The fires were a function of drought.  As of summer’s end, 2011 was the driest year in 117 years of record keeping for New Mexico, Texas, and Louisiana, and the second driest for Oklahoma. Those fires also resulted from record heat.  It was the hottest summer ever recorded for New Mexico, Texas, Oklahoma, and Louisiana, as well as the hottest August ever for those states, plus Arizona and Colorado.

Virtually every city in the region experienced unprecedented temperatures, with Phoenix, as usual, leading the march toward unlivability. This past summer, the so-called Valley of the Sun set a new record of 33 days when the mercury reached a shoe-melting 110º F or higher. (The previous record of 32 days was set in 2007.)

In fact, this year the weather elsewhere rode to the rescue, and the news for the Southwest was good where it really mattered.  Since January, the biggest reservoir in the United States, Lake Mead, backed up by the Hoover Dam and just 30 miles southwest of Las Vegas, has risen almost 40 feet. That lake is crucial when it comes to watering lawns or taking showers from Arizona to California.  And the near 40-foot surge of extra water offered a significant upward nudge to the Southwest’s water reserves.

The Colorado River, which the reservoir impounds, supplies all or part of the water on which nearly 30 million people depend, most of them living downstream of Lake Mead in Los Angeles, San Diego, Phoenix, Tucson, Tijuana, and scores of smaller communities in the United States and Mexico.

Back in 1999, the lake was full. Patricia Mulroy, who heads the water utility serving Las Vegas, rues the optimism of those bygone days.  “We had a fifty-year, reliable water supply,” she says. “By 2002, we had no water supply. We were out. We were done. I swore to myself we’d never do that again.”

In 2000, the lake began to fall -- like a boulder off a cliff, bouncing a couple of times on the way down. Its water level dropped a staggering 130 feet, stopping less than seven feet above the stage that would have triggered reductions in downstream deliveries. Then -- and here’s the good news, just in case you were wondering -- last winter, it snowed prodigiously up north in Colorado, Utah, and Wyoming.

The spring and summer run-off from those snowpacks brought enormous relief. It renewed what we in the Southwest like to call the Hydro-Illogic cycle: when drought comes, everybody wrings their hands and promises to institute needed reform, if only it would rain a little. Then the drought breaks or eases and we all return to business as usual, until the cycle comes around to drought again.

So don’t be fooled.  One day, perhaps soon, Lake Mead will renew its downward plunge.  That’s a certainty, the experts tell us.  And here’s the thing: the next time, a sudden rescue by heavy snows in the northern Rockies might not come. If the snowpacks of the future are merely ordinary, let alone puny, then you’ll know that we really are entering a new age.

And climate change will be a major reason, but we’ll have done a good job of aiding and abetting it. The states of the so-called Lower Basin of the Colorado River -- California, Arizona, and Nevada -- have been living beyond their water means for years. Any departure from recent decades of hydrological abundance, even a return to long-term average flows in the Colorado River, would produce a painful reckoning for the Lower Basin states.  And even worse is surely on the way.

Just think of the coming Age of Thirst in the American Southwest and West as a three-act tragedy of Shakespearean dimensions.

The Age of Thirst: Act I

The curtain in this play would surely rise on the Colorado River Compact of 1922, which divided the river’s water equally between the Upper and Lower Basins, allocating to each annually 7.5 million acre-feet, also known by its acronym "maf." (An acre-foot suffices to support three or four families for a year.) Unfortunately, the architects of the compact, drawing on data from an anomalously wet historical period, assumed the river’s average annual flow to be about 17 maf per year.  Based on reconstructions that now stretch back more than 1,000 years, the river’s long-term average is closer to 14.7 maf.  Factor in evaporation from reservoirs (1.5 maf per year) and our treaty obligation to Mexico (another 1.5 maf), and the math doesn’t favor a water-guzzling society.

Nonetheless, the states of the Lower Basin have been taking their allotment as if nothing were wrong and consequently overdrafting their account by up to 1.3 maf annually.  At this rate, even under unrealistically favorable scenarios, the Lower Basin will eventually drain Lake Mead and cutbacks will begin, possibly as soon as in the next few years.  And then things will get dicier because California, the water behemoth of the West, won’t have to absorb any of those cutbacks.

Here’s one of the screwiest quirks in western water law: to win Congressional approval for the building of a monumental aqueduct, the Central Arizona Project (CAP), which would bring Colorado River water to Phoenix and Tucson, Arizona agreed to subordinate its Colorado River water rights to California’s.  In that way, the $4 billion, 336-mile-long CAP was born, and for it Arizona paid a heavy price. The state obliged itself to absorb not just its own losses in a cutback situation, but California’s as well.

Worst case scenario: the CAP aqueduct, now a lifeline for millions, could become as dry as the desert it runs through, while California continues to bathe. Imagine Phoenix curling and cracking around the edges, while lawn sprinklers hiss in Malibu. The contrast will upset a lot of Arizonans.

Worse yet, the prospective schedule of cutbacks now in place for the coming bad times is too puny to save Lake Mead.

The Age of Thirst: Act II

While that Arizona-California relationship guarantees full employment for battalions of water lawyers, a far bigger problem looms: climate change. Models for the Southwest have been predicting a 4ºC (7.2ºF) increase in mean temperature by century’s end, and events seem to be outpacing the predictions.

We have already experienced close to 1º C of that increase, which accounts, at least in part, for last summer’s colossal fires and record-setting temperatures -- and it’s now clear that we’re just getting started.

The simple rule of thumb for climate change is that wet places will get wetter and dry places drier. One reason the dry places will dry is that higher temperatures mean more evaporation. In other words, there will be ever less water in the rivers that keep the region’s cities (and much else) alive. Modeling already suggests that by mid-century surface stream-flow will decline by 10% to 30%.

Independent studies at the Scripps Oceanographic Institute in California and the University of Colorado evaluated the viability of Lake Mead and eventually arrived at similar conclusions: after about 2026, the risk of “failure” at Lake Mead, according to a member of the Colorado group, “just skyrockets.” Failure in this context would mean water levels lower than the dam’s lowest intake, no water heading downstream, and the lake becoming a “dead pool.”

If -- perhaps “when” is the more appropriate word -- that happens, California’s Colorado River Aqueduct, which supplies water to Los Angeles, San Diego, and the All-American Canal, which sustains the Imperial and Coachella Valleys, will go just as dry as the Central Arizona Project aqueduct. Meanwhile, if climate change is affecting the Colorado River’s watershed that harshly, it will undoubtedly also be hitting the Sierra Nevada mountain range.

The aptly named Lester Snow, a recent director of California’s Department of Water Resources, understood this. His future water planning assumed a 40% decline in runoff from the Sierras, which feeds the California Aqueduct. None of his contemplated scenarios were happy ones. The Colorado River Aqueduct and the California Aqueduct make the urban conglomerations of southern California possible. If both fail at once, the result will be, as promised, the greatest water crisis in the history of civilization.

Only Patricia Mulroy has an endgame strategy for the demise of Lake Mead. The Southern Nevada Water Authority is, even now, tunneling under the lake to install the equivalent of a bathtub drain at close to its lowest point. At a cost of more than $800 million, it will drain the dregs of Lake Mead for Las Vegas.

Admittedly, water quality will be a problem, as the dead pool will concentrate pollutants. The good news, according to the standard joke among those who chronicle Sin City’s improbable history, is that the hard-partying residents and over-stimulated tourists who sip from Lake Mead’s last waters will no longer need to purchase anti-depressants. They’ll get all the Zoloft and Xanax they need from their tap water.

And only now do we arrive at the third act of this expanding tragedy.

The Age of Thirst: Act III

Those who believe in American exceptionalism hold that the historical patterns shaping the fate of other empires and nations don’t apply to the United States. Be that as it may, we are certainly on track to test whether the U.S. is similarly inoculated against the patterns of environmental history.

Because tree rings record growing conditions year by year, the people who study them have been able to reconstruct climate over very long spans of time. One of their biggest discoveries is that droughts more severe and far longer than anything known in recent centuries have occurred repeatedly in the American Southwest. The droughts of the Dust Bowl in the 1930s, of the 1950s, and of the period from 1998 to 2004 are remembered in the region, yet none lasted a full decade.

By contrast, the drought that brought the civilization of the ancestral Puebloans, or Anasazi, centered at Chaco Canyon, to its knees in the twelfth century, by contrast, lasted more than 30 years. The one that finished off Mesa Verdean culture in the thirteenth century was similarly a “megadrought.”

Jonathan Overpeck, a climate scientist at the University of Arizona who played a major role in the Nobel-Prize-winning work of the Intergovernmental Panel on Climate Change, tells me that the prospect of 130° F days in Phoenix worries him far less than the prospect of decades of acute dryness. “If anything is scary, the scariest is that we could trip across a transition into a megadrought.” He adds, “You can probably bet your house that, unless we do something about these greenhouse gas emissions, the megadroughts of the future are going to be a lot hotter than the ones of the past.”

Other scientists believe that the Southwest is already making the transition to a “new climatology,” a new normal that will at least bring to mind the aridity of the Dust Bowl years. Richard Seager of Columbia University, for instance, suggests that “the cycle of natural dry periods and wet periods will continue, but… around a mean that gets drier. So the depths -- the dry parts of the naturally occurring droughts -- will be drier than we’re used to, and the wet parts won’t be as wet.”

Drought affects people differently from other disasters. After something terrible happens -- tornados, earthquakes, hurricanes -- people regularly come together in memorable ways, rising above the things that divide them. In a drought, however, what is terrible is that nothing happens. By the time you know you’re in one, you’ve already had an extended opportunity to meditate on the shortcomings of your neighbors. You wait for what does not arrive. You thirst. You never experience the rush of compassion that helps you behave well. Drought brings out the worst in us.

After the Chacoan drought, corn-farming ancestral Puebloans still remained in the Four Corners area of the Southwest. They hung on, even if at lower population densities. After the Mesa Verdean drought, everybody left.

By the number of smashed crania and other broken bones in the ruins of the region’s beautiful stone villages, archaeologists judge that the aridifying world of the Mesa Verdeans was fatally afflicted by violence. Warfare and societal breakdown, evidently driven by the changing climate, helped end that culture.

So it matters what we do. Within the limits imposed by the environment, the history we make is contingent, not fated. But we are not exactly off to a good start in dealing with the challenges ahead. The problem of water consumption in the Southwest is remarkably similar to the problem of greenhouse gas pollution. First, people haggle to exhaustion over the need to take action; then, they haggle over inadequate and largely symbolic reductions. For a host of well-considered, eminently understandable, and ultimately erroneous reasons, inaction becomes the main achievement. For this drama, think Hamlet. Or if the lobbyists who argue for business as usual out west and in Congress spring to mind first, think Iago.

We know at least one big thing about how this particular tragedy will turn out: the so-called civilization of the Southwest will not survive the present century, not at its present scale anyway. The question yet to be answered is how much it will have to shrink, and at what cost. Stay tuned. It will be one of the greatest, if grimmest, shows on Earth.