This Is How Much Hotter Nights in Phoenix Are Because of A/C

Waste heat from air conditioners causes a significant amount of warming outside

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Thermal image of window mounted AC units in an apartment building.  Tyrone Turner/National Geographic Society/Corbis

Air conditioners can put ridiculously large demands on the electrical grid—accounting, in some extreme case, for as much as 65 percent of a region's electricity use. Yet making buildings cool inside means moving heat outside, and, due to mechanical inefficiencies, the air conditioners themselves add even more heat to mix. In a new study, researchers have figured out just how big of an effect air conditioners can make on the temperature outside.

In July 2009 a heatwave hit Phoenix, Ariz. The temperature outside soared, and those fortunate enough to have it hunkered down with their A/C.

According to the new study, led by Arizona State University's Francisco Salamanca, the heat output from all those buzzing air conditioners was highest during the day. But, compared to the blistering heat from the sun, it didn't make an appreciable difference.

At night, though, was another story. It can be hard to sleep in the heat, and many people keep the A/C running all night. Waste heat from all those air conditioners caused a noticeable amount of warming outside, the scientists say:

During the night (from 08 pm to 05 am), the effect of the AC systems was significant, increasing the mean 2 [meter]-air temperature between 1 °C [1.8 Fahrenheit] and 1.5 °C [2.7 Fahrenheit] for most of the urban area.

The excess heat churned out by the air conditioners raised the temperature by nearly 2 degrees, making the air conditioners themselves need to work that much harder.

“Our results demonstrate that releasing waste heat into the ambient environment exacerbates the nocturnal urban heat island and increases cooling demands,” the scientists write. The two degrees of warming from air conditioners seen in Phoenix lines up with previous research—A/C units in Tokyo, Paris, and Madrid are responsible for 1.8 to 2.3 degrees of warming.

During a heat wave everyone is fighting to stay cool. For some, their lives actually depend on it. Yet in a world where the climate is warming, how we choose to hide from the heat matters more than you might think. Air conditioners use power, and power typically means adding greenhouse gases to the atmosphere. Air conditioners might be the most effective way to cool down, but they're not the most efficient--especially if they're just fighting themselves.

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