The storm, which had swept away asphalt streets in Pasadena, Calif., also doused most of the western countryside from Colorado to the Virgin River Canyon in Arizona. Its rain, though devastating in some parts, was welcome. The southwestern United States has been in the grip of a drought that might be a record-setter - depending upon how one interprets the numbers.

Last week's rainfall notwithstanding, the drought is fueling arguments of both those who would say the climate is experiencing only natural variability, and those who contend such events are being induced by human-caused, greenhouse warming.

If regional precipitation patterns indeed are changing - whether or not a result of global warming - that is a critical issue in the arid West. Water is a scarce commodity there and it is hoarded in huge reservoirs for use during shortages.

The question of when and where water falls from the sky is of more than academic interest to residents of Los Angeles, San Diego, Phoenix, Denver, Tucson and other thirsty metropolitan areas in the region.

The western United States is experiencing a severe, multi-year drought that is unprecedented in some hydroclimate records, said Edward Cook, with the tree ring laboratory at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y.

If the climate is warming, these droughts could get worse, he said.

The critical thing is that the current drought has been going on for five years, Cook told UPI's Climate. It may end tomorrow or it may continue - we don't know.

Looking back about 1,000 years, however, to the so-called medieval warm period between about 900 A.D. and 1300 A.D., reconstructions indicate a much longer period of elevated aridity. At any given time, it was not more severe than we're seeing now, but the duration was greater, Cooke said.

These very long drought periods are possible because they have happened in the past, he added.

In a paper published in the journal Science in early October, Cook and colleagues wrote, If elevated aridity is a natural response to climate warming, then any trend toward warmer temperatures in the future could lead to a serious, long-term increase in aridity over western North America.

Cook said the drought appears to be related to the El Nino-Southern Oscillation of the tropical Pacific. The increased warming in the 20th century has changed the easterly trade winds, upsetting the east-west temperature gradient of the ocean by upwelling the deep currents, bringing colder water to the surface.

The cold (water) temperatures are correlated well with drought formation in the West, he explained. Recent warming is partly due to greenhouse gases, and leading to a more persistent development of La Nina conditions (the cold phase of El Nino) in the tropical Pacific.

As the oscillation part of ENSO implies, the tropical Pacific shifts from one phase to another, and these changes will not eliminate this shift. In fact, Cook said, It will probably shift back to warm.

Nevertheless, the long-term trends based on the historical data are worrisome.

In another paper, scheduled to be published soon in the Journal of Climate, Cook and colleagues report they have run a computer model based on solar forcing - the effect of the sun's activity on climate - back over time to try to duplicate the historical temperature record. They found the warming - which would have been natural in the past, rather than human caused - increased drought in the region.

This was a natural forcing, but it looks like it applies, Cook said. Any trend toward warming in the future - either natural or anthropogenic (human caused) could very well lead to more frequent and persistent drought in the United States.

This view about the greater potential of drought to grip the West is consistent with other research into the subject. For example, recent climate science indicates the current drought could last for decades, according to Jonathan Overpeck, director of the Institute for the Study of Planet Earth at the University of Arizona in Tucson.

The climate system can cross a threshold, as it has done many times in the past, Overpeck told Climate. The climate after the threshold is very different - drought and hydrologic changes can occur on a continental scale.

This could be precisely what is happening in the Southwest.

The drought we're now getting in the West may have crossed this threshold, Overpeck said. It can mean a drought that lasts as long as 25 or 30 years.

Recent climate modeling results from NASA's Goddard Space Flight Center in Greenbelt, Md., give some insight into why this change may be coming about. Siegfried Schubert, a senior research scientist at Goddard, has been able to connect the Great Plains drought cycle of the past 100 years with the observed record of sea surface temperatures.

If we know the sea surface temperatures, we found that the model does reproduce the major events in the Great Plains - in particular the Dust Bowl of the 1930s, Schubert told Climate. The combination of a cold Pacific and warm tropical Atlantic drove the Dust Bowl conditions.

Regarding the Southwest drought, Overpeck said: It is related to anomalous sea surface temperatures in the tropical Pacific and the Indian Ocean. It is pretty clear that those oceans have been warming steadily since pre-industrial times.

That has led some of my colleagues to conclude that the drought related to the sea surface temperatures might be unprecedented in post-industrial times.

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