July 2021 was the earth’s hottest month on record, according to data from the National Oceanic and Atmospheric Administration (NOAA). The ocean surface temperature was 1.67 degrees above average.
One city in the Southwest region of the United States is trying to find ways to keep the ground cool.
“It looks basically like cured concrete,” said Ryan Stevens, a civil engineer with the City of Phoenix.
Ryan Stevens, along with other engineers, are working to lower asphalt temperatures.
“Really, it gives that illusion of putting on the product like sunscreen onto our roads, and that’s really what it’s doing is insulating our roads from the daytime solar energy,” he said.
The city pilots its “Cool Pavement Program” in nine locations across Phoenix. It’s a lighter coating on top of the roads already in place.
“It’s not paint. “It’s not paint,” we hear people saying. We actually used an asphalt-based product,” said Kini Knudson, director of Street Transportation for the City of Phoenix.
The program started last year, and now covers 36 miles of asphalt throughout the city.
“If it’s something that makes sense from an environmental climate perspective but also makes our pavement last longer, for me, it’s a no-brainer,” he said.
“The idea is to keep the energy out of the pavement because at night the pavement is the last thing to cool down. About 30 to 40 percent of the urban area is paved surfaces,” Stevens said.
Similar problems are caused by paved surfaces in American cities. Larger populations and more buildings created a higher urban heat island index — it’s a concept called the urban heat island effect.
“The urban heat island is a situation that has been studied for about 200 years. Because buildings can store heat differently to the natural surface, it happens. So a building will hold onto heat and hold it on through the night, and what that does is it keeps the nighttime temperatures warmer so we have this situation where we’re not cooling off at night,” Erinanne Saffell, Arizona’s state climatologist, said.
“That energy, the sunlight comes down and is stored in the surfaces,” she explained. “The more reflective a surface, the less energy it’s going to store and so it’s not going to be as hot.”
Saffell likens it to the way a person might feel when wearing a dark shirt in the heat sun.
“If you’re wearing a light-colored shirt that reflects the heat, that’s not going to get you as hot,” she said.
“We know that heat is a challenge for our community, we want our community to be comfortable,” Kate Gallego, the Mayor of Phoenix, said. “We’ve noticed it’s not getting as cool as night, we’re retaining a lot of heat.”
According to results from the first year, the cool asphalt had an average surface temperature between 10.5 and 12 degrees Fahrenheit below regular asphalt at noon and in the afternoon. The cool asphalt was cooler than regular asphalt at all times of day.
“That’s enough temperature distance or change that you can feel it if you’re walking in a community,” Gallego said.
However, Stevens said it’s still being looked at how this impacts the air around the asphalt.
“The degree of which that air mixing, does it feel hotter or not, is something we still need to look at,” he said.
The city also plans to explore different colors and hues.
Steve Hamburg claims that asphalt and buildings can be kept lighter for a long time.
“If you go to the Mediterranean, you would never see dark roofs. You’re always going to see light roofs, because they want to reflect the heat off the roofs so it doesn’t warm the structure…people have been known to do that for thousands of years,” Steven Hamburg, chief scientist at the Environmental Defense Fund, said.
The Phoenix project will continue to investigate what is most effective to reduce the urban heat island effect, and to keep our planet cool.
“It’s going to take a few more years to be able to fully evaluate the impact and the benefits of it,” Knudson said.