Air conditioning turns up city heat

Tim Radford in the Daily Climate: Researchers in the United States have identified how city-dwellers inadvertently stoke the heat of the night: By installing air conditioners. At night, heat from air conditioning systems now raises some urban temperatures by more than 1ºC.

Because the cities are getting hotter as the climate changes, residents are increasingly investing in air conditioning systems − which discharge heat from offices and apartment blocks into the city air. The vicious circle effect is that cities get still warmer, making air conditioning all the more attractive.

According to scientists at Arizona State University, the air conditioning feedback loop is now having a measurable effect. During the days, the systems emit waste heat, but because the days are hot anyway, the difference is negligible. At night, heat from air conditioning systems now raises some urban temperatures by more than 1 degree Celsius, they report in the Journal of Geophysical Research Atmospheres.

The team focused on the role of air conditioning systems in the Phoenix metropolitan area in Arizona's Sonora Desert, where summer conditions are harsh there anyway. But worldwide, normally warm countries are experiencing increasing extremes of heat, and conditions in cities have on occasion become lethal.

To cap this, cities are inevitably hotspots – and not just because of global warming. The concentration of traffic, pavement, lighting, central heating, industry, buildings and activity associated with millions of people can raise temperatures as much as 5°C above the surrounding countryside.

At present, 87 percent of U.S. households have air conditioning, and the United States – which is not one of the warmer nations – uses more electricity to keep cool than all other countries of the world combined. To keep Phoenix residents cool during periods of extreme heat, air conditioning systems can consume more than half of the region's total electricity needs, straining power grids....

Airconditioning units atop the Fairmont Hotel in Dallas, Texas, shot by Joe Mabel, Wikimedia Commons, under the Creative Commons license 3.0