Global Warming Brings Perpetual Drought to U.S. Southwest

NEW YORK, New York, April 5, 2007 (ENS) - Human-caused climate change is likely to lead to long periods of extreme drought throughout the American Southwest starting early this century, finds a new study released today by the Lamont-Doherty Earth Observatory, a member of The Earth Institute at Columbia University.

The researchers compared the coming drought to the Dustbowl of the 1930s that sent millions of environmental refugees fleeing to California from across the Great Plains.

In contrast to past droughts, future drying is not linked to any particular pattern of change in sea surface temperature but seems to be the result of "an overall surface warming driven by rising greenhouse gases," researchers said.

"The arid lands of southwestern North America will imminently become even more arid as a result of human-induced climate change just at the time that population growth is increasing demand for water, most of which is still used by agriculture," said Richard Seager, senior research scientist at the Lamont-Doherty Earth Observatory and one of the lead authors of the study.


Sprinklers moisten a field in arid Arizona, where agriculture depends on irrigation. (Photo courtesy USGS)
Projections of climate change caused by human activities conducted by 19 different climate modeling groups around the world, using different climate models, show widespread agreement that southwestern North America, and the subtropics in general, are heading toward a climate even more arid than it is today.

Appearing today in the journal "Science," the research shows that there is a broad consensus amongst climate models that this region will dry in the 21st Century and that the transition to a more arid climate may already be underway.

If these models are correct, the levels of aridity of the recent multiyear drought, or the Dustbowl and 1950s droughts, will, "within the coming years to decades, become the new climatology of the American Southwest," the researchers said.

"Our study emphasizes the fact that global warming not only causes water shortage through early snow melt, which leads to significant water shortage in the summer over the Southwest, but it also aggregates the problem by reducing precipitation," said Mingfang Ting, one of the study’s co-authors.


Desert near El Paso, Texas (Photo courtesy National Weather Service)
As the planet warms, the study documented how the Hadley Cell, which links together rising air near the Equator and descending air in the subtropics, expands toward the poles.

Descending air suppresses precipitation by drying the lower atmosphere, so this process expands the subtropical dry zones.

At the same time, and related to this, the rain-bearing mid-latitude storm tracks also shift poleward.

Both changes in atmospheric circulation, which are not fully understood, cause the poleward flanks of the subtropics to dry.

Other land regions expected to be affected by subtropical drying include southern Europe, North Africa and the Middle East as well as parts of South America.

"The West, and in particular, the United States and Mexico, need to plan for this right now, coming up with new, well-informed and fair deals for allocation of declining water resources," warned author Seager.

Las Vegas Valley Water District spokesman Scott Huntley says the study reinforces what Colorado River water users like the city of Las Vegas are experiencing right now. "Ground zero is right here in Las Vegas," he said.

"This really is the wake-up call to all of the desert Southwest to cooperate to develop solutions," said Huntley. "We have seen the effects in Lake Powell and in Lake Mead - we've seen that the usage of water along the Colorado has been greater than the flows for seven or eight years now. This has to be the incentive for states and water users to come together."


This 2003 photo of the white border around Lake Mead, a water storage reservoir behind the Hoover Dam, shows how full the reservoir once was. The lake level has fallen even farther since then. (Photo courtesy Las Vegas Valley Water District)
Agriculture is still by far the dominant water user - 85 percent of water useage in the desert Southwest is for agriculture, Huntley explained.

While water used to turn the arid Southwest into fertile fields brings vegetables, grains and fruits to market, up to 90 percent of the water used for irrigation in the Southwest is lost. It evaporates from the soil or transpires from crops and is not returned to its source for reuse.

Huntley says many solutions are possible if there is cooperation between states and between agriculture and municipalities.

Conservation and efficiency are important, he said, desalination is a possibility, and municipalities might purchase water rights from farmers to keep their fields fallow.

In addition, he said, states might go outside of the Colorado River basin to utilize groundwater.

Roger Manning, executive director of the Arizona Municipal Water Users Association, AMWUA, says there is already a drought in Arizona, but the association's member communities - Chandler, Gilbert, Glendale, Goodyear, Peoria, Phoenix, Mesa, Tempe, and Scottsdale - have been taking steps over the past few years to ensure that the worst effects of the drought will be mitigated.


Phoenix, Arizona is a a city of 1.46 million people. (Photo courtesy PDPhoto)
"The Phoenix metropolitan area has avoided a major water shortage this past year due to foresight of the Valley's water managers who, for the past 20 years, have been planning on how to address the inevitable drought," Manning says. "We are in better shape than most of the rest of the state, because we have four sources of water here in the Valley - pumped groundwater; surface water from the Salt, Verde, and Agua Fria rivers; Colorado River water; and reclaimed wastewater."

Legal restrictions on the use of pumped groundwater has resulted in additional reliance on the Colorado River as a water supply source, Manning explains.

"Here in central Arizona, we live in an area with the most progressive and rigorous groundwater code in the nation," he says.

In central and southern Arizona and the Prescott area, "municipal water conservation programs are required, and new residential developments for which there is not a 100-year assured water supply from primarily non-groundwater sources are not allowed," Manning says.

Since 1992, only low water use plumbing fixtures can be sold and installed in Arizona. These kinds of fixtures are used in all new construction and remodeling projects.

"Over the years, millions of dollars have been spent constructing water treatment plants, installing pipelines, drilling wells, and developing underground storage projects," said Manning.

Recently, the AMWUA has purchased water from the Central Arizona Project and stored it underground for recovery and use during a drought.

"We are treating wastewater to as high a level as possible so that it can be used to recharge the Valley's aquifers and to irrigate golf courses, parks, and other turf facilities. In fact," said Manning, "the biggest nuclear power plant in the country, the Palo Verde Nuclear Generating Station, uses treated wastewater for cooling purposes."

The Lamont-Doherty study, "Model projections of an imminent transition to a more arid climate in southwestern North America," is online at: