Was 20th Century Unusually Wet?

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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