Worst drought in a millennium predicted for central and southwest U.S.

Regions will become drier than any previous time on record, study says

California reservoir

DRYING UP  By the end of the century, drought conditions in the Southwest and Central Plains of the United States could be unprecedented. It’s already pretty bad in places like California, where water levels are dropping in reservoirs like this one

Don DeBold/Flickr (CC BY 2.0)

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

Researchers from New York 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 Southwest U.S. could experience the driest conditions in nearly a millennium. The results were published February 12 in Science Advances, the new open-access journal of the American Association for the Advancement of Science, and presented at the AAAS annual meeting.

“These droughts at the end of 21st century are going to be unlike anything in our modern experience,” says study coauthor Benjamin Cook, a climate scientist at NASA’s Goddard Institute for Space Studies in New York City. “It’s very likely that we will get a megadrought at the end of the century.”

Scientists have previously predicted that the 21st century will become much drier, but it has been hard to put the severity of the drought predictions in context with conditions that humans have already experienced. Reconstructions of past droughts provide that perspective. 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 is evaporated 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 past drought conditions dating back to 1000. The reconstructions are based on tree-ring measurements. A tree grows 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 isn’t visible at all.

The tree-ring data create a comprehensive history of drought conditions in the Southwest and Central Plains from about 1000 years ago to 2005 and show a severely dry period in the 1100s, which may have contributed to the decline of ancient Pueblo peoples in the Colorado Plateau in the late 13th century.

GETTING DRIER This graph shows the variability in soil moisture. The brown line shows how moisture levels have changed since the year 1000. The lower the line dips, the drier the conditions. Colored lines to the right of the graph show predictions of soil moisture through 2099 and an unprecedented trend toward severe dryness. NADA is a tree-ring data bank. PDSI is a drought severity index based on soil moisture. Cook et al/Science Advances 2015

Cook and his 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, some 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, there’s an 80 percent chance of a megadrought at the end of the century, the team reports.

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

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

He says that 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 says. The authors make a convincing argument for an exceptional drought at the end of the century, Dai says.

He notes, 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 says, could either enhance or reduce the drought by the end of the 21st century, depending on what kind of natural swings there are by that time.

Editor’s note: This story was updated February 20, 2015, to correct the labels in the soil moisture graph.

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