US cities roll out ‘cool pavements’ to fight dangerous heatwaves

The coatings reflect the hot summer sun to make roads and surrounding neighbourhoods cooler

As the climate crisis causes summer temperatures to soar across the US, some cities are introducing innovative measures to keep their citizens cool.

Cities like Los Angeles, California and Phoenix, Arizona are covering some roads in coatings designed to cool down black asphalt - which absorbs, and then radiates, a ton of heat.

In Phoenix, for example, temperatures regularly reach over 100 degrees Fahrenheit (38 degrees Celsius), causing black asphalt can reach temperatures between 120-150F (48-67C), according to the Environmental Protection Agency (EPA).

One way to make streets cooler is to have them absorb less sunlight. In Phoenix, the city has applied a coating to some roads that reflects more sunlight than standard asphalt and making them noticeably less hot as a result.

According to the city, roads with the coating can be up to 12F cooler during the afternoon than standard roads. And the benefits didn’t just affect roads — they made their surroundings cooler, too.

Even night-time air temperatures were about half a degree cooler where roads had coatings, the city said. Roads accumulate heat during the day which gets released overnight — making the whole neighbourhood hotter during what is supposed to be the cooler hours of the day.

Night-time drops in temperature are beneficial to human health, says the Center for Disease Control and Prevention (CDC).

The climate crisis is causing hotter nights, with nighttime temperatures in many parts of the world warming even faster than days.

This leads to increased risk of conditions like heat stress, which can impact vulnerable populations like the elderly and those with pre-existing conditions. Other potential benefits include reducing the amount of energy needed to cool down buildings and lowering the temperature of stormwater runoff, which can benefit aquatic wildlife.

Reflective coatings are being applied in some LA neighbourhoods and trialled in other parts of the country, like Charleston, South Carolina. In New York City, a similar reflective substance has been painted onto building roofs to help bounce off more sunlight and stay cool.

However there is one potential downside to so-called “cool pavements” - they can make people feel hotter during the day. In 2019, Bloomberg reported that studies in Los Angeles found that temperatures during the day can feel about 7F hotter, and Phoenix officials say that people can feel 5.5F hotter when standing on cool pavement coatings.

That’s because the surfaces are more reflective and instead of absorbing heat into the ground, sunlight is reflected out where it can feel hotter to a person on the street.

“Cool” surfaces are just one tool that cities are deploying to counteract the “urban heat island” effect. Cities are hubs of concrete, asphalt and glass which can get much hotter than surrounding areas covered in dirt, trees and grass.

During the day, temperatures in cities can be up to 7F hotter than surrounding areas, and up to 5F hotter at night, says the EPA.

This effect can be especially dangerous during heatwaves. In 2020, over 300 people died from heat-associated causes in Maricopa County, home to Phoenix.

In addition to more reflective roads, adding more trees and green spaces to cities cools neighbourhoods. For example, in some shadier areas of Baltimore, temperatures on a summer day can be nearly 15F cooler than areas with fewer trees, according to The New York Times.

So-called green roofs, which are covered in vegetation like succulents and grasses, can also help buildings cope with more heat.

For Southwestern cities like Phoenix, located in the second-fastest warming region in the US, these adaptations will become increasingly necessary.

Summertime temperatures in Phoenix now regularly break 100F (38C). Since 1970, the average summer temperature in the Arizona city has increased by about 3.6F, with around 80 more days per year spent above 90F (32C), according to the non-profit Climate Central.