On July 17, 1902, a 26 year old Willis Haviland Carrier flipped the switch on the first modern air conditioner at the Sackett-Wilhelms Lithographing & Publishing Company of Brooklyn, where humidity was causing problems with their paper stock. Carrier subsequently figured out the theory of everything for air conditioning, what Wikipedia says became known as ""Magna Carta of Psychrometrics." This document tied together the concepts of relative humidity, absolute humidity, and dew-point temperature, thus making it possible to design air-conditioning systems to precisely fit the requirements at hand."
It took a while to get the bugs out, to develop more efficient centrifugal chillers, to replace ammonia with something less toxic, and to expose air conditioning to the general public, which ate it up.
The first air conditioned movie theater, the Rivoli in New York City, was a sensation. Soon they spread across America.
It was a big deal for retailers; people loved hanging around in air conditioned stores. Some say it contributed as much to the death of downtown and the growth of the suburban mall as the car did.
The real air conditioning revolution happened after World War II, with the introduction of affordable residential air conditioning. This changed everything; suddenly much of the southern and southwest United States, which was previously almost uninhabitable in summer, became comfortable year round. Tony Long wrote in Wired a few years ago:
The impact of air conditioning can’t be overstated. Sun Belt cities and other places where stifling hot weather is a factor enjoyed an economic boom as people settled in large numbers, protected from the elements by Carrier’s invention. That population shift in turn changed the political balance of the nation. Even the nature of architectural design changed, with perhaps the most conspicuous example being the smoky glass-clad skyscrapers that now dot almost every big-city skyline.
John Steele Gordon notes how it may affect the election this year in The Wall Street Journal:
In the first half of the 20th century, the South was an economically backward part of the country with a large net emigration, mostly to the North. In 1950, the states of the old Confederacy had only 127 electoral votes.
But with the end of segregation, the growth of the global economy and air conditioning, the New South began to rise. Today the Confederate states have 160 electoral votes. Texas gained four votes in the last 10 years, Florida two. The outcome of presidential races this year and in the future could well turn on the fact that Willis Carrier invented air conditioning a century ago
There are many, not all Democrats, who would say that this is not necessarily a good thing. As I have noted many times, the problem is not the air conditioning itself, which everyone acknowledges is a lifesaver, and for many, a necessity.
The problem is that we have forgotten how people designed before it, where the architecture of houses and buildings was adapted to climate. Now we just throw electricity at it. A house in a Florida or Phoenix suburb looks pretty much like a house in New England; office buildings and factories are indistinguishable. We made what was once a luxury into a necessity. That is great for the development industry and a whole lot of hack architects and engineers, but not much else.