More concrete, more heat in Phoenix

Wednesday, Aug. 29, 2007 | 10 p.m.

PHOENIX -- Arizona is poised to take another record. It's about as unwelcome as a couple of other firsts -- No. 1 in the nation for illegal immigrants crossing the border and No. 1 in the nation for identity thefts.

This “one” directly corresponds with another No. 1 -- its status as the fastest-growing state in the nation. While news of global warming becomes as common as the wheeze of air conditioners here, Phoenix is fighting a different, if related, problem. In part because of heavy growth, heat is being reflected, trapped and absorbed in concrete, rooftops and a maze of buildings that blocks wind. At the same time, there's little vegetation to absorb the heat, and high energy use generates more.

It's called the “urban heat-island effect,” and, whatever the effect of global warming, this phenomenon is sending the mercury rising. On Tuesday, Phoenix tied the all-time record of 28 days at 110 degrees or greater in one summer, reached in 1979 and again in 2002. If the temperature rises to 110 degrees one more day this year, Phoenix will set a record.

“We're forecasting ... 109 for Thursday and 110 again on Friday,” says Keith Kincaid, a National Weather Service forecaster. But if the temperature doesn't hit 110 on those days, he adds, “we have had 110-degree days in September before.”

This summer is hot elsewhere, to be sure. But in few places can you fry an egg on a sidewalk as quickly and thoroughly as you can here. Experts say the main reason the number of 110-degree-or-higher days has risen so steadily -- and steeply -- is rapid growth. In the 1950s, the temperature rose to 110 or higher an average of 6.7 days per year. In the 1960s it was 10.3 days per year; in the 1980s it was 19 days per year; and in the 2000s (through Aug. 21), 21.9 per year, according to the Weather Service.

For Westerners, it's about as much fun as an earthquake, a drought or, well, a 110-degree day. But it does have people's attention. True, it's not as difficult as this summer's devastating floods or fires elsewhere in the U.S. Many people have swimming pools, and most have air conditioning. But that adds to the heat-island effect, experts say.

“Every time you use that mechanical air conditioner, you're throwing hot air back into the environment,” says Jay Golden, an expert on urban climate and energy at Arizona State University. “It's not only the sun and the pavement, but we're generating more heat because of human adaptation.” And that's where global warming comes in: The hotter it is, the more we need to cool off; and the more we try to cool off -- with air conditioning, for instance -- the more heat-trapping greenhouse gases and “waste energy” we create, feeding both phenomena.

The lows at night are rising, too. Three decades ago, the nighttime low was about 30 degrees cooler than the day's. Today, it is on average only 20 degrees cooler. That's because cities are slower to cool at night, retaining heat in roads and buildings.

Golden points to differing temperatures between downtown Phoenix and a rural weather station at the Casa Grande National Monument, about 50 miles southeast. In 1950, he says, it was only 6 degrees warmer in Phoenix than at the monument. By 2000, the temperature in Phoenix was 12 degrees higher. Now, it is almost 14 degrees higher in the city than in rural areas.

That has a huge effect on water consumption and electricity generation, he says. Researchers at Arizona State recently calculated the correlation between nighttime temperatures and water consumption.

“A 1-degree nighttime increase equals 677 gallons more on average per household per year,” he says -- as much because of evaporation from pools, irrigation and agriculture as to human consumption. Golden and his colleagues study these rises in temperatures for urban areas from Phoenix to London and Beijing.

“We are trying to do two things,” Golden says. “One is to quantify the impacts from this national trend of climate change in the broad context … Then, we try to provide policymakers sound science and engineering to understand what the impacts are.”

In the Phoenix area, for example, 40 percent of the heat-island effect is because of paved surfaces, Golden said. “We're trying to transition to pervious pavement, which would allow for water penetration.”

That, he adds, would support the growth of urban vegetation, which is typically removed for new building projects. And urban vegetation planted at intervals, as well as the water pervious pavement retains, would lead to cooler temperatures at night.