When a national group of urban planning historians was asked to rank the most important influences on the American metropolis in the last half of the 20th century, their top 10 list focused on social, economic and political developments. Of the many technological devices that helped shape our lives during that era, only two made the list: the automobile and the air conditioner.

The car culture’s impact on cities and suburbs has been the subject of heated debate for decades. But all the while, humming away inconspicuously in the background, air conditioning also has been changing the way we live and changing us, and not always for the better.

In 1960, air conditioning was found in only 12% of U.S. homes and 20% of cars. Even in the South, only 18% of homes had it. Today, 85% of homes nationwide have air conditioning, and virtually no new house or car is built without it.

The ascendancy of air conditioning has had its most obvious impact on our choice of where to live. Between 1960 and 2009, the population of the Northeast Census region grew by 23% and that of the Midwest by 28%. Meanwhile, the South swelled by 96% and the West ballooned by 143%. A mass migration-within-a-migration led millions of families to the vast suburbs that sprouted on the cheap land ringing Sunbelt cities — and air conditioning made hot, humid and mosquito-infested regions more habitable.

Suburbia’s original appeal came wrapped in visions of green earth, clear skies and backyard bliss. But to fulfill the dreams of home buyers on modest incomes, developers cut back on costly structural features such as movable window sashes, screens, awnings and eaves, high ceilings, thermal mass, cross-ventilated designs and attic fans. They bulldozed shade trees and began building instead for mechanical climate control. Families responded by spending more time indoors.

Not everyone embraced the concept of October weather in July. Asked by researchers conducting a 1990s survey on air conditioner usage why she left hers turned off, a California apartment resident responded, “Because it makes it too hot outside.” She had a point. With less exposure to heat, studies show, our physical and mental tolerance for heat declines — and our fondness for the air conditioner grows.

In every region of the country, business-world standards of dress and appearance have been designed specifically for a cool, dry office atmosphere. And vehicle air conditioners ensure that commuters don’t arrive on the job sweat-soaked and windblown. Mechanical cooling also allows a company to concentrate large numbers of employees in the inexpensive, windowless space deep within an office block. One result is that many shivering employees are carrying sweaters and space heaters to work in summer.

Heavy-handed climate control makes good business sense. Like computers, economies run faster when they’re neither overheated nor frozen. A 2008 National Bureau of Economic Research study concluded that in the era of air conditioning, the general tendency of hot weather to depress economic activity no longer affects the world’s wealthier nations. That 70% of U.S. economic growth since 1960 has occurred in the South and West bears this out.

But that growth has come at a high cost. The long commuting time that lies between an affordable mortgage and a desirable paycheck has been growing year by year, with the problem most acute in the nation’s hotter cities. On average, the overcooked drivers of traffic-choked Los Angeles, Atlanta, Dallas-Fort Worth, Houston and Miami are stuck in traffic delays more than three times as many hours per year as drivers in Pittsburgh, Cleveland, Detroit, Cincinnati and Buffalo.

The relentless intensification of work and commuting patterns could not have been sustained summer after summer without the air-conditioned pipeline that conveys employees in cool vehicles from cool homes to cool offices and back, maybe with a stop at an even chillier supermarket or mall.

But the parking lots, roadways and buildings that support that efficient delivery system also trap and re-radiate solar energy, creating the so-called urban heat island effect. Cities and freeways now stay several degrees hotter than the surrounding countryside during the day and as much as 20 degrees warmer at night.

Running full blast, a car’s air conditioner dramatically increases levels of noxious exhaust in the surrounding air, guaranteeing that other drivers will have to keep their windows closed and the air running. In that, as in many other ways — by aggravating global warming, by encouraging poor building ventilation, by increasing our own biological susceptibility to heat — dependence on air conditioning always seems to generate demand for more air conditioning.

Air conditioning buildings and cars in the United States has the climate impact of half a billion metric tons of carbon dioxide a year. That exceeds the total annual carbon dioxide emissions of any one of these nations: Australia, France, Brazil or Indonesia. In an effort to reduce energy use and curb greenhouse emissions, industry and government are pursuing more efficient cooling technologies for cars and buildings. But greater efficiency can’t reverse the unsustainable living, working and transportation patterns that air conditioning has helped foster.

Greener building designs that favor natural ventilation will help, but in the millions of existing homes, workplaces and schools that we’ll be using for decades to come, the most important adjustment will be not in our thermostats but in our own comfort expectations.

Businesses can help. Studies find that the majority of office employees are already dissatisfied with their workplace temperature, and that the most important improvement employers can make is to give workers more control over windows, shades, air movement, clothing, position and location.

The key to reducing the impact of mobile air conditioning is to keep as many cars as possible at home and switched off. That will mean restructuring cities and suburbs as pedestrian havens, discouraging car travel (and keeping cities cooler) by replacing parking lots with parks, and launching a crash expansion of inexpensive, convenient and cool mass transportation.

In other words, we need to back out of the ecological dead-end alley we’ve been traveling down for half a century. It won’t be easy. With air conditioning so thoroughly integrated into American society, we’re going to have trouble finding reverse gear. But it’s there.

Stan Cox is the author of, most recently, “Losing Our Cool: Uncomfortable Truths About Our Air-Conditioned World (and Finding New Ways to Get Through the Summer).”