"The 30s and 50s droughts lasted at most eight or nine years. We're talking about something here that is a drier overall climate," Richard Seager, the study's lead author, and a research scientist at the Lamont-Doherty Earth Observatory in New York, said in an interview.

"Once it's established, you're not going to be expecting precipitation to be turning back to levels we are familiar with in the later part of the 20th century."

Any long droughts could stress the agriculture industry, already suffering from dryness that began in 1999, and cities including Las Vegas and Phoenix. It could also affect cross border US-Mexico relations, Seager said.

Water is already in short supply in the region with some places, such as Tucson, Arizona, relying primarily on supplies left over from the last ice age some 20,000 years ago.

The effects of long and severe drought on the largest water users in the region, agriculture and cities, will depend on how dwindling water resources are managed, said Seager.

Billions of dollars in water projects are planned or underway in the region.

The study looked at 19 global climate models used in the latest UN's Intergovernmental Panel on Climate Change report on global warming. It assumed emissions of tailpipe and smokestack greenhouse gases would increase until 2050 and then decrease moderately.

As the planet warms, wet climate systems and the Hadley Cell, which carries heat and moisture from the tropics to the northern and southern mid-latitudes, shift toward the Earth's poles, leaving the Southwest drier, according to the study.

A sudden sweeping shift to clean technologies such as wind and solar power or rapid success in burying CO2 emissions from coal plants in places like the United States, China and India could mean C02 emissions fall sooner and ease drought threats.

But that is unlikely given the infancy of such technologies. Washington predicts fossil fuels will remain the main source of energy until at least the mid-century.