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　　While I was reading William deBuys's new book, A Great Aridness, two massive dust storms reminiscent of the 1930s raged across the skies of Phoenix and of Lubbock, Texas. Newspapers blamed them on the current drought in the West, which is proximately true. But what ultimately is causing this drought, and why would any drought produce such terrifying clouds of dust? The answer is that they may be portents of a more threatening world that we humans are unwittingly creating. As deBuys explains, "Because arid lands tend to be underdressed in terms of vegetation, they are naturally dusty. Humans make them dustier."

　　Agriculture is the main reason for those dust storms—the clearing of native grasslands or sagebrush to grow cotton or wheat, which die quickly when drought occurs and leave the soil unprotected. Phoenix and Lubbock are both caught in severe drought, and it is going to get much worse. We may see many such storms in the decades ahead, along with species extinctions, radical disturbance of ecosystems, and intensified social conflict over land and water. Welcome to the Anthropocene, the epoch when humans have become a major geological and climatic force.

　　DeBuys is an acclaimed historian turned conservationist in his adopted home of the Southwest. A Great Aridness is his most disturbing book, a jeremiad that ought to be required reading for politicians, economists, real-estate developers and anyone thinking about migrating to the Sunbelt. In the early chapters he reports on the science of how and why precipitation and ecology are changing, not predictably but in nonlinear ways that make the future very uncertain and dark. In later chapters he visits ancient pueblo ruins left behind by earlier civilizations that were destroyed by drought, and he follows the grim trail of migrants crossing the border from Mexico, stirring up a controversy that climate change can only exacerbate. The book is an eclectic mix of personal experience, scientific analysis and environmental history.

　　Smoke as well as dust is spoiling the southwestern skies. As deBuys points out, forest fires are getting much bigger. In June 2002 the Rodeo and Chediski fires erupted on Arizona's Mogollon Plateau, soon merging into a single conflagration that consumed nearly 500,000 acres. It was Arizona's largest fire—until the Wallow Fire eclipsed it in June 2011. Another devastating effect of climate change has been the explosion of bark beetles among western pines, which in turn contributes to the new fire regime; in 2003, dead trees covered 2.6 million acres in Arizona and New Mexico. Could anything be more demoralizing than the sight of green forests turned a grisly brown, then bursting into flame and left charred and black?

　　Even more depressing than declining forests are mountains bare of snow. When future springs arrive, the sound of running water will be much diminished. The biggest environmental catastrophe for the Southwest, already our most arid region, is losing the melting runoff from snowpacks into rivers, canals and irrigation ditches. An ominous chapter in the book examines the future of the Colorado River, which for decades has been the "blood" of the Southwest's oasis civilization. In the 1920s Americans divided the river between upper and lower basins, allocating to each a share of the annual flow. California, which contributes almost nothing to the river, sucks up the largest share of any state, spreading it across the Imperial Valley's agricultural fields and diverting the rest to Los Angeles. Years ago policy makers assumed that the river carried about 17 million acre-feet of water per year—that is, enough water to cover 17 million acres to a depth of one foot. They overestimated, as people tend to do when hope and greed outrun the facts. Now comes a drier and hotter future, when the Colorado River will carry even less water—perhaps as little as 11 million acre-feet.

　　Tim Barnett and David Pierce of the Scripps Institution of Oceanography estimate that to adjust to a sustainable level of supply, consumers of Colorado River water will have to get along with 20 percent less water than they use today. That is still a lot of water to lose, but the loss may not be catastrophic. Urban users are already conserving about as much as they can per capita. Farmers, on the other hand, who consume about 80 percent of the western water supply, including in California, are wasting much through inefficient management and low-value crops. Half of the water goes to raise alfalfa to feed cattle, and much of the rest evaporates or soaks into the sand. If some of agriculture's share could be diverted to cities, there might be enough to sustain the current population. Rural communities would decline, some lucky farmers would retire with a potful of money, and the public would have to figure out where to get its lettuce, tomatoes, oranges and meat. The cost of water would go up dramatically, and those without money would go thirsty and leave. New hierarchies would take the place of old ones.

　　Thirty million people now depend on the Colorado River. Perhaps they can manage to adjust to a diminished flow and to declines in domestic food supplies and hydroelectric power. But more people are on the way: Demographers calculate that the population of the Southwest may increase by 10 or 20 million between now and 2050. Some of those people will come from other parts of the country, some from Mexico and Central America, and some from other nations that are coping poorly with their current problems or are overwhelmed by climate change. Whatever their origin, the new arrivals will go to the familiar oases, hoping to find the good life with a swimming pool and a green lawn.

　　Developers are eager to make money by selling homes to these newcomers. The political and economic culture of the Southwest is dead set against any acknowledgment of limits to growth. In the last few chapters of the book, deBuys shows that even now those in power refuse to accept any check to expansion; business must be free to do business. Others say that they are helpless to stop the influx: Patricia Mulroy, general manager of the Southern Nevada Water Authority in Las Vegas, declares, "You can't take a community as thriving as this one and put a stop sign out there. The train will run right over you." Her solution is to create an expensive "straw" to extract water from a shrinking Lake Mead, drawing on the "dead pool" that will be left below the intakes for generating electricity. She doesn't have the money to build that straw right now, but she is working hard to keep her improbable city from drying up and becoming a casualty like ancient Mesopotamia. Similarly, Phoenix continues to issue building permits helter-skelter and counts on "augmenting the supply" of water sometime in the future. But where will the state and city go for more supply, and how will they bring it cheaply over mountains and plains to keep Phoenix sprawling into the sunset?

　　DeBuys gathers enough scientific evidence to make a convincing case against that growth mentality. A similar case could be made against growth in the rest of the United States, although in the East the threat may be too much water, not too little, and too many storms, not too much smoke and dust. The past warns us that ancient peoples once failed to adapt and survive. Will theirs be America's fate? Perhaps. But past human behavior may not be a reliable indicator of how people will behave in the future. If the environment is becoming nonlinear and unpredictable, as deBuys argues, then human cultures may also become nonlinear and unpredictable. No other people have had as much scientific knowledge to illuminate their condition. What we will do with that knowledge is the biggest imponderable of all.