Vicious cycle: Air conditioning is making your city even hotter

As temperatures rise around the world, air conditioning demands will increase. This of course means big energy demands, which often means more carbon emissions, which means more warming—in other words, cooling your apartment is making it hotter outside. But that’s just the long-term effect; new research suggests that turning on your AC unit actually sends the mercury skyward on a short time scale as well, a cruel trick of physics that is helping to bake our cities.

“General AC systems absorb heat (cooling the indoor air) from the interior of the buildings and release heat into the surrounding outdoor environment,” write researchers from Arizona State University in the Journal of Geophysical Research: Atmospheres. An individual unit obviously doesn’t produce all that much ambient heat, but when you combine all the cooling in cities where it’s hot enough to cook dinner on the pavement, you can make a real difference.

The researchers modeled a system using both building energy schemes and climate inputs, and simulated a 10-day “extreme heat period” in the already-scorching city of Phoenix. They found that during the day, the contribution of AC ambient heat to city temperatures was negligible, but at night it played a significant role. When a building’s floor area was fully air conditioned, that raised the nearby temperatures by as much as 1.5 degrees Celsius (2.7ºF) during nighttime hours. When a more realistic 65 percent of a building was cooled, it still resulted in up to 1 degree of warming outside. In a place like Phoenix, every building has to be air conditioned, which means that heat gets added all over the city.

This extra warmth gets piled on top of the urban heat island effect, a concept which describes how cities tend to generate their own little heat bubble. A city of 1 million people can be as much as 3ºC hotter than the area immediately around it; obviously, the AC may be contributing to that already, but the new paper’s authors write that the effect they saw likely exacerbates it. It truly is a vicious cycle, with our need to cool off making it harder and harder to do so.

And importantly, it highlights the need for some creative thinking: “Sustainable development and optimization of electricity consumption in cities would require turning ‘wasted heat’ from AC into ‘useful energy’ which can be utilized inside houses for various purposes including, for example, in water heaters.” The study authors calculated that Phoenix could save as much as 1,300 megawatt-hours of energy every day with some of those creative AC reduction strategies; that’s enough to power around 130 average American homes—for an entire year. – Dave Levitan | July 29 2014

Source: Salamanca F, Georgescu M, Mahalove A, et al (2014). Anthropogenic heating of the urban environment due to air conditioning, Journal of Geophysical Research: Atmospheres, 119, 5949-5965. DOI: 10.1002/2013JD021225

Image:, NikD90