Global Warming Hits Southwest

The polar bear on its shrinking ice floe has become the urgent icon of global warming and runaway climate change. Even the flat-earther in the White House now concedes that the magnificent bears may be doomed to extinction as the sea ice melts and the Arctic Ocean is transformed into open blue water for the first time in millions of years. Humanity's "great geophysical experiment," as the oceanographer Roger Revelle long ago characterized the steeply rising curve of carbon dioxide emission, has knocked nature off its Holocene foundations in the circumpolar lands.

But the Arctic is not the only theater of spectacular and unequivocal climate change, nor are the polar bears the only heralds of a new age of chaos. Consider, for example, some of Ursus maritimus's distant relatives: the black bears that forage happily but ominously in the fabled Chisos Mountains of Texas's Big Bend National Park. They may be the messengers of an environmental transformation in the Borderlands almost as radical as that taking place in Alaska or Greenland.

While hiking en route to Emory Peak on a preternaturally warm day in January 2002, with my mind still haunted by the apocalyptic images of the previous September, I made the nodding acquaintance of an antic and harmless young bear in a trail camp. Apparitions of bears are always slightly magical, and I presumed the encounter was an affirmation of a still largely unspoiled wilderness. In fact, as I was startled to learn from a ranger the next day, the young bear was, so to speak, a mojado -- the offspring of recent undocumented immigrants from the other side of the Rio Grande.

Black bears had been common in the Chisos when it was the quasi-mythical redoubt of Mescalero Apache and Comanche raiders in the seventeenth and eighteenth centuries, but ranchers relentlessly hunted them to extinction in the early twentieth century. Then, almost miraculously in the early 1980s, bears reappeared amid the madrone and pine of Emory Peak. Astonished wildlife biologists surmised that the bears had migrated from the Sierra del Carmen in Coahuila, swimming the Rio Grande and crossing forty miles of furnace-hot desert to reach the Chisos, a promised land of docile deer and abundant garbage.

Like the jaguars that have re-established themselves in the border mountains of Arizona in recent years or, for that matter, the blood-sucking chupacabra of norteño folklore who has reputedly been seen in the suburbs of Los Angeles, the black bears are part of an epic migration of wildlife as well as people al otro lado. Although no one knows exactly why the bears, big cats and legendary vampires are moving northward, one plausible hypothesis is that they are adjusting their ranges and populations to a new reign of drought in northern Mexico and the US Southwest.

The human case is clear-cut: Abandoned ranchitos and near-ghost towns throughout Coahuila, Chihuahua and Sonora testify to the relentless succession of dry years -- beginning in the 1980s but assuming truly catastrophic intensity in the late 1990s -- that has pushed hundreds of thousands of poor rural people toward the sweatshops of Ciudad Juárez and the barrios of Los Angeles.

In some years, "exceptional drought" has engulfed the entire Plains from Canada to Mexico; in other years, crimson conflagrations on weather maps have crept down the Gulf Coast to Louisiana or crossed the Rockies to the interior Northwest. But the semipermanent epicenters have remained the basins of the Colorado and Rio Grande rivers, as well as northern Mexico.

By 2003, for example, Lake Powell had fallen by nearly eighty feet in three years, and crucial reservoirs along the Rio Grande were barely more than mud puddles. The Southwestern winter of 2005-06, meanwhile, was one of the driest on record, and Phoenix went 143 days without a single drop of rain. Rare interruptions in the drought, like the Noachian monsoon of last summer (parts of El Paso received an incredible thirty inches of rain), have been insufficient to adequately recharge aquifers or refill reservoirs, and in 2006 both Arizona and Texas reported the worst drought losses to crops and herds in history (about $7 billion altogether).