Wednesday, December 26, 2007

2007 one of Tucson's warmest

Arizona Daily Star: You wouldn't have known it if you just flew into Tucson for a vacation last weekend, but this year was one of the hottest on record here. The region's long-term drought, on the other hand, slacked off a bit this year, but Tucson is a long way from proclaiming that it is over.

…A longtime weather service meteorologist said the continued warm weather here is primarily a result of the urban heat-island effect, in which growth increases the number of buildings that block out the cooling air from night skies and provide multiple surfaces for absorbing and reflecting the sun's heat. Six of the 11 warmest years on record came since 1990. Three of the five warmest years came since 2000.

But two University of Arizona climate scientists said that long-term global warming caused by continued emissions of carbon dioxide into the air is another likely cause of the warming weather, although they cautioned that it's too early to say that for sure until more climate information rolls in.

Barring an unlikely year-end deluge, 2007 will end with with 9.78 inches of rain. That's down from 11.81 inches in 2006 and an annual average of 12.17 inches. Compared to spells when wildlife and trees were dying in the desert, 2007 seemed relatively benign, droughtwise. This is only the 38th-driest year on record in Tucson. But it will be the seventh straight year of below-normal rainfall here, said John Glueck, a National Weather Service meteorologist in Tucson.

More troublesome is that winter rainfall totals have run further below normal than those in the summer over the past decade or so. Because winter rains are more likely to seep into the ground than the fast-moving summer rains that swim quickly down Tucson's washes and streams, the lack of winter rains makes the forests more vulnerable to fire. Lesser winter rainfalls also mean there's less water replenishing the Tucson underground aquifer, Glueck said.

…If the city is going to be in a long-term drought, officials must plan for it, said Glueck, adding, "It's going to be the big issue for the next five to 10 to 15 to 20 years. We've got to figure out something to do with the water."

…But it isn't just Tucson that's warming, it's the Southwest in general, including non-urban areas, countered Jonathan Overpeck, a climate scientist and director of the University of Arizona Institute for the Study of Planet Earth.

"There is little doubt that what we're seeing is something broader than can be ascribed to urban heat-island warming alone," Overpeck said in an e-mail interview. "The big question relates to what is driving the regional warming — is it human-caused global warming, or some natural variability?"

It's more likely human-caused warming because the higher temperatures are being accompanied by retreat of spring snowpack, decline in springtime river flows and drying of forests, said Overpeck, one of the scientists involved in the warming studies of the United Nations Intergovernmental Panel on Climate Change, which recently shared the Nobel Peace Prize with former Vice President Al Gore. All those phenomena are consistent with what global computer models have projected will happen because of the buildup of human-induced greenhouse gases, he said.

"Natural variability could be playing a role, too, but it's not clear what it is, and thus I suspect it is small," he said. "There is no known natural cycle that should be giving us the warming trend of the last couple decades, both in and outside of urban areas."…

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