Environment: Why the West Is Burning

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This is important because in the West most precipitation falls as snow at higher elevations. Thus, a city like Reno, Nevada, gets, on average, just over 7 in. of precipitation a year, vs. some 70 in. at the top of nearby Mount Rose. During the 1950s drought, for example, a very large portion of the West, along with a big chunk of the Southeast and Great Plains, experienced long-term shortfalls of both winter snows and summer rains. "This is the kind of drought we worry about a lot," says Betancourt--and it's the kind of drought that the present configuration of sea surface temperatures in the North Pacific and North Atlantic seems primed to produce.

WHAT HAPPENS NEXT?

That question gnaws at Douglas Kenney, a professor of natural resources law at the University of Colorado in Boulder. As he sees it, "Everyone's pretty clear that the earth's getting warmer, but it's unclear just what that means. It might mean a wetter future or a dryer future. It might even mean a wetter future with no net gain." How is that possible? The answer lies in the impact rising temperatures are likely to have on the vast reservoirs of water locked up in the mountains in the form of snow and ice.

Scientists have documented a troubling shrinkage of the snowpack across the West, owing at least in part to the fact that rising temperatures are inexorably forcing the snow line higher. They have also found that the snowmelt is starting earlier in spring, as many as four weeks earlier in the Sierra Nevada and the Cascades. And that likewise poses a problem. Why? Snow and ice are like natural dams that hold water back during the winter months, when the risk of flooding is highest, and then melt and release it during the dry months of summer when moisture is at a premium.

"Drought is more than a precipitation deficit," observes University of Washington climatologist Philip Mote. The real problem, he says, "is that you don't have as much water as you'd like at a given point in time." And that goes for plants as well as people. For accompanying an earlier snowmelt, scientists note, is an earlier start to the growing season, which means that the demand for water by forests, marshes and grasslands--not to mention agricultural crops, lawns and putting greens--is bound to rise. In this context, a "normal" amount of precipitation may not be sufficient; and when precipitation drops below normal, as it has in recent years, the stress on trees and plants is all the more extreme.

Consider, for example, the massive forest diebacks occurring across the West. "People tend to think of forests as pretty slow changing," says Craig Allen, an ecologist with the U.S. Geological Survey. "But once certain thresholds are exceeded, very rapid changes can occur." In some cases, thirsting trees perish because their circulatory systems--the long tubular columns in the trunk that transport water from the roots to the crown--collapse. In other cases, the trees become so weak they can no longer fend off insects and disease.