This is how much hotter urban sprawl makes US cities feel

Nearly 34 million people across 65 US cities — roughly one in 10 Americans — live in a place where the built environment makes temperatures feel at least 8 degrees Fahrenheit hotter than it would without that urban sprawl. That’s according to a new study by the nonprofit Climate Central, which mapped the impact of a phenomenon called the urban heat island effect in major cities in the US.

Dark, paved surfaces with little greenery tend to absorb heat. Hot exhaust from heavy traffic and industrial facilities can also make certain neighborhoods warmer than others. That’s how the urban heat island effect can make a heatwave feel even worse for some communities. In the US, a history of segregation and redlining that resulted in chronic underinvestment in many neighborhoods of color also created heat islands in cities.

It’s a problem that often flies under the radar because a typical weather forecast for a city misses these hyperlocal nuances. To get a better picture of how heat impacts a city from neighborhood to neighborhood, Climate Central documented the urban heat island effect down to a more granular level.

“When you look across the country, this is everywhere. This is not a big city problem. This is a big development problem. And that can happen anywhere,” says Climate Central senior data analyst and research manager Jennifer Brady.

Brady and her team scrutinized 37,094 census block groups in the US. A single block group might be home to between 600 to 3,000 residents, an even smaller range than a census tract. “The finer and finer detail you can get, the more accurate the estimates are going to be because what we’re looking at are land cover type,” Brady says.

They compared maps of those census block groups with an existing National Land Cover Database to assess what the makeup of the neighborhood was — whether it was crowded with tall or low-rise buildings or any tree cover.

They used methods previously published in the journal Scientific Reports to estimate how much those factors might warm or cool a neighborhood. That allowed them to calculate an urban heat island (UHI) index for each block group, a measure of how much hotter these areas are because of what the built environment looks like.

The average per capita UHI index for all the locations studied was 8 degrees Fahrenheit, but some places can feel even hotter. You can explore Climate Central’s interactive map online to see how this plays out nationally and at the census block group level in each city.

In New York City, for instance, the per capita average UHI index reached 9.7 degrees — meaning the way many neighborhoods were built makes them feel close to 10 degrees hotter than they would without the same urban sprawl. NYC’s main weather observation station happens to be located in Central Park, where tree cover and greenery can have a cooling effect in comparison. In 2021, The Verge explored New York City with thermal cameras and similarly found big differences in temperatures from neighborhood to neighborhood.

The good news is that we know how to undo the urban heat island effect. Simple measures like painting rooftops and other surfaces lighter colors, planting trees, and greening roofs can all help neighborhoods cool down. That can make an even bigger difference in the future as climate change makes heatwaves more frequent and intense.

“It’s very easy to go out and say, ‘Oh, this area’s hotter.’ But I think the logic behind this is very explicitly: why is it hotter?” Brady says. “I think then that leads to your solution.”