Ways of cooling buildings have existed for millennia. The ancient Egyptians hung damp reeds over their windows and placed water-filled pots in hallways. The Romans collected snow in donkey carts and stored it in pits for the summer. In the Middle East, wind towers and underground water channels were used to channel cool breezes through homes. In the mid-19th century, New York theatres used big fans to blow ice transported from New England through ducts towards their audiences. But the birth of modern air conditioning is usually dated to 1902 when it came about almost accidentally as the solution to a different problem.

And what was that problem?
The Sackett & Wilhelms printing company in Brooklyn wanted a system to control humidity at its colour printing press; the paper would expand and contract as humidity varied, ruining the product. A young engineer named Willis Carrier discovered that if you circulated air over coils chilled by compressing ammonia, it would reduce humidity by condensing water vapour; it also, incidentally, made the air much cooler. Carrier later sold his technology to other manufacturers that needed their air to be drier (flour mills and razor manufacturers) or cooler (dairy and meatpacking). But he soon realised that “comfort” applications were even more promising. An early commission was to cool Philadelphia’s Masonic Temple. And from the 1920s on, he sold his “Weathermaker” to cinemas, department stores, restaurants and offices across America.

How did Air Conditioning affect American life?
In the past, cinemas closed throughout the hot summer weather. But from the 1920s on, the cool air became part of the attraction: the tradition of the Hollywood summer blockbuster dates back to this era; so, too, does the rise of the shopping mall. Air conditioning changed architecture profoundly. Very high buildings – previously difficult to ventilate because wind increases with altitude – were suddenly a viable option, as were glass-fronted skyscrapers. And in the postwar period, smaller domestic air con units became available: between 1962 and 1992, the proportion of US houses with air con rose from 12% to 64%. This not only transformed houses, which no longer needed to have thick walls, high ceilings and sun porches, but also changed the country’s demographics. The Sun Belt – from Southern California to Florida – boomed from the 1960s; people and industry moved there as its hot summers became more bearable; its share of the population rose from 28% in 1950 to 40% in 2000. Some political scientists argue that Ronald Reagan would never have been elected if the large-scale migration of Republican pensioners to the Sun Belt had not occurred.

And the rest of the world?
Other developed economies with hot summers have seen similar patterns. According to recent estimates, about 90% of Japanese homes are air conditioned – the same as in the US. China now has at least 35% of the world’s air conditioners. Even more dramatic is the development of mega-cities, such as Singapore and Dubai, in previously very inhospitable climates. In 1950, fewer than 500,000 people lived on the southern coast of the Persian Gulf. Today, it is home to 20 million. In Saudi Arabia, 70% of electricity is used for air conditioning. In India, summer blackouts have been blamed on the new air conditioning units of the middle class.