Texas almost certainly faces a future of perpetual drought as bad as the record dry years of the 1950s because of global warming, climate scientists say in a study published Thursday.

The trend toward a drier, hotter southwestern United States, including all of Texas, probably has already begun and could become strikingly noticeable within about 15 years, according to a study led by Richard Seager of Columbia University's Lamont-Doherty Earth Observatory.

Drought conditions are expected to resemble the Dust Bowl years of the 1930s and Texas' worst-ever drought of the 1950s, Dr. Seager said. Unlike those droughts, however, the new conditions won't be temporary, the study found.

Lamont-Doherty Earth Observatory

A map shows the projected drought area, with darker colors noting more severe effects.

"This time, once it's in, it's in for good," Dr. Seager said.

The drought projections come one day before the Intergovernmental Panel on Climate Change, the world body on global warming, releases its latest report on the effects of a changing climate. The panel's reports summarize published science on global temperature trends and environmental effects.

The latest IPCC report is expected to outline already observed global trends, such as the loss of polar ice, changes in rainfall and hurricanes, and shifts in crops and natural habitats, as well as projecting future trends based on increasingly sophisticated computer projections of the coming climate.

What the future climate will be remains uncertain, since many factors such as greenhouse gas emissions trends could change. For that reason, climate scientists call their scenarios for the future projections instead of predictions.

The IPCC, reflecting the bulk of scientific findings, concluded that it is "very likely" -- defined as greater than 90 percent -- that the observed warming since the mid-20th century was due to human influences. The panel says atmospheric concentrations of carbon dioxide, the chief greenhouse gas, are higher than at any time in the past 650,000 years, and probably 800,000 years.

The IPCC projects a further increase in global average temperatures of 3.2 to 7.2 degrees Fahrenheit during this century.

Previous studies have projected an increase in drought conditions in the arid southwestern United States and northern Mexico as average temperatures rise and large-scale humidity patterns change, but the Columbia-led study, published in Thursday's issue of the journal Science, fills in numerous details. Researchers from the federal government's National Oceanic and Atmospheric Administration, Princeton University, the private National Center for Atmospheric Research, and Tel Aviv University also participated.

CLINT GRANT / DMN

White Rock Lake was dry at the height of a 1956 North Texas drought.

The Columbia research team examined the output of 19 climate models that made a total of 49 projections of future rainfall, temperatures and evaporation in the Southwest. All but the three of the projections concluded that the region will face a serious increase in drought conditions as early as 2021.

"The models agree strongly on this," Dr. Seager said.

The study examines the expected changes in precipitation in North America in 2021-2040 compared to 1950-2000. It projects much wetter conditions in the polar regions, with the precipitation decreasing steadily toward the southwest.

All of Texas would receive significantly less rain, while the deficit in Arizona, western Mexico, the Yucatan peninsula and nearly all of Central America would see extreme drought.

Texas' chief water planner said the drought projections do not change the state's forecasts of future water needs. State law already requires the Texas Water Development Board to assume that future conditions will match the worst drought on record, the 1950s dry spell, said William Mullican, the agency's planning director.

"Besides, the plan is reviewed and updated every five years, so if there's a change, we can adjust," he said.

However, a leading opponent of the water board's extensive plans to build new reservoirs said a hotter, dryer Texas should dictate a new emphasis on conservation instead of creating more lakes to catch rain that will become increasingly scarce. Opponents contend that the state water plan far exaggerates future water needs and underplays the potential for reducing water use.

"The water plan, as it is structured now, has virtually no conservation," said Janice Bezanson, executive director of the Texas Committee on Natural Resources, an Austin-based nonprofit group. "If we're going to have a permanent drought, people really will do what's necessary to conserve."