Two summers ago, California firefighters were brought in to battle a blaze in Oak Creek Canyon, north of Sedona. Climate change is to blame for the prevalence of dry-season wildfires in the Southwest, UA researchers say, explaining that a storm-track shift has lengthened the dry period before the summer monsoon.

Kelly Presnell / Arizona Daily Star 2006

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The same climate shift that is melting polar ice is depriving the Southwest of critical spring moisture — lengthening the time between the last winter rain and the first summer thunderstorm, two UA researchers say.

The University of Arizona researchers have documented a decrease in late-winter and spring storms in the Southwest and have tied it to a previously documented northward shift in the storm track.

The wind shift and the precipitation decrease are subtle, but they're capable of producing dramatic change in a region that receives so little rain, said Stephanie A. McAfee, a doctoral candidate in geosciences.

Losing a storm or two each March and April lengthens the seasonal drought that puckers the cacti, melts mountain snow and dries out wildfire fuels, she said.

McAfee and Joellen L. Russell published their findings in Geophysical Research Letters, a journal of the American Geophysical Union.

A widening gap between storms at that critical time would explain "at least in part the increasing number of really catastrophic fires in the Southwest," said noted fire researcher Tom Swetnam, director of the UA Laboratory of Tree-Ring Research.

"Those last winter storms, while not numerically important for our annual rainfall total, are critical for the distance we have to go," said Russell, an assistant professor of geosciences at the UA.

McAfee said she wasn't surprised to find a decrease in rain. "It just stands to reason that if you're moving storms, you're moving precipitation," she said. What surprised her was that the effects of the shift are already easily measured.

McAfee's study examined storm and precipitation data in Southern California, Arizona, Nevada, Utah, western Colorado and western New Mexico. She chose a 20-year period because of the availability of good data, she said.

"I looked at the period from 1978 to 1998, looking at March and April in the Southwest. The overall decreases are not huge. There is not that much rain to start with. It's on the order of one or two storms a year."

One or two storms, though, can have a huge impact in arid climates, she said.

She now wants to study how that lengthening dry season affects the region's plants, using satellite photography and tree-ring measurements.

There are many reasons for the recorded increase in large forest fires in the West, Swetnam said, including suppression of fires over the past century that caused a buildup in burnable material. But warmer, drier springs have a "profound impact" on the potential for fire, he said.

Swetnam said he hasn't read McAfee and Russell's full report, but he said its conclusions are consistent with studies he and others have done. "Indeed, we've seen in fire-occurrence records across the West that it's correlated with earlier and earlier arrival of spring, and snowmelt occurring earlier."

The "scary part," Russell said, is that computer models predict the storm track will continue to shift northward.

The research was funded by the National Oceanic and Atmospheric Administration through Climas, the UA's Climate Assessment for the Southwest.