In the summer, the temperature in New York City is about 7 degrees Fahrenheit higher than in its surrounding areas, according to NASA. That is not unusual. Cities are often warmer than their suburbs because of a phenomenon called "the heat island effect." The way a city is designed — the building materials used, the way streets are arranged, the lack of canopy — can actually sequester heat.

More than half of the world's population (and growing) live in cities, so interest in figuring out how to cool them down might be growing, too.

Learn more about NPR's look at our warming world with this series. Curious what action Denver is taking? One response to the heat island effect locally has been the green roof initiative.