Cities tend to be hotter than surrounding suburban or rural areas, creating “heat islands.” This happens because cities have less trees and vegetation, which cool the air, and more buildings and roads, which usually heat the air. The temperature inside a city is generally 1-7°F hotter than outside the city, according to the Environmental Protection Agency.
Individual neighborhoods can get even hotter. Areas that were subject to racist housing policies in the past, like Brooklyn’s Brownsville neighborhood, have less vegetation and more built surfaces, and are hotter today than other parts of their cities. These formerly redlined neighborhoods are still home mostly to low-income communities of color, where many households can’t afford air conditioning.
In extreme cases, the urban heat island effect can turn deadly. Cities have a higher baseline temperature to start with, so during heat waves they get especially hot. Over the past 30 years in the United States, extreme heat has killed more people than any other kind of weather. Along with those who lack air conditioning, people who work outside (like construction workers) are especially at risk. Even if it doesn’t kill, too much heat can exacerbate pre-existing illnesses or make people sick through dehydration and heat stroke.
Cities like Louisville, Kentucky are trying to shrink the urban heat island effect by planting more trees and subsidizing more reflective roofs or green roofs on buildings. These physical interventions help, but they aren’t the only things cities can do to protect their residents, said Juan Declet-Barreto, a social scientist who studies environmental hazards at the Union of Concerned Scientists. For example, city governments can also help residents weatherize their homes or pay for air conditioning. They can offer public cooling centers, and prevent people from being evicted or losing their utilities during the hottest parts of the year.
And of course, cities are suffering from more extreme heat as climate change goes on. “There’s only so much that we can do to adapt,” said Declet-Barreto. To prevent climate change from cranking the urban heat island effect to a broil, he said we also need rapid reductions in greenhouse gas emissions.