A new study by Arizona State researchers, published on May 28 in the Journal of Geophysical Research, concluded that that so much wasted heat is emitted by air conditioning units in Phoenix that it actually raises the city’s outdoor temperature at night by between 1 and 2.7 degrees.

Like many cities, Phoenix is an urban heat island, which experiences significantly higher temperatures than its less-developed surroundings. One of the prime causes is the replacement of trees and grass with rooftops and pavement, which make up 60 percent of the surface area of a typical city. Those usually dark surfaces absorb about 80 percent of sunlight, causing them to heat up.

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In addition, human activities that generate heat, such as industrial plants and vehicles, also contribute to the effect.

This image, created by data from a NASA satellite in 2000, shows the concentration of rooftops and paved surfaces (in blue) in the Phoenix urban area. Vegetation is green and soil is red.NASA

But Phoenix is an example of a paradoxical aspect of the urban heat island, in which urban dwellers actually heat up the outside environment as a result of our efforts to stay cool indoors, and expend enormous amounts of energy in the process. That effect is particularly noticeable at night.

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According to this analysis by Arizona State’s Global Institute of Sustainability, nighttime temperatures in Phoenix have risen significantly over the past 40 years. Prior to 1970, there were no recorded instances of nighttime low temperatures exceeding 90 degrees Fahrenheit or higher after dusk in Phoenix. Between 2000 and 2009, there were 50 such hot nights.

The authors of the new Journal of Geophysical Research article write that Phoenix could save as much as 1,300 megawatt-hours per day by reducing the heat island effect.

Photo: Thinkstock