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Megadroughts predicted for U.S.: some states may see driest conditions in a millennium.

Record-setting droughts are in the forecast for the central and southwestern United States, a study comparing past and predicted drought conditions shows.

Researchers compared drought predictions for the second half of the 21st century with reconstructions of drought conditions dating back to the 11th century and found that the Central Plains and the Southwest could experience the driest conditions in nearly a millennium. The results were presented February 12 and published in Science Advances.

"These droughts at the end of the 21st century are going to be unlike anything in our modern experience," said study coauthor Benjamin Cook, a climate scientist at NASAs Goddard Institute for Space Studies in New York City. "Its very likely that we will get a megadrought at the end of the century."

Scientists have previously predicted that many regions will become much drier during the 21st century, but it has been hard to put the severity of the drought predictions in context with conditions that people have already experienced. The new study is the first to compare the severity of droughts at the end of the 21st century with past drought conditions dating back to the year 1000.

The severity of a drought is based on soil moisture, specifically how much is added when it rains and how much evaporates as temperatures increase. In the study, the team looked at three kinds of soil moisture metrics from 17 models for climate from 2050 to 2099 and at reconstructions of drought conditions going back a millennium. The reconstructions are based on tree-ring measurements. In North America, trees grow a new ring each year, and how wide the ring grows depends on the amount of water in the soil. If the year was extremely dry, the resulting ring isnt visible at all.

The tree ring data create a comprehensive history of drought conditions in the Southwest and Central Plains from about 1,000 years ago to 2005 and show a severely dry period in the 1100s, which may have contributed to the decline of ancient Pueblo peoples of the Colorado Plateau in the late 13th century.

Cook and colleagues looked at the severity of future drought in two different scenarios. In one, greenhouse gas emissions continue at the current levels. In the other, attempts are made to reduce them. Greenhouse gas emissions contribute to rising temperatures, which affect soil moisture and ultimately the severity of a drought. Under both scenarios, the end of the century will be much drier than the medieval megadrought. Under the "business as usual" model, theres an 80 percent chance of a megadrought at the end of the century.

California is now experiencing extreme drought, the worst since the year 800 (SN: 1/10/15, p. 16). The predicted megadroughts would take current conditions in California and extend them for decades, Cook said.

"This could likely happen if we do nothing to slow down global warming," said climate scientist Aiguo Dai of the State University of New York at Albany, who was not involved in the study.

Reconstructing past periods of dryness provides a range of the natural swings in drought severity. "If the future drought change induced by greenhouse gases is outside of this range, then we know the future drought conditions will be unprecedented and troublesome," he said. The authors make a convincing argument for an exceptional drought at the end of the century, Dai said.

He noted, however, that the team did not factor in natural swings in drought severity in the future. These natural variations, such as what was seen in the medieval megadrought, will be superimposed on top of the future changes due to greenhouse gases. The combination of these two, Dai said, could either enhance or reduce the drought by the end of the 21st century, depending on the nature and extent of those swings in the future.

Caption: By the end of the 21st century, drought conditions could become unprecedented in the U.S. Central Plains and Southwest (a reservoir in drought-stricken California is shown).


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Title Annotation:EARTH & ENVIRONMENT
Author:Yeager, Ashley
Publication:Science News
Geographic Code:1USA
Date:Mar 7, 2015
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