There's a commonly held heuristic that the least sustainable cities in America are the Southern ones. They are generally newer metros built in the age of the car (in large part because no one wanted to live there before we also invented the air conditioner). Particularly in the Southwest, there's never enough water to go around. And it takes tons of energy to keep the people who live in these places cool. Right?
A closer look at the math of indoor climate control suggests that, at least on this last point, southern cities may in fact be more sustainable than their older, bitterly colder brethren in the north. We're looking at you, Minneapolis. Michael Sivak, a research professor at the University of Michigan, compared Minnesota's largest city (and the coldest major metro in the U.S.) with Miami (our warmest metro on average), looking at the energy it takes for the two just to keep themselves at livable temperatures.
Minneapolis – just talking here about heating and cooling – is three-and-a-half times as energy demanding as Miami, a finding that will likely shock people there (or in Milwaukee or Buffalo) who've long prided themselves on life without A/C.
"The main story is counter-intuitive to me as well because we hear all the time about how unsustainable it is to live in the desert," Sivak says. "This doesn’t actually argue that that's not the case. It just says look, there's another side of the coin as well, and we should pay attention to the other extreme."
Sivak wasn't looking at the actual energy consumption of residents in these two cities (based, for instance, on their utility bills). Rather, in a paper published in Environmental Research Letters, he calculated differences in energy demand by looking at "heating and cooling degree days" in each location, the energy efficiency of heating and cooling appliances, and the efficiency of power plants (air conditioning almost always runs on electricity while most heating is fueled by other sources).
The ideal temperature inside your home is considered to be about 21 degrees Celsius (or 70 degrees Fahrenheit), the point at which you'd probably be perfectly comfortable without heating or cooling. But researchers look instead at the benchmark of 18 degrees Celsius, because humans naturally heat a house a few degrees just by living in it (and running the shower and cooking dinner).
"Think of it this way," Sivak says. "Let’s say you would like to have 70 degrees indoors. Think of how cold it can get in Minneapolis or Chicago or Ann Arbor. It can get down to zero." But on a really hot day in Miami, maybe the temperature tops out at 100. It takes a lot more energy to heat a room by 70 degrees than to cool a room by 30. In fact, it takes more energy to heat a room by one degree than to cool it by the same amount. And the typical air conditioner is about four times more energy efficient than the typical furnace or boiler.
Sivak freely admits that he's looking here at only one small piece of the sustainability picture, which also includes things like water consumption, transportation and air quality (within the realm of heating and cooling, he also doesn't factor in the energy needed to extract the natural resources that feed power plants to provide electricity to your air conditioner). And he's not considering whether buildings in Minneapolis are better insulated than those in Miami. But this does suggest that colder places aren't more sustainable simply by virtue of not being warm. "What I would like," Sivak says, "is for people to start thinking about both extremes."