This view of the southern Arizona Sonoran Desert is not as bleak as it might be; there is actually a little green vegetation visible. This image, from the Desert Biomes page at Marietta College, must have been taken in June or earlier, when it is possible to wander about in the Sonora without getting heat stroke by noontime. By August the green is gone, as the plants await the winter rains.
Some years the rains never arrive. When average rainfall is less than a foot (30 cm) per year, it doesn't take much variability to really dry things out. Variability is the name of the game in arid landscapes. William deBuys investigates that variability, and in particular where the range of variation is expected to go in future decades with a warming climate, in A Great Aridness: Climate Change and the Future of the American Southwest. Whether you believe "global warming" is caused by people and their CO2 or not, it is clear that the global climate is warming. This warming is adding more energy to the atmosphere. How does this added energy manifest itself?
Thank George Hadley, colleague and rival of Edmond Halley (as in Halley's Comet), who nearly 300 years ago figured out that tropical heat powers atmospheric circulation in a band centered on the Equator. Warm, wet air rises (and rains copiously), spreads south- and northward, cools (and rains some more), then descends at around 30°N and S latitudes. This is now named for him, the Hadley Cells and Hadley Circulation. The descended air returns to the tropics, picks up a westerly trend from the Coriolis effect, and powers the trade winds. But what happens when cool, semi-dry air descends on those latitudes at the northern and southern limits of the Hadley Cells? It warms and its relative humidity drops dramatically. The result? The Sahara, Mojave, Sonora, Chihuahua, and Atacama Deserts, among others. The descending air makes it hard for moist air masses to relieve desert aridity. More energy in the atmosphere will mean larger Hadley Cells and drier deserts.
Author deBuys's book focuses on North America's great southwestern deserts: The Mojave, Sonoran and Chihuahuan Deserts of Arizona, New Mexico, Nevada and northern Mexico. The consensus of analysis related to a warming climate warns that these areas will get even drier as this century progresses, while the humid areas of the American east and northeast get wetter. The southwestern dryness will be the worst in Arizona. When you get away from over-irrigated Phoenix, Arizona is right about dead center in every concept of "desertness", with just under 10 inches (25 cm) of rainfall, very little vegetation, and year-round warmth that gets positively scorching in mid- to late summer.
A large part of the book centers on Arizona as the major element of the Colorado River watershed. By the time I left the Los Angeles area for the third time, in 1978, the tap water was getting pretty bad, being rather salty remnants of the Colorado River, picked out of one of the reservoirs (Lake Mead or Lake Powell), after having traveled 300 miles or so (~500 km) through the Grand Canyon, evaporating all the while, then evaporating even more while sitting in the reservoir. All that evaporating concentrates trace salts into a rather unpalatable excuse for something to put in your pipes. My most recent visit confirms that it continues to get worse. Angelenos are a huge market for bottled water from anywhere else.
What is happening to the Colorado River? This chart tells part of the tale.
This is Figure 2 from David Meko et al, "Medieval drought in the upper Colorado River Basin," Geophysical Research Letters 34, (2007), and downloadable here. The caption reads:
Time series plot of 25-year running mean of reconstructed flows. Flows are plotted as percentage of the 1906–2004 mean of observed natural flows (18.53 billion cubic meters, or 15.03 million acre-ft). Confidence interval derived from 0.10 and 0.90 probability points of ensemble of 1000 noise-added reconstructions. Horizontal dashed line is lowest 25-year running mean of observed flows (1953–1977).The key datum here is the bold line at the right end of the chart, the observed flow of the Colorado River. It starts up quite high in 1906, drops quite low in the 1950s to '70s, rebounds to a lesser high in the 1990s, then begins to drop through 2004. It has been dropping ever since. Water policy for parceling out the river's water was set 100 years ago, based on measurements taken during the three wettest years, which led the policy makers to set a baseline flow as 17.5 maf (maf = million acre-feet) per year. Except for a couple of years in the 1990s, the river's flow has seldom exceeded 15 maf yearly. During the periods of original filling of Lake Mead and Lake Powell, not all the water was being used yet, so the lakes filled. They have been emptying for more than a decade, however. They are about half gone.
What will result from a more energetic climate? For one thing, the Hadley Cell will get a little larger, moving the northern limits of the southwestern desert northward a couple of degrees (~100 mi or 160 km). We are already seeing massive tree die-off in northern Arizona and New Mexico and southern Colorado. The trees are succumbing to a one-two punch: conditions called "drought" because they are drier than in the past have stressed the trees. Stressed trees are more susceptible to attack from boring insects. Too many insects leads to girdling of a tree, which then dies. Millions of trees have died. Millions more are expected to die.
A poignant note is found in a late chapter that tells of Mount Graham, one of several "sky islands". This isolated cluster of peaks is a good place to put telescopes. It is also the host to an isolated biome, and the only known home of an endangered species of small red squirrel. The whole species numbers about 300 individuals. Recent fires have reduced the squirrel's habitat by half (they also threatened a couple of the telescopes). A warming climate is making it harder for the animal's winter larder to stay fresh enough to last until springtime. Further warming may extinct the Mt. Graham red squirrel, making moot the years of litigation that preceded telescope construction. Not a happy outcome.
What does the author suggest? Time and time again he warns that the recent "drought" conditions are the new normal, and going forward, the cycle will include much deeper droughts, such as that reported by Meko and his co-authors. A larger, more energetic Hadley Cell will make the Sonoran Desert and her sister deserts possibly as dry as parts of the Atacama, where several years can pass with no rain at all. With this in mind, the ongoing rush of people into these regions is simply insupportable. Early in the book he suggests that cities like Tucson may find it necessary to forbid the installation of new water meters. "Y'wanna build the house, go ahead, but you'll have to truck in your water. No new meters." But late in the book he reports on one California town that banned installation of new meters. The market in existing meter permits took off, to the point that one sold for $300,000. Would you pay half or more the cost of your new home for its water rights? Somebody did just that!
I have been warning for a few years that, where the Twentieth Century was characterized by wars about territory, and later about energy, the Twenty-First will be the century of water wars. The political and regulatory situation in the American southwest, on a collision course with the drying out of the watersheds, could well lead to a new civil war in this country. Yes, I mean a shooting war: guns, bombs, and laser-guided cruise missiles and all.
It doesn't matter to what cause you ascribe the warming climate. Read this book to learn what is likely to ensue. And retire to someplace wetter than Arizona. I love the desert, particularly in winter. I like the feel of warm, dry air. But I also need to drink, and to bathe occasionally. I'll stay here in the humid northeast, thank you.