The 50 years ending in 2000 were the Southwest's warmest for at least 600 years, underscoring studies showing that human-caused climate change is heating up the region.

But the area's climate experts can't say the area's continued drought is also due to long-term climate change.

While the Southwest suffered its worst drought from 2000 to 2010 since at least 1900, several much longer and more severe "megadroughts" lasting up to 60 years occurred between the years 100 and about 1600. These are conclusions of the first Southwest Climate Assessment, a document prepared by more than 100 researchers around the West and released in summary form this week. The assessment is a regional version of global climate reports issued regularly by the International Panel on Climate Change, a group established by the United Nations.

The new report was released at a Southwest Climate Summit here, attended by about 120 representatives from federal and state governments, nonprofit groups, tribes and universities. The full version will be released in August.

Despite uncertainty about the current drought's causes, the new climate report predicts that climate change will push the region into more severe droughts in this century. They will bring continued declines in river flows, late winter snowpack and soil moisture.

In fact, Brad Udall, a Colorado researcher and native Tucsonan who wrote a chapter for the new assessment, warns that declines in the Colorado River could be worse than what has been predicted in studies to date. The river, which supplies much of Tucson's drinking water, showed a sign of vulnerability to drought two years ago when for the first time on record, demand for water from the 25 million people living in this region exceeded the Colorado's annual supply.


The report compares late-20th-century temperatures to those shown by tree-ring data over the previous 600 years in a half-dozen studies, including one from the San Francisco Peaks area north of Flagstaff. These results don't cover the decade ending in 2010 - which was hotter than any decade in the 20th century. For the future, the report sees temperatures rising by a range of 1 to 4 degrees from 2021 to 2050, 1 to 6 degrees from 2041 to 2070 and 2 to 9 degrees from 2070 to 2099.

These findings were no surprise to Jonathan Overpeck, director of the University of Arizona's Institute of the Environment and the study's lead author.

"This part of the country is warming faster than any other part outside of Alaska in the last 100 years," Overpeck said.

The findings don't by themselves prove global warming. But they bolster confidence in numerous studies done around the world that have used computer models to show that greenhouse gas emissions are linked to warming weather, said Jeff Lukas, a University of Colorado scientist who worked on the new report. Its results go back only 600 years because of a problem in getting tree-ring data relating to temperatures farther back, he said.


Martin Hoerling, a federal meteorologist who spoke at the conference, said indications show this current drought is more likely due to cyclical causes than climate change. That's partly because the drop in rainfall over the past 30 years follows a very wet period, he said. The global climate studies used by the international climate change panel to project future climates showed no sign that a drying trend would occur this soon, he said. "This is not what the models say climate change will do at this point in history," said Hoerling, adding that the recent drought's strength appears consistent with natural phenomena such as El Niño and La Niña.

Yet this current drought has the "heavy fingerprint of warming," said the UA's Overpeck, since it is warmer than the known megadroughts. In recent years there's been increased springtime, northward movement of storm patterns away from this region - a phenomenon predicted by climate computer studies, he said.

"We had it this year very clearly when the storm tracks went north of Colorado and Nevada. Colorado had its driest spring in the instrumental record," he said. Because the Central Arizona Project bringing water to Tucson gets much of its supply from Northern states, "that matters big-time to us - more than what is going on down here," Overpeck said.


In a talk to the climate summit, Udall, director of the federally funded Western Water Assessment, pointed to five studies since 2004 that predicted annual river flows would decline by 4 percent to 22 percent by 2050. An engineer and native Tucsonan whose father, Rep. Morris K. Udall, helped bring the CAP to Tucson, Udall said he feels strongly that the science used to produce these studies "is not informing us of the potential future risks" to the river's supply.

One reason, he said, is that none of the studies predicts that climate change will ever lower the river's flows below the lowest that have occurred in recorded history. Second, the northward movement of storms that causes today's dry weather also occurred in the megadroughts, suggesting that current flows could decline more, he said.

Third, in Australia, which recently had a 10-year drought, stream flows were reduced around 50 percent, even with "relatively modest" declines in precipitation of 10 percent to 20 percent, Udall said. While Australia lies in a different hemisphere, he said both areas sit within a longitudinal zone that is prone to dry weather.

Summing up, he said the current river studies have too narrow a range of possible future flows, both on the dry and wet sides, "and the risk on the Colorado is on the dry side. I think we trust these computer models too much. There is ample reason to believe, based on fundamental climate science, that the future could be significantly worse."

In response, a federal scientist who has worked on river studies, Michael Dettinger, said that because natural river systems on which the studies are based are so complex, it can be harder to get below the past low water levels when making predictions. While not criticizing Udall's stance, Dettinger, of the U.S. Geological Survey, said these climate computer models are so complex that a river's decline will not be the same, proportionately, for a 1-degree temperature warming as for a 9-degree warming.

Contact reporter Tony Davis at or 806-7746.