The drought settling over the American heartland this summer would be historic by any measure. According to the U.S. Department of Agriculture, it is the most widespread drought in half a century, covering more than half the country and affecting nearly 90 percent of the nation’s corn and soy crops. In a single July afternoon, U.S. Secretary of Agriculture Tom Vilsack declared more than a thousand counties “disaster areas,” marking the largest such designation in history and encompassing three quarters of Kansas, along with nearly all of Colorado, New Mexico, Oklahoma, and Texas. Even if the drought had come in the midst of a boom cycle, this would be a backbreaking year.

But these are hardly boom times. As “Broken Heartland,” my cover story for the July issue of Harper’s, explained, the collapse of agriculture on the Great Plains has been looming for a long time. It began, by most measures, in the 1930s, when decades of agricultural tilling left the ground vulnerable to howling winds that whipped the sky black with dust and inspired an exodus on main streets from the Dakotas to West Texas. In the eight decades since that flight, the population of the rural plains has continued to fall, so that nearly a million square miles today meet the definition of “frontier” used by the Census Bureau a century ago. In Kansas alone, some 6,000 towns have vanished altogether.

Despite this population plunge, farming on the plains persists, largely through advances in irrigation and technology. With the aid of modern tractors, combines, and irrigation systems, as well as an endless supply of pesticide, herbicide, fertilizer, and well water, a single farmer today can produce as much grain as a whole town a few decades ago. One might say that, on the plains, it doesn’t take a village.

But it does take water. Despite advances in technology and dryland farming, the fact remains that most of today’s megafarms on the plains rely on a single aquifer, the Ogallala, which lies beneath eight states, including those at the heart of today’s drought. Unfortunately, unlike some other aquifers, the Ogallala does not refill very much when it rains. For sixty years, its gush of water, which provides nearly a third of all U.S. irrigation, has come from the same finite reserve trapped in prehistoric times. Whenever that supply is gone, it’s gone, and experts say it’s going fast. As Kevin Mulligan, one of the leading USDA researchers on the Ogallala, told me as we looked over a map of the dwindling aquifer, “There will be very little irrigated agriculture on the high plains twenty years from now.”

And that was before the summer drought.

It may be tempting to think that, because the Ogallala does not refill from rainfall, the lack of rainfall will not hurt it. Would that it were so. In fact, rainfall is the single most important way to protect the aquifer, as it drastically reduces the need for pumping. According to records kept by the High Plains Underground Water Conservation District near Lubbock, farmers use about three times as much irrigation water in a drought as they do in average times, and during last year’s dry spell some of the monitoring wells in West Texas dropped by twenty-five feet, about ten times the average annual rate. With less than a hundred feet of aquifer remaining in most parts of the region, one shudders to imagine how many more droughts the farmers of West Texas can take.

Few people outside the office of Senator James Inhofe (R., Okla.) deny the reality of global warming anymore, and those of us who witnessed the violent derecho storm last month (Tropical Storm Inhofe, I call it) bore witness to just how terrifying the coming extremes of weather will be. Whatever else climate change entails, we know this much on the plains: The torrential downpours will do nothing to refill the aquifer or offset prolonged periods of drought; rather, the two will converge into an even nastier blow, as the droughts turn cropland brown and rob the soil of its meager roots, laying the earth naked before the opening skies.