The invention of the air conditioner is still shaping the future growth of global metros.

As U.S. cities are experiencing some of the warmest temperatures on record, it's easy to take for granted the ability to feel almost instant relief from oppressive heat with the push of a button or the flip of a switch. Today about 87 percent of U.S. households have an air conditioning unit.  

But at the turn of the 20th century, when Willis Carrier of New York invented the first modern electrical air conditioning unit, the goal wasn't to keep homes and offices cool on hot summer days. Rather, the device became popular with industries from textiles to cotton to tobacco. It wasn't until some decades later that human comfort became an important use, especially in the South; first with air-conditioned movie theaters and then in department stores.

It wasn't until the beginning of World War II that homes in southern U.S. cities began using air conditioning units. By 1955, one in every 22 American homes had air conditioning. In the South, that number was about 1 in 10, according to the historian Raymond Arsenault [PDF]. Since this increase in air conditioning use, many of these Southern cities experienced a population boom. 

I took a look at the metro areas in the U.S. with more than 1 million people and found which have historically been the hottest, based on the number of cooling degree days per year -- a statistic used to measure how much and how many days the outside temperature in a certain location is above 65 degrees. Using numbers from NOAA, I found that between 1971-2000, six big cities in the South had an average of at least 3,000 cooling degree days. I also compared the 1940 metro population (when available) to the metro population in 2010. From the time just before air conditioning became popular in the South to today, population growth in the region has skyrocketed. This raises the question: would these hot Southern cities be around, at least in their present form, if air conditioning hadn't been invented? 

But, of course, there are bigger, hotter cities across the globe. In fact, seven of the largest metros in the world have an average high temperature above 90 degrees Fahrenheit. 

Not surprisingly, all of these cities are found in developing countries. As Michael Sivak, a professor at the University of Michigan notes, only two of the warmest 30 global metros can be found in developed countries. With the middle class growing in warm metros in countries like India, demand for air conditioning is increasing. A recent New York Times article reported that sales of air conditioning units in India and China are growing 20 percent per year and are fast becoming a middle-class status symbol. Last year, 55 percent of new air conditioners were sold in the Asia Pacific region.

Unfortunately, we'll have to take the good (increased comfort) with the bad. That increased demand will also have a major impact on energy use in these global cities. As Sivak's research shows, the potential cooling demand from Mumbai alone is one-quarter of the demand for the entire United States. Increased innovation in more energy efficient air conditioning units might play a role in mitigating energy demand, but as the Times points out, gases from air conditioners "contribute to global warming thousands of times more than does carbon dioxide."

The air conditioner played a role in shaping the Southern metros we see today in the United States, and air conditioned mega cities will eventually shape the climate of the planet. 

Top image: Riyadh's skyline (Reuters/Zainal Abd Halim)

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