Matt Crenson, Reconstructions

SN Prime | August 1, 2011 | Vol. 1, No. 7

Anything but lush, the U.S. Southwest has been especially parched lately. About a decade ago a cycle of droughts began; the latest one has dried much of the region to a degree that meteorologists expect only twice a century.

But look back a millennium or more, and you’ll find signs that today’s conditions are not all that unusual. Studies of ancient climate suggest that the last decade’s water crisis, and even the 1930s Dust Bowl, pale in comparison to a series of droughts that struck the Southwest 700 to 1,100 years ago.

Temperature was the driving force behind those ancient droughts; solar activity warmed western North America by up to 1 degree Celsius above the long-term average. With temperatures today rising once again — this time due not to  solar variation but to increasing atmospheric carbon dioxide levels — there’s every indication the Southwest is about to get even drier.

It’s already warmer there than it was during those ancient droughts. Climate models suggest that it will warm another 3 to 5 degrees by the end of the 21st century. By then, the kind of crop-withering, water-rationing conditions that are considered extreme today will be business as usual.

The news is especially bad for cities in the Southwest that draw water from the only major river basin in the region, the Colorado. That means pretty much all of them: Phoenix, Tucson, Los Angeles, Denver and Las Vegas all get water from the Colorado. It also means trouble for places like California’s Imperial Valley, where irrigation allows farmers to grow thirsty crops like cotton and alfalfa on land that gets only a few inches of precipitation annually.

Tree ring records suggest that during the worst drought of the last 2,000 years in the Southwest, which peaked in the mid-12th century, the mean annual flow of the Colorado River at Lees Ferry, Utah, was about 14.2 billion cubic meters, compared with an average 20th century value of 18.3 billion cubic meters. That drought’s effects were felt as far east as Missouri — conditions there were drier between 1148 and 1208 than in any extended period of the last century, researchers reported last year in Agricultural and Forest Meteorology. A 2010 paper in Aeolian Research described evidence that windblown sand dunes were being deposited at that time outside Abilene, Kan.

Such conditions would stress present-day water resources to their limits, researchers from Arizona, California, Arkansas and New York wrote in December in the Proceedings of the National Academy of Sciences. Major urban areas would go thirsty. Farming would be untenable not only in the Southwest, but also in much of the U.S. corn belt.

But even that can’t compare to bigger droughts that prevailed in the Southwest between ice ages hundreds of thousands of years ago. A sediment core drilled 82 meters into the Valles Caldera, an extinct volcano in the mountains overlooking Los Alamos in northern New Mexico, suggests that extreme drought conditions in the Southwest can go on not just for years or decades, but for millennia. The core shows that while the Northern Hemisphere was in the grip of an ice age, temperatures abruptly warmed for a few millennia several times between about 385,000 years ago and 425,000 years ago.

One warm snap, about 420,000 years ago, is of particular interest because it was climatically similar to the 10,000 years of warm, stable conditions that have allowed humans to develop agriculture, found civilizations and build a global economy. The sediments in the core indicate the presence of a lake in the caldera at that time, and pollen grains show that grasses and trees like oak and juniper were growing nearby. Conditions weren’t exactly like they are today, but they were similarly hospitable. After 50 millennia of pleasant living, things cooled off by about 2 degrees in the Southwest. In the April 27 Nature, the researchers say present-day climate might be expected to remain stable for a comparable period — in the absence of human-caused climate change.

With increasing carbon dioxide, however, things are likely to be more like the warmest periods recorded by the Valles Caldera core. During those intervals the core reveals mud cracks and minerals like calcite, left behind when bodies of water evaporate. Signs of decreased soil quality suggest that the basin’s grasses died off, probably due to diminished summer rainfall.

This year those summer thunderstorms — a crucial source of growing-season moisture in New Mexico — have failed to materialize. The Valles Caldera sediment core suggests that global warming might cut off that moisture supply for good, leading to permanent aridity in a place that’s already plenty dry.