Derek Thompson: Humans wanted to keep cool long before Carrier’s invention. But it’s sort of pathetic how we tried to do it. You describe the early 19th-century business of New England companies shipping large carved ice cubes insulated with sawdust around the country. New England literally exported ice the way Georgia exports peaches. There were even shortages during mild winters—“ice famines.”

Tim Harford: It was really hard to cool things! Before the invention of air-conditioning, you had to take something that was very cold and move it to places that were hot. And there were fascinating problems. For example, when the bodies of water that supplied the ice, like lakes, started getting polluted, the pollutants would be trapped in the pieces of ice. When they melted at their destination, it filled the air with unpleasant smells.

Thompson: Truly, thank God for Willis Carrier. The global effects of air-conditioning that you describe are mind-blowing. Air-conditioning transformed cities’ skylines, allowing for tall glassy skyscrapers that didn’t broil people in the top floors. It transformed demographics, allowing for migration in the U.S. to the Sun Belt, to Atlanta and Phoenix. By allowing politically conservative retirees to move south and west, you quote the author Steven Johnson saying that air-conditioning elected Ronald Reagan.

Harford: Yes, and it’s key to have a global perspective, too. This didn’t just reshape America. Air-conditioning reshaped the world. Places like Singapore and Shanghai are miserable when they’re hot and humid, but today they are global metropolises. There are studies saying that human productivity peaks around 70 degrees. That means that air-conditioning made us more productive, but also, by creating density in Singapore, it allows people to work longer and keep making the world a rich place. There is also the dark side of air-conditioning. You cool the temperature inside, but these units are energy-hungry, and they contribute to global warming.

Thompson: The first invention in the book is the plow, which facilitated the agricultural revolution. You write that “agricultural abundance creates rulers and the ruled, masters and servants, and inequality of wealth unheard of in hunter-gatherer societies.”

The second invention in the book is the gramophone, which allowed the most popular singers to reverberate in houses around the world, turning local stars into global superstars. That, too, creates inequality of wealth unheard of in pre-gramophone societies. Do you think technology inherently creates superstar effects and inequality?

Harford: There are two questions in there. First, does tech always increase inequality? Second, can politics offset that?

First, no. The famous example of technology displacing workers is the mechanized loom, famously smashed by the Luddites. These machines made their owners rich. But I think it’s possible that those looms actually decreased income inequality, because the new looms could be used by lower-skilled workers, who earned more money. On the other hand, I do think that much technology today is biased toward people who already have skills, which exacerbates winner-take-all effects.