Was 20th Century Unusually Wet?

-- The American Southwest is in the seventh year of a drought that could have a profound impact across the entire country and, in the end, stand as a stark monument to human exploitation of a land of limited resources.

Droughts have struck that region often down through the centuries, contributing to the collapse of entire societies like that of the remarkable Anasazi Indians who built six-story housing developments hundreds of years ago and wide roads in a region with no vehicles.

But this drought could be even worse, plaguing the most rapidly growing area in North America. Several lines of evidence suggest that the past few decades have been wetter than normal, and severe droughts lasting tens of years are more likely the norm rather than the exception.

Dry History

The story is told in tree ring records that tell of the variation in growth of trees over hundreds of years due primarily to rainfall, and soil analysis that tells of the presence of charcoal and thus vegetation, and records of changing ocean temperatures as far away as the North Atlantic. They all point in the same direction.

In a word, scientists are already starting to call it a "megadrought." But more than likely it's a return to normal weather patterns for a huge chunk of the nation ranging from the snow-covered peaks of the Colorado plateau to the arid deserts of the Southwest.

Someday, not too far down the road, water may be more valuable than oil in megalopolises from Los Angeles to Las Vegas to Phoenix to Denver and beyond. Even a couple of years of normal rainfall, if they come soon, won't change the fundamental fact that some very hard decisions are going to have to be made in the near future.

And adding to all of that is the concern voiced by many scientists that global warming will make the picture even bleaker.

The details lie along the entire route of the once mighty Colorado River that carved the Grand Canyon as it coursed down from the highlands toward Mexico, which it doesn't even reach any more. For decades water from the river has been siphoned off to reservoirs where it could be fed to the growing populations of the Southwest.

The two major reservoirs, Lake Mead, formed by the construction of Hoover Dam, and Lake Powell, long detested by environmentalists because it wiped out one of the most spectacular canyons in the United States, are ghosts of their own pasts.

The water level in Lake Powell has dropped 130 feet since 1999, leaving one "waterfront" lodge a third of a mile from the lake. Lake Mead has shrunk so much that signs that once dotted the shoreline stand high and dry hundreds of feet from the water. An ice cream parlor that had once been part of the old town of St. Thomas recently poked through the surface of the lake. It hadn't been seen for decades because it had been buried under 64 feet of water.

And here's a scary statistic. Water officials say that if the drought continues, within three or four years water in both lakes will be so low that hydroelectric turbines in the Glen Canyon and Hoover dams will cease to function, dimming the lights in much of the Southwest unless expensive power can be purchased elsewhere.

So is this an extraordinary drought, or is it nature's way of returning to business as usual?

Tree Rings, Sediment Cores Point to Same Verdict

Scientists from several institutions have been studying 602 tree ring chronologies from the western United States covering the past 2,000 years. Wider rings correspond to wetter years, and by overlapping the patterns of rings of one tree with the rings of another, scientists can reconstruct periods that are longer than the lives of the individual trees.

The research shows that the 20th century was much wetter than normal. More typical were the droughts of the middle ages, which many experts believe led to the downfall of the Anasazi culture around 1300. Interestingly, the most intense droughts occurred during a 400-year span that corresponds to a well-documented European phenomenon called "Medieval Warm Period" when temperatures were warmer throughout much of Europe.

That has led some experts to fear that global warming could lead to even more severe droughts in the Southwest since the weather patterns seem to be linked to what is happening in Europe, much of which is growing warmer.

Meanwhile, scientists led by Duke University have been looking at sediment cores throughout the region struck by the 1930s Dust Bowl drought. One pattern leaped out at them: a clear variation in charcoal deposits.

High levels of charcoal signaled the presence of prairie fires because there was plenty of grass, thus indicating plenty of rainfall. Low levels indicated drought.

Again, the record indicates that from 5,000 to 8,000 years ago, droughts were the norm for a wide region extending from the Dakotas to Minnesota. That's not part of the Southwest, of course, but it indicates that less rain fell in past centuries across a wide swath the country.

Warm Atlantic Leads to Dry Southwest

Why these patterns persist is open to widespread debate, but one line of evidence shows just how complex this issue is, and it, too, bodes ill for the Southwest. Scientists with the U.S. Geological Survey have been studying evidence of varying sea surface temperatures of the far-off North Atlantic Ocean over the past century.

What they have found is that warmer temperatures in the North Atlantic correspond to drought conditions in the United States, and the researchers suggest that the Southwestern drought may be due in large part to the currently warmer North Atlantic waters. That's all very tentative, because that subject has not been studied extensively, but it's a bit foreboding.

The warmer North Atlantic is expected to continue for at least several years, so if they are right, the Southwest is in for a very difficult time.

The impact could be global. Farmers will have less water, and thus able to grow fewer crops. Competition for electricity will soar, straining systems that are already stressed. And water wars will return to the Southwest, this time in the courts as jurisdictions fight over dwindling supplies.

What could change all that, of course, is rain. Lots of rain. For several years.

Lee Dye's column appears weekly on ABCNEWS.com. A former science writer for the "Los Angeles Times," he now lives in Juneau, Alaska.

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