More people, more concrete, and lots more heat in Phoenix

An 'urban heat island' effect, fed by the city's growth, is trapping heat and making temperatures soar.

By , Staff writer of The Christian Science Monitor

Arizona is poised to take another record. It's about as unwelcome as a couple of other firsts – No. 1 in the nation for most illegal immigrants crossing the border, or 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 – particularly in the Phoenix metro area – 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 usage generates more.

It's called the "urban heat-island effect," and whatever the impact of global warming here, 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.

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"We're forecasting 111 for Wednesday, 109 for Thursday, and 110 again on Friday," says Keith Kincaid, a forecaster with the National Weather Service here. 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. And you'd have to fry a lot of them: 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, for example, 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, 2007), 21.9 per year, according to the National Weather Service.

For Westerners living here, 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 US. Many people have swimming pools, and most have air conditioning. But that, too, adds to the problem of 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 in Tempe. "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.

No escape in the Phoenix nights

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

Dr. 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 six degrees warmer in Phoenix than at the Casa Grande Monument. By 2000, the temperature in Phoenix was 12 degrees higher. Now, it is almost 14 degrees warmer in the city than in the adjacent rural areas.

That has a huge impact on water consumption and electricity generation, he says. Researchers in his department recently calculated the correlation between nighttime temperatures and water consumption. "A one-degree nighttime [temperature] increase equals 677 gallons more on average per household per year," he says – due as much to evaporation from pools, irrigation, and agriculture as to human consumption. Golden and his colleagues study these rises in temperatures for urban areas from here 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."

Looking toward solutions

Here in the Phoenix area, for example, 40 percent of the heat-island effect is due to paved surfaces, according to Golden. "We're trying to transition to pervious pavement, which would allow for water penetration," he says.

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.

"If we were to take all the surfaced parking lots in this city and cover them with 50 percent tree cover," that would significantly decrease the surface temperatures, he says. His department is also studying the survival methods of this area's early inhabitants, such as the Hohokam with their earthen structures.

Today, two-story houses are popular, he says. But what if policymakers were to ban future building of two-story houses – or at least upper floors – in order to make buildings shorter, and less prone to trapping heat. Instead, housing plans could include basements, he says, which would naturally remain cooler – though the prospect of lower levels has long been considered too expensive or difficult, despite the plethora of inground pools.

The good news about these rises in temperatures, if there is any, Golden says, is that local governments are beginning to pay attention to how they design cities, how closely they space houses, and how much forestry and agriculture they plan.

Phoenix, for example, is pushing for more open-space parks with trees downtown. And the city of Mesa is offering $500 rebates to residents who convert their yards from lawns to xeriscape, including desert trees that provide canopy shade.

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