Megadrought predicted for U.S. Southwest and Central Plains

For the past four years, California has experienced a drought that has placed 27 percent of the state in “extreme drought” conditions and—even worse—a staggering 41 percent in “exceptional drought” conditions.

Cities are calling residents to limit their water use and many are in the process of instituting water theft as a criminal offense.

In a study published this month in the research journal Science Advances, researchers at NASA, Cornell and Columbia universities warn that current contributions to global warming will cause the Southwest and Central Plains’ drought to worsen into a “megadrought,” an arid period spanning decades, in the second half of the 21st century.

In effect, these states will experience a drought more severe than any in the past 1,000 years that is expected to last at least 30 years.

Lead researcher Benjamin Cook and colleagues examined data from different methods of climate monitoring that used soil moisture indices of two depths and tree ring analysis along with computer models such as General Circulation Models (GCMs) to reconstruct the global climate patterns of the last millennia (1000 to 2005).

Beyond one millennia, these measurements’ accuracy degrades. Cook and colleagues write, “This would also allow us to establish the potential risk of future conditions matching or exceeding the severest droughts of the last millennium.”

GCMs take into account a number of factors like ocean movement, atmospheric data, soil moisture and more. For factors that have less direct measurements, such as tree ring data, the scientists relied upon two metrics called the Palmer Drought Severity Index (PDSI) and the North American Drought Atlas (NADA).

Meteorologists often use the data to forecast weather; researchers use it to predict and understand climate change.

“Here, we have demonstrated that the mean state of drought in the late 21st century over the Central Plains and Southwest will likely exceed even the most severe megadrought periods of the Medieval era in both high and moderate future emissions scenarios,” Cook said.

The team constructed 17 GCM models based on the collected data to find this trend. Tree ring analysis reveals the amounts of precipitation and soil moisture found in the environments of ages past. The study’s findings indicate that the Southwest and Central Plains have encountered megadroughts before, but the coming megadrought will be more severe than any previous ones in the last millennium.

Many reservoirs are already showing massive bathtub-like rings on their sides, showing the drop in water levels over the years.

The combination of dropping water levels in reservoirs like Arizona’s Lake Powell, depleting groundwater sources, and higher temperatures have many Californians and other residents in the Southwest and Central Plains on edge about the future.

“Combined with the likelihood of a much drier future and increased demand, the loss of groundwater and higher temperatures will likely exacerbate the impacts of future droughts, presenting a major adaptation challenge,” said Cook.