Droughts in the U.S. Southwest and Central Plains during the second half of the 21st century could be drier and longer than any droughts seen in those regions during the past 1,000 years. In a new study published in the journal Science Advances on Feb. 12, 2015, scientists found that continued increases in manmade greenhouse gas emissions will drive up the risk of severe droughts. The study is based on projections from several climate models, including one sponsored by NASA.
“Natural droughts like the 1930s Dust Bowl and the current drought in the Southwest have historically lasted maybe a decade or a little less,” said Ben Cook, climate scientist at NASA’s Goddard Institute for Space Studies and at Columbia University. “What these results are saying is we’re going to get a drought similar to those events, but it’s probably going to last at least 30 to 35 years.”
According to Cook and colleagues, the current likelihood of a megadrought—one lasting more than three decades—is 12 percent. If greenhouse gas emissions level off by the middle of the 21st century, the likelihood of megadrought will be more than 60 percent. If greenhouse gas emissions continue to increase along current trajectories throughout the 21st century, there’s an 80 percent likelihood of a decades-long megadrought in the Southwest and Central Plains between the years 2050 and 2099.
The scientists ran 17 climate models using both emissions scenarios and then analyzed a drought severity index and two soil moisture data sets. The high-emissions scenario projects an atmospheric carbon dioxide concentration of 1,370 parts per million (ppm) by 2100, whereas the moderate emissions scenario projects the equivalent of 650 ppm by 2100. Currently, the atmosphere contains 400 ppm of carbon dioxide.
In the American Southwest, climate change would likely cause reduced rainfall and increased temperatures that will evaporate more water from the soil. In the Central Plains, drying would largely be caused by the same temperature-driven increase in evaporation.
“What I think really stands out in the paper is the consistency between different metrics of soil moisture and the findings across all the different climate models,” said Kevin Anchukaitis, a climate scientist at the Woods Hole Oceanographic Institution who wasn’t involved in the study. “It’s rare to see all signs pointing so unwaveringly toward the same result.”
Cook’s study is the first to compare future drought projections directly to drought records from the past 1,000 years. Modern measurements of drought indicators go back about 150 years. Cook and his colleagues used a well-established database of tree-ring measurements from centuries-old trees to study older droughts. Tree species like oak and bristle-cone pines grow more in wet years, leaving wider rings; drought years bring narrow rings.