Already the fire season in Southern California is breaking records. Last year was bad enough; this year is outpacing it in both the number of fires started (2,749 vs. 2,453) and the amount of acreage consumed (69,167 vs. 38,523). And Southern California is not alone. A fast-moving wildfire exploded in a canyon on the outskirts of Las Vegas two weeks ago, forcing the evacuation of 75 Girl Scouts from a campground in the Spring Mountains this on top of a fire that threatened the capital of Nevada and another that nearly destroyed a $200 million astronomical observatory in Arizona. Just a few more big ones could easily turn 2004 into one of the West's worst fire years on record.
And no one knows when the drought will end. Scientists believe this dry spell, which has plagued a broad swath of the West since 1999, is more typical of the region than its 60 million inhabitants would care to admit. As Charles Ester, chief hydrologist for Arizona's Salt River Project, a major provider of water and electricity, puts it, "What we took as a period of normal rainfall in the past century was actually a period of abundance."
Consider, for example, the 1922 compact that determines the allocation of water from the Colorado River. Scientists have shown, by studying tree rings and other historical evidence, that the allocation was based on water flows that were the highest they had been for more than 475 years. By contrast, the flows since 1999 rank among the lowest. As a result, Lake Powell, the giant reservoir created on the Colorado by the Glen Canyon Dam, stands some 60% below capacity and seems destined to fall even lower. No wonder that states like Colorado whose rights to that water are trumped by the rights of California, Nevada and Arizona are anxiously bracing for a crisis.
At risk are not only natural ecosystems and agricultural enterprises but also the multiple amenities that people living in the West have for so long taken for granted: ski resorts and golf courses, green lawns and lush gardens, swimming pools and hot tubs, not to mention such modern necessities as dishwashers and flush toilets and the hydropower that keeps refrigerators and home computers humming. Caught off guard, political leaders and water-resource managers have been turning to scientists for help. What do researchers know about patterns of drought in North America? What do they think occurred in the mid-1990s when a big chunk of the West abruptly veered from wet to dry? And do they believe that the current shortfall of precipitation is just a temporary dry spell or an ominous realignment of the earth's climate system?
Secrets of Tree Rings
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.