Rising temperatures will combine with decreased rainfall in the Southwest to create droughts that will be worse than the historic "Dust Bowl" of the 20th century and last far longer. The Dust Bowl lasted eight years and affected 100 million acres around the Texas and Oklahoma panhandles and adjacent lands in Kansas, Colorado and New Mexico. Dust storms swept through large swaths of farmland, depositing dust as far east as Chicago, New York and Washington. It is estimated that more than half a million people were made homeless.
|[Tributary of Colorado River]|
Currently, 62 percent of California—home to 39 million people—is under severe drought conditions. The state is entering its sixth year of drought, with record-low levels of precipitation and snowpack. Moderate or worse drought covers 45 percent of Arizona and 37 percent of Nevada. The water level in many of California's lakes and reservoirs remains below historic averages.
Much of the Southwest relies on the Colorado River and its tributaries for some or all of its water. The Colorado feeds critical farmland, public water supplies and helps generate hydroelectric power. Historically, the Colorado emptied into the Gulf of California. Today, what little remains of the Colorado River when it reaches Mexico has been diverted to irrigate the farms of Mexicali Valley. The Colorado River is one of the most dammed and diverted rivers on the planet.
The Colorado River supplies 55 to 65 percent of water for Southern California. It was reported last year that more people are entitled to Colorado River water than the river can supply.
In California's San Joaquin Valley, over-pumping of groundwater for irrigation has caused the land to sink—in some places, as much as one meter. Toxic algal blooms have affected more than 40 lakes and waterways this year. A shift in electric power generation in the state away from dwindling hydropower sources to natural gas has cost Californians $2 billion.
A megadrought occurring again in the Southwest in the coming decades would impose unprecedented stresses on water resources of the region. The report calls for a mix of policies, including reducing demand, greater water efficiency, and shifts to groundwater supplies, as well as further reductions in greenhouse gas emissions.
It might all be too little too late.
 Ault et al: Relative impacts of mitigation, temperature, and precipitation on 21st-century megadrought risk in the American Southwest in Science Advances – 2016