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Hot cities more sustainable than cold ones, study says

A woman walks her dog in Minneapolis. Indoor energy demands in the chilly city are higher than cooling demands in Miami, according to a new study.
A woman walks her dog in Minneapolis. Indoor energy demands in the chilly city are higher than cooling demands in Miami, according to a new study.Reuters file

When it's hot outside, people crank up air conditioners that usually suck electricity from coal- and natural gas-fired power plants at the root of human-caused global warming. This seems like a recipe for disaster, but it's more sustainable than living in a cold climate and cranking up the heat, a new paper suggests.

"The traditional view that living in hot desert areas is not sustainable should be re-examined," Michael Sivak, a research professor at the University of Michigan, Ann Arbor, told NBC News. "Because my data suggest that from this point of view — mainly a climate control point of view — living in very cold areas is less sustainable than hot areas."

He compared the energy demands for indoor heating and cooling in Minneapolis, Minn., the coldest metropolitan area in the country, with those in Miami, Fla., the warmest big city. He found the demands are 3.5 times greater in Minnesota.

The biggest factor in his comparison is the number of heating or cooling days per year, which reflects the demand for energy needed to heat or cool a building. The measure is calculated by comparing the mean daily outdoor temperature with 18 degrees Celsius (64.4 degree Fahrenheit). So, for example, a 10 degree Celsius day corresponds to 8 heating degree days. A 25 degree Celsius day corresponds to 7 cooling degree days. In earlier research, Sivak found that Minneapolis has 4,376 heating degree days and Miami has 2,423 cooling degree days per year.

"The need for heating in Minneapolis is more energy demanding than cooling in Miami because the difference of the ambient temperature from the desired temperature is greater in Minneapolis than in Miami," Sivak explained.

His comparison also included:

  • the efficiencies of heating and cooling appliances (a typical air conditioner is about four times more energy efficient than a typical furnace or boiler primarily because it takes more energy to heat up a room than it does to cool it); 
  • and the efficiencies of power plants, which generate nearly all the electricity used in cooling and 7 percent for heating. ("In terms of power plant efficiencies, cooling is worse than heating," he noted). 

When all three parameters are taken into consideration, including cooling days in Minnesota and heating days in Miami, Sivak found that Minneapolis is 3.5 times as energy-demanding as Miami.

The study doesn't examine what happens as the planet warms, and thus fewer heating days are needed in places such as Minnesota, Buffalo, N.Y., and Portland, Ore., and more cooling days are required in Miami, Phoenix and Las Vegas, but the finding may be a silver lining of global warming.

"Proportionately, you would be shifting the needs," Sivak said. "You would be heating less and you would be cooling more."

In fact, he noted in a paper published Wednesday in Environmental Research Letters, the impact of warm-city living may be even more pronounced than suggested by his calculations since "people are generally more tolerant of heat than of cold."

In other words, people are more likely to turn on their heater when there's a nip in the air than they are their AC when the temperatures begin to rise.

While all of this sounds reasonable, "you run up against basic physical constraints in a hot place that you don't in a cold place," Austin Troy, director of the transportation research center at the University of Vermont, told NBC News. Troy is also the author of The Very Hungry City, a book that illustrates the energy demands of living in warm climates.

For example, in a cold place you can build an passive solar house that uses very little energy to heat it, but similar options are lacking for people living in hot climates. And as the climate warms, in the "sun belt there'll be significantly increased cooling demands for the summer," he added.

John Roach is a contributing writer for NBC News. To learn more about him, visit his website