Air conditioning turns up city heat


Phoenix at night. Air conditioning is making the hot city even hotter at night, scientists say. Photo by Alan Stark/flickr.

June 9, 2014

Increasing use of air conditioners to stay cool is having the vicious circle effect – especially at night – of worsening the problem of cities getting hotter as the climate changes, say US researchers

By Tim Radford
Climate News Network

LONDON − 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.

Lethal heat

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.

Biggest difference always at night

Arizona scientists simulated a 10-day period of unusually hot weather in July 2009 and used computer models and detailed readings from weather records to analyze the effect of air conditioning systems on local temperatures. While the biggest demand for air conditioning was in the daytime, the biggest difference was always at night, they found.

"Our work demonstrates one Celsius degree local heating of urban atmospheres in hot and dry cities due to air conditioning use at night time," said Francisco Salamanca, the report's lead author. "This increase in outside air temperature in turn results in additional demands for air conditioning.

"Sustainable development and optimization of electricity consumption would require turning wasted heat from air conditioning into useful energy, which can be used inside houses for various purposes − including, for example, water heaters."

Such actions would reduce local air temperatures: In Phoenix alone, they could directly save more than 1,200 Megawatt hours of electricity per day.

More hot days ahead

In 2012, the United States experienced a set of record-breaking temperatures, and the U.S. Department of Energy has warned that days of extreme heat are expected to become more frequent and more intense because of climate change.

But this seems already to be a pattern worldwide, according to recent analyses of climate patterns. And the demand for air conditioning is expected to accelerate in India, China and other emerging economies.

Tim Radford is an editor of Climate News Network, a journalism news service delivering news and commentary about climate change for free to media outlets worldwide. 

The Daily Climate is an independent, foundation-funded news service covering energy, the environment and climate change. Find us on Twitter @TheDailyClimate or email editor Douglas Fischer at dfischer [at]

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