, the projections hedge mightily.
Brad Udall, director of the Western Water Assessment, which includes a consortium of government and academic scientists, warns against placing too much confidence even in what he calls consensus science. “Having said that, I sure wouldn’t be betting money on more flow out of the Colorado River,” he adds. “A prudent person would expect less flow, starting at 10 percent and getting worse.”
Plenty of scientists have predicted worse. University of Washington scientists Niklas Christensen and Dennis P. Lettenmaier have predicted that, with relatively modest warming of 1.7 degrees Celsius by mid-century, stream flow in the Colorado will decline 18 percent but water storage will drop 32 percent. Martin Hoerling of NOAA and Jon Eischeid of the University of Colorado foresee a more parched future yet, with the Colorado River carrying only 7 million acre-feet per year as the norm, less than half the volume on which the Colorado River Compact is based.
Even the more minimal reductions in water pose major challenges for the growing Southwest. Unquestioned is population expansion. Nevada’s Clark County, home to Las Vegas, has surpassed Manhattan in population and is now aiming to serve 3 million people. Already, the Southern Nevada Water Authority is paying homeowners to rip up their yards, offering two dollars per square foot of sod. Lawns are banned altogether in the front yard of new homes. Las Vegas may be the poster child of growth in the West, but it’s hardly alone. Demographers are predicting the Southwest will continue to lead the nation in population growth, with at least 16 million more residents arriving in the next several decades. The conventional wisdom is that farms will be purchased by growing cities for their water. Agriculture uses more than 80 percent of water in the West, and those water-rights transfers are already well under way.
The drought has raised questions considered moot only a few years ago. Illustrating this new nexus of uncertainty is Colorado. Development-minded officials always assumed the state still had, because of the compacts, the rights to make use of an additional 500,000 acre-feet of water per year from the Colorado River, and possibly more. Two new and competing proposals for piping water to the state make the same assumption. Each pipeline—one of them from Flaming Gorge Reservoir in Utah—would cost billions of dollars, pumping water hundreds of miles over or through several mountain ranges to the Front Range urban corridor. At least in theory, this new supply of water would keep farmers on the land while also satisfying cities. But is it prudent to build this expensive new infrastructure given the strengthening evidence that water is unlikely to be available? Eric Kuhn, manager of the Colorado River Water Conservation District, based in Glenwood Springs, is doubtful. “They are based on the traditional way of planning, which is that the future will look like the past,” he says.
In Denver, water manager Barry takes the same information from the climatologists and draws another conclusion. If the future exaggerates the present—general aridity but with great variability—that only strengthens the argument for more storage. “You need more storage to store what is going to be apparently higher highs to get you through lower lows,” he says. He explains that a hypothetical dam that now fills every eight years might fill only once every twenty years in a warmer and perhaps drier climate. “I’m not sure that means you shouldn’t build it,” he says.
Columbia University’s Seager sees a big gamble under way in the American West.
“In developing countries, such as those in Sahelian Africa, drought can immediately cause death and colossal suffering,” he says. “In rich countries such as the United States, the effects of drought are different. No one will starve as a result of continued drought in the West. But over the twentieth century the West has emerged into the most advanced civilization the world has ever seen, based on irrigation and water resource engineering. Plumbing on a continental scale supports massive agricultural, industrial and cultural production. Just how vulnerable is such a complex, water-dependent society in an arid region to climate? We do not know, because we have not been at this point before. But, as man changes the climate, we may be about to find out.”