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March 08, 2011

Ancient mega droughts are sign of dry times to come

by John D. Cox

Across the American Southwest, the idea of drought never seems very far away. There are good years and bad, of course, but it only takes a 10 percent loss of rainfall to bring on severe water-supply trouble. They don't call it desert for nothing.

Now a a team of federal and university researchers have published a remarkably detailed record of the region's distant hydrological past that is a must-read for anyone contemplating the populous region's future. Their analysis of ancient lakebed sediments is published in the current issue of the journal Nature.

The team took an 80-meter-long sediment core drilled from the volcanic lakebed of Valles Caldera in northern New Mexico. The core exposed laminations that give scientists roughly 200,000 years of climate history, between 370,000 and 550,000 years ago. Temperature and precipitation patterns rise and fall over such a great expanse of time, in response to periodic changes in Earth's orbit and atmospheric circulation. But the most striking features in the core are enormously long "megadroughts" lasting thousands of years.

Especially interesting to researchers is a 50,000-year warm period that began about 435,000 years ago when Earth's orbital and climate conditions were pretty close to the way they are now. Within this period are two cool-wet phases and three warm-dry phases -- including a southwestern drought lasting several thousand years that began and ended abruptly.

The team "found that the driest conditions occurred during the warmest phases" of the warm periods, when average annual temperature "was comparable to or higher" than modern temperatures, reported lead author paleoclimatologist Peter Fawcett of the University of New Mexico in the paper's abstract.

In a separate analysis in Nature, John Williams of the University of Wisconsin pondered the question of whether this earlier warm period is a good analogue of what the southwestern U.S. can expect of the current warm-climate era that began 11,000 years ago.

"If so, southwestern climates might naturally be trending towards a somewhat cooler and wetter stage -- except that other factors (us) are affecting climate. Knowing the likely natural trends thus helps us to diagnose the causes of twenty-first-century hydrological trends. And perhaps, just perhaps, these natural trends will partially mitigate the projected drying in the southwest."

But other researchers are not optimistic. One noted specialist, Richard Seager at Lamont-Doherty Earth Observatory in Palisades, New York, told Nature's Quirin Schiemeier: "The drying we expect for the twenty-first century is entirely the result of increased greenhouse forcing. Any natural variations in orbital forcing and incoming sunlight will hardly have a noticeable role in the near future."

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