雅思阅读材料：A Drier and Hotter Future

　　A Drier and Hotter Future

　　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.