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 over the past 1,000 years. In a new study published in the journal Science Advances on February 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 is 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 is 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, while the moderate emissions scenario projects the equivalent of 650 ppm by 2100. Currently, the atmosphere contains 400 ppm of CO2.
The maps above depict soil moisture at 30 centimeters below the land surface in the years 2090 to 2999. The top image shows a high emissions, “business as usual” scenario, while the second image reflects reductions in greenhouse gas emissions by the middle of the 21st century. Brown areas are is drier than the 20th century average, while blue areas are wetter. The soil moisture data are standardized to the Palmer Drought Severity Index.
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 Institutionwho was not involved in the study. “It is 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.
“We can’t really understand the full variability and the full dynamics of drought over western North America by focusing only on the last century or so,” Cook said. “We have to go to the paleoclimate record—looking at these much longer timescales, when much more extreme and extensive drought events happened—to really come up with an appreciation for the full potential drought dynamics.”
Cook and colleagues were particularly interested in megadroughts that took place in North America between 1100 and 1300. These medieval-period droughts, on a year-to-year basis, were no worse than droughts in the more recent past. But they lasted, in some cases, 30 to 50 years. When these past megadroughts are compared side-by-side with computer model projections of the 21st century, both the moderate and business-as-usual emissions scenarios are drier and the risk of long droughts increases significantly.
“Those [medieval] droughts had profound ramifications for societies living in North America at the time,” Anchukaitis said. “These findings require us to think about how we would adapt if even more severe droughts lasting over a decade were to occur in our future.”
More than 64 million people are directly affected by drought in the Southwest and Southern Plains, and far more are indirectly affected because of the vast number of farms, orchards, and ranches that supply the rest of the United States.