Environment: Why the West Is Burning

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Some of this evidence comes from corals that grow in the tropical Pacific. Just like trees, observes Georgia Tech paleoclimatologist Kim Cobb, coral polyps lay down growth rings that encode information about temperature and precipitation. About a year ago, Cobb and her colleagues spliced together a 1,000-year-long climate record from corals collected on Palmyra Atoll, 1,000 miles south of Hawaii. The record reveals a sequence of long-term shifts in sea surface temperatures and precipitation that seem reminiscent of El Nino and La Nina. Cobb speculates that extreme La Nina--like conditions recorded by a 10th century coral might have been connected to the drought that occurred in the Sierra Nevada at that time.

Scientists are still trying to ferret out the physical mechanisms capable of causing long-term changes in the tropical Pacific. Among the suspects are volcanic eruptions, which temporarily cool the surface of the planet, and cyclical ups and downs in the sun's luminosity that occur on time scales ranging from decades to centuries.

Climate models run by the University of Miami's climatologist Amy Clement and her colleagues suggest that the key to the puzzle may lie not in the eastern basin of the tropical Pacific--the area El Nino and La Nina so profoundly affect--but rather in a pool of warm water positioned to the west. In the models, when the so-called western warm pool cools off just a bit or when it warms a tad more, the eastern basin of the tropical Pacific tends to respond by doing the opposite. In recent years, it so happens, the western basin of the tropical Pacific--along with the neighboring Indian Ocean--has been the warmest it has been for at least 150 years. And what that adds up to, says Martin Hoerling, a research meteorologist with the National Oceanic and Atmospheric Administration, is "the perfect ocean for drought."

But there is more to the story than just the tropical Pacific, thinks paleoclimatologist Julio Betancourt of the U.S. Geological Survey's Desert Laboratory in Tucson, Ariz. Along with several colleagues, Betancourt has investigated the influence on Western drought of sea surface temperatures outside the tropics. The North Atlantic, they have found, seems to play a central role. When the surface of the North Atlantic warms, the stage appears to be set for both 1930s-and 1950s-style droughts. Tree-ring records taken from the Atlantic basin suggest that one such warming occurred in the late 1500s, coinciding with the 16th century megadrought.

What tips the balance between a 1930s-and a 1950s-style drought? Betancourt, for one, believes the answer may lie off the West Coast of the U.S. Sea surface temperatures in the North Pacific, an accumulating body of evidence suggests, undergo distinctive patterns of warming and cooling every 20 to 30 years (in response, some think, to long-term changes in the tropics). Accompanying these changes in sea surface temperatures are seesaws in atmospheric pressure that alter storm tracks across the North American continent. When waters off the coast of California warm in synch with those of the North Atlantic, as happened in the 1930s, summer rainfall tends to fail, particularly in the Plains states. But when these same waters cool, as occurred in the 1950s and again in the late 1990s, winter precipitation can falter as well.