US faces worst droughts in 1,000 years, predict scientists

Climate change is likely to cause decade-long mega-droughts across US south-west and Great Plains, new study shows
Suzanne Goldenberg The Guardian 12 Feb 15;

The US south-west and the Great Plains will face decade-long droughts far worse than any experienced over the last 1,000 years because of climate change, researchers said on Thursday.

The coming drought age – caused by higher temperatures under climate change – will make it nearly impossible to carry on with current life-as-normal conditions across a vast swathe of the country.

The droughts will be far worse than the one in California – or those seen in ancient times, such as the calamity that led to the decline of the Anasazi civilizations in the 13th century, the researchers said.

“The 21st-century projections make the [previous] mega-droughts seem like quaint walks through the garden of Eden,” said Jason Smerdon, a co-author and climate scientist at Columbia University’s Lamont-Doherty Earth Observatory.

Researchers have long known that the south-west and Great Plains will dry out over the second half of the 21st century because of rising temperatures under climate change.

But this was the first time researchers found those droughts would be far worse even than those seen over the millennia.

The years since 2000 give only a small indication of the punishment ahead. In parts of Arizona, California, Nevada, New Mexico, Oklahoma and Texas, 11 of those years have been drought years.

As many as 64 million people were affected by those droughts, according to Nasa projections.

Those conditions have produced lasting consequences. In California, now undergoing its fourth year of drought – and the worst dry spell in 1,200 years, farmers have sold off herds. Growers have abandoned fields. Cities have imposed water rationing.

But future droughts could be even more disruptive, because they will likely drag on for decades, not years.

“We haven’t seen this kind of prolonged drought even certainly in modern US history,” Smerdon said. “What this study has shown is the likelihood that multi-decadal events comprising year after year after year of extreme dry events could be something in our future.”

The study, Unprecedented 21st-Century Drought Risk in the American Southwest and Central Plains, was published in a new online journal Science Advances.

The researchers said the effects of drought would likely be exacerbated by population growth in the south-west and rising demands for water.

Already current demands for water – for agriculture and for daily life – have drastically reduced groundwater sources in California and across the south-west.

Under the current warming trajectory, the south-west and Great Plains could expect to see chronic water shortages, making it impossible to carry out farming and ranching under current methods.

“Given the likelihood of a much drier future and increasing water resources demand, groundwater loss and higher temperatures will likely exacerbate the impacts of future droughts, presenting a major adaptation challenge,” the paper said.

The researchers used data derived from tree rings, whose growth patterns show the effects of dry and wet years, sampled across North America, and soil moisture, rainfall and evaporation records, and 17 climate models to study the effects of future temperature rise on the region.

Study sees even bigger longer droughts for much of US West
SETH BORENSTEIN Associated Press Yahoo News 12 Feb 15;

SAN JOSE, California (AP) — As bad as recent droughts in California, the Southwest and the Midwest have been, scientists say far worse "megadroughts" are coming — and they're bound to last for decades.

"Unprecedented drought conditions" — the worst in more than 1,000 years — are likely to come to the Southwest and Central Plains after 2050 and stick around because of global warming, according to a new study in the journal Science Advances on Thursday.

"Nearly every year is going to be dry toward the end of the 21st century compared to what we think of as normal conditions now," said study lead author Benjamin Cook, a NASA atmospheric scientist. "We're going to have to think about a much drier future in western North America."

There's more than an 80 percent chance that much of the central and western United States will have a 35-year-or-longer "megadrought" later this century, said study co-author Toby Ault of Cornell University, adding that "water in the Southwest is going to become more precious than it already is."

Megadroughts last for decades instead of just a few years. The 1930s Dust Bowl went on for more than 35 years, Ault said.

The study is based on current increasing rate of rising emissions of carbon dioxide and complex simulations run by 17 different computer models, which generally agreed on the outcome, Cook said.

The regions Cook looked at include California, Nevada, Utah, Colorado, New Mexico, Arizona, northern Texas, Oklahoma, Kansas, Nebraska, South Dakota, most of Iowa, southern Minnesota, western Missouri, western Arkansas, and northwestern Louisiana.

Looking back in records trapped in tree ring and other data, there were megadroughts in the Southwest and Central Plains in the 1100s and 1200s that lasted several decades, but these will be worse, Cook said. Those were natural and not caused by climate change, unlike those forecast for the future, Cook said.

Because of changes in the climate, the Southwest will see less rain. But for both regions the biggest problem will be the heat, which will increase evaporation and dry out the soil. The result is a vicious cycle: The air grows even drier, and hotter, Cook said.

Scientists had already figured that climate change would increase the odds of worse droughts in the future, but this study makes it look worse and adds to a chorus of strong research, said Jonathan Overpeck, co-director of the Institute of the Environment at the University of Arizona.

"These results are not surprising, but are eye-opening nonetheless," said Overpeck, who wasn't part of the research, in email.

Science Advances: http://www.scienceadvances.org