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Winter nights have warmed significantly across Arizona over 70 years, raising questions about whether human-caused global warming is part of the cause, said a University of Arizona researcher.

From 1931 to 2001, average wintertime low temperatures rose by as little as 0.03 a of a degree per decade in Safford to as much as 1.11 degrees in Mesa, according to the UA-analyzed data.

Tucson's average winter low rose by about 0.7 of a degree per decade — or nearly 5 degrees over 70 years. That placed it sixth among 25 cities and towns analyzed.

All but Miami, Tombstone and Douglas showed higher temperatures over this period, according to the new analysis by UA's federally financed Climate Assessment for the Southwest program.

The warming's effects could ultimately range from the benign to the serious, according to several research-ers. There already are indications that growing seasons have lengthened and that a winter's first frost comes later and the last frost comes earlier in some parts of the state.

At Organ Pipe National Monument, for instance, the frost-free period has increased by a month since 1945. Eight of the 25 stations monitored for 70-year temperatures were found to have statistically significant longer growing seasons.

But researchers are also concerned that warming can aggravate the effects of drought by inducing more evaporation, and reducing the amount of moisture in soil. They say it could increase the length of the wildfire season and reduce the amount of runoff into rivers and streams and the recharge of rainfall into the aquifer.

"It's not just water. It's not just ecology. It's not just fire," said Gregg Garfin, a climatologist and program manager for the UA's Institute for the Study of Planet Earth, which monitors and studies climate change. "It's when you add it up. It's mutually reinforcing."

He plans to write up his temperature analysis for a report to be submitted later this year by a Forest Health Advisory Committee to Gov. Janet Napolitano's office.

 That temperatures have jumped in big cities is no surprise. Researchers already know that buildings, roads and other man-made structures can cause an "urban heat island effect" when steel, concrete and other materials absorb the sun's heat. That causes temperatures to rise.

But in this case, the winter temperatures rose in many towns as sparsely populated as Sacaton and Seligman. Only two of the 12 top-ranked locations, Mesa and Tucson, were in urban areas.

The heat island effect can't be ruled out as a factor, said Garfin. Part of the increase can be explained by natural, decade-to-decade variations in the weather caused in part by shifting ocean currents.

But separate tree-ring studies of the Colorado Plateau in Northern Arizona and southern Utah, and of an area stretching across the Southwest from Texas to California, found that the region's weather in the late 20th century was one of the warmest periods in hundreds of years, Garfin said.

Studies of snowmelt patterns across the West also show that natural variations aren't the whole cause, he said. If ocean currents were the only cause, temperatures in the Southwest and Northwest would be going in different directions, but they aren't, he said.

Andrew Ellis, Arizona's official state climatologist, said he believes that the heat island effect is a bigger factor than global warming in raising Arizona's temperatures. Even some rural temperature increases could have been caused by development right around the temperature-monitoring stations, he said.

But he said that global warming is a real concern for Arizona, particularly because of its potential effects on snowpack. While some computer models predict that warming temperatures may bring Arizona more future precipitation, all models that have looked at warming agree that less of it will be in the form of snow because of warmer temperatures, he said.

 "Liquid water from rain evaporates more easily than does snow," said Ellis, an Arizona State University geography professor. "If you have precipitation on the ground in the form of snow rather than water, the chance of being able to protect it from the atmosphere is greater."

Although the causes and significance of warming temperatures have been a subject of scientific debate for years, today most climate scientists in the United States agree that long-term warming is occurring and that at least some has human causes.

Even a longtime global-warming skeptic at ASU is today more sympathetic to the idea than before. A decade ago, climatologist Robert Balling was writing books questioning whether greenhouse gases had played a major role in warming temperatures.

This month, he said in an interview that he is "very comfortable" with saying that the buildup of greenhouse gases has caused global warming over the past 30 years and that there is a 100 percent chance that at least some of the warming in the West over the same period was human-caused.

But on a global scale, "you could argue we are recovering from a little Ice Age," he said. "Five hundred years ago, the world plunged into a little Ice Age and about 150 years ago we began to recover from the little Ice Age."

Legislators representing some of the rural areas affected by the warming Arizona temperatures had different reactions to them.

Rep. Tom O'Halleran, R-Sedona, said it's clear that the state needs to do a much better job in recognizing trends in both temperature and water supply.

"Any climate change will have a profound impact on our ability to provide the natural resources necessary to deal with population growth over the next 20 to 30 years," he said.

Sen. Robert Cannell, D-Yuma, said he is skeptical that global warming is human-caused but agreed that a temperature increase of this scale over 70 years is significant.

"I'm not saying it doesn't exist," Cannell said. "I do think that people who get grants from the government have an ax to grind from global warming. Maybe if they do research that says it has nothing to do with global warming or greenhouse gases, they do not get funded.

"I am trying to stay open to different ideas. Maybe this data will change my mind."

 

On StarNet Read the latest news about the environment and wildlife in Arizona at azstarnet.com/ wildlife