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.

from the August 30, 2007 edition

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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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