But there’s an inconvenient truth: You can make it cooler inside only by making it warmer outside. Air-conditioning units pump hot air out of buildings. A study in Phoenix, Arizona, found that this effect increased the city’s night-time temperature by two degrees.20 Of course, that only makes air-conditioning units work harder, making the outside hotter still. On underground metro systems, cooling the trains can lead to swelteringly hot platforms.
