Environment: Why the West Is Burning

(2 of 5)

That the West is a semiarid region subject to episodic droughts has been understood for some time. What's new is the detailed picture of those droughts that is emerging from a vastly improved network of North American tree-ring records that extend back more than 1,000 years. Those records--some 835 in all--are based on the growth rings laid down by multiple species of long-lived trees, including blue oaks, giant sequoia and bristlecone and ponderosa pines. Interpreting the rings takes skill, but the basics are simple. The rings are wide when moisture is sufficient, narrow when it is not.

Dendrochronologists, as scientists who study tree rings are called, are using these records to probe the patterns of drought over space and time. Droughts like the 1930s Dust Bowl, which was focused in the northern Rockies and Great Plains, appear to be quite rare, with only two somewhat close analogues over the past 500 years. But 1950s-style droughts, centered on the Southwest, are more common, recurring, on average, twice each century. An event that scientists have dubbed the 16th century megadrought resembled a combination of the droughts of the 1930s and 1950s.

And even the 16th century megadrought pales in comparison to earlier dry spells that California State University geographer Scott Stine has documented in the Sierra Nevada. Between A.D. 900 and 1350, he has established, trees and shrubs grew in lake basins and riverbeds that are now filled with water. By dating the wood from their stumps, Stine has identified two droughts that were separated by an extremely wet period. One of these droughts lasted 140 years, the other more than two centuries.

A prolonged dearth of precipitation during this time affected other areas as well. Across Mexico, Guatemala and the Four Corners region of the Southwest, unreliable rains threw the ancient Maya and Anasazi civilizations into crisis. In the Great Plains, peat marshes dried up and sand dunes resumed their wind-driven march. In Nevada's Great Basin, hardy bristlecone pines throttled back their growth, waiting for a wetter clime. "If the future resembles the past," says Stine, "then we're in for a lot of trouble."

What causes drought?

What could trigger droughts that persist for decades and even centuries? Topping the list of suspects are swings in sea surface temperatures like those associated with El Nino and La Nina. Every three to seven years, a distinctive warming of the eastern basin of the tropical Pacific (El Nino) influences North American weather patterns, often in ways that send winter storms farther south than usual. As a result, California and the Southwest tend to get quite wet. By contrast, the cooling of the sea's surface (known as La Nina) tilts the odds toward drought in this same region. And there is evidence that the swings between El Nino and La Nina are mirrored by longer-term changes in the tropical Pacific that are less extreme but much more persistent.