By the 1950s, Americans had adopted a lifestyle that was profoundly shaped by artificial cold, buying frozen dinners purchased in the refrigerated aisles of the local supermarket, and stacking them up in the deep freeze of their new Frigidaires, featuring the latest in ice-making technology.

In that iconic 1950s American household, the most novel cold-producing device was not storing fish filets for dinner or making ice for the martinis.

It was cooling down (and dehumidifying) the entire house.

The first ‘apparatus for treating air’ had been dreamed up by a young engineer named Willis Carrier in 1902.

He had been hired by a printing company in Brooklyn, New York, to devise a scheme that would help them keep the ink from smearing in the humid summer months.

His invention not only removed the humidity from the printing room - it also chilled the air.

Carrier noticed that everyone suddenly wanted to have lunch next to the printing presses, and he began to design contraptions that would be deliberately built to regulate the humidity and temperature in an interior space.

The first great test for air-conditioning came over Memorial Day weekend of 1925, when Carrier debuted an experimental AC system in Paramount Pictures’ new flagship Manhattan movie theatre, the Rivoli.

He even persuaded Adolph Zukor, the legendary chief of Paramount, that there was money to be made by investing in central air for his theatres. Zukor himself showed up for the Memorial Day weekend test.

Carrier and his team had some technical difficulties getting the AC up and running. The room was filled with hand fans waving furiously before the picture started. Carrier later recalled:

“It takes time to pull down the temperature in a quickly filled theatre on a hot day but gradually, almost imperceptibly, the fans dropped into laps as the effects of the air conditioning system became evident. We then went into the lobby and waited for Mr Zukor. When he saw us, he said tersely, ‘Yes, the people are going to like it.’”

Between 1925 and 1950, most Americans experienced air-conditioning only in large commercial spaces such as movie theatres, department stores, hotels or office buildings.

Carrier knew that AC was headed for the domestic sphere, but the machines were simply too large and expensive for a middle-class home.

By the late 1940s air-conditioning finally made its way to the home front, with the first in‑window portable units appearing on the market.

Within half a decade, Americans were installing more than a million units a year. Places that had been intolerably hot and humid were suddenly tolerable.

By 1964, the historic flow of people from South to North that had characterised the post Civil War era had been reversed.

The Sun Belt expanded with new immigrants from colder states, who could put up with the tropical humidity or blazing desert climates thanks to domestic air-conditioning.

Tucson rocketed from 45,000 people to 210,000 in just 10 years. Houston expanded from 600,000 to 940,000 in the same decade. Carrier’s invention circulated more than just molecules of oxygen and water. It ended up circulating people as well.

But the rise of the Sun Belt in the United States was just a dress rehearsal for what is now happening on a planetary scale.

All around the world, the fastest growing megacities are predominantly in tropical climates: Chennai, Bangkok, Manila, Jakarta, Karachi, Lagos, Dubai, Rio de Janeiro.

Demographers predict that these hot cities will have more than a billion new residents by 2025.