One of the last places in the country you wanted to be on July 11 was Houston, Texas. Roasting under a heat dome, Houston topped 105ºF that day, continuing a punishing trend that has already seen the city hit over 90°F on 46 days in 2023.
Houston isn’t alone. Record highs have been reached this summer in Tucson, Ariz.; Tampa, Fla.; Corpus Christi, Texas.; and both Stockton and Sacramento, Calif., which on July 1 posted twin readings of 109ºF. Climate change is surely playing a role in the rise of such incinerating heat, but it is no coincidence either that the greatest suffering has been endured not in the outlying suburbs, exurbs, or countryside, but in city centers, characterized by what experts call urban heat islands.
Strip away natural tree cover and other foliage; lay down asphalt parking lots and ribbons of highway; construct buildings tall enough to cut off natural wind flow—and you create urban ovens, which absorb heat during the day and slowly radiate it back out at night. Even after sundown, there is no relief to be found.
Read more: What Extreme Heat Does to the Human Body
On average, according to the Environmental Protection Agency (EPA), cities range from one to seven degrees hotter than the countryside during the day and two to five degrees hotter at night. And that’s nothing compared to the differences within cities themselves, some parts of which are planted with tree cover and parkland, and others of which are denuded of green, and encased in asphalt and concrete.
“In some studies,” says Hunter Jones, program manager of the National Integrated Heat Health Information System at the National Oceanic and Atmospheric Administration (NOAA), “we’re finding that different parts of the same city have temperature disparities of up to 20 degrees.”
Houston is a case study. Only 18% of the city has any appreciable tree cover, and not all Houstonians get their share: There is a 14% discrepancy between the green cover in wealthier parts of the city compared to poorer ones. To fix this, Houston aims to plant no fewer than 4.6 million trees by 2030.
Until then, to cope with the current heat wave, Houston has implemented its heat emergency plan opening 22 cooling centers (such as libraries, YMCAs, and community centers); urging the use of some two dozen city pools; warning residents about the importance of staying hydrated and avoiding caffeine and alcohol, which cause dehydration; and encouraging the elderly, the young, and anyone with a chronic disease to stay inside air-conditioned buildings between 1:00 p.m. and 5:00 p.m.
These efforts may help in the short-term, but more can be done by cities like Houston to combat the heat island effect. The first step for many cities, Jones says, is planting trees and establishing parks wherever possible. Reflective rooftops can reduce the amount of heat buildings absorb during the day. And coating concrete and asphalt surfaces with titanium dioxide—which is also found in sunscreen—can help keep their temperature down.
“There are a variety of other coatings too that have been developed that can reflect a lot of that [solar] energy,” says Jones. In some cases, merely painting streets a reflective shade of gray can help as well.
To help better understand how heat is affecting cities the federal government has been studying the heat island problem. Since 2017, NOAA has been conducting a citizen-scientist heat island mapping campaign, under which volunteers with heat sensors on their cars or bicycles travel through their neighborhoods in the morning, afternoon, and evening, recording location and temperature readings and sending them back to NOAA for collation and eventual remedial action. This year, the campaign is taking place in 15 different cities across 14 states; since 2017, more than 60 cities have been mapped.
“This has been a really fantastic opportunity to assist communities in collecting temperature and humidity data,” says Jones. “We then use machine learning to generate maps to show them where the intensity and the most severe heating is.”
The problem of urban heat islands, however, is not going away any time soon—and with 56% of the human population living in cities, it affects the majority of us. Curbing climate change is the ultimate, long-term, solution. Until that happens, adapting is the answer to the mess we’ve made—and suffering is the price.
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