One of the ironies of climate change is that as heat waves become more common, people use more air conditioning–and those air conditioners help drive more climate change, and make things hotter. By the middle of the century, as more people around the world can afford air conditioners, the number of units could more than triple and end up using as much electricity as China uses today for its entire economy.
A team of Columbia University researchers thinks that painting buildings with a new type of coating could help. Some cities already use white roofs to try to cool buildings by reflecting sunlight (Los Angeles is also experimenting with painting roads white; painting buildings and roads can both help cool down entire neighborhoods). But the new coating developed by the scientists can keep buildings even cooler.
“It’s like snow,” says Yuan Yang, an assistant professor of materials science and engineering at Columbia. Snow, he explains, is filled with micron-sized crystals of ice that create tiny pockets of air that scatter light–the effect that makes snow white rather than transparent like ice, and highly reflective. The researchers created a polymer coating that also has nano-to-micro-scale pockets of air that turn it white. “Our paint can be much more reflective than normal paint,” he says.
Regular white paint might reflect 85% of the sunlight that hits it. The new coating, depending on its thickness, could reflect as much as 99%. (If used with colored paint, it’s less effective, but also keeps buildings cooler than they would otherwise be; the coating is also less effective in areas with high humidity.) In a hot, dry city like Phoenix, the white coating could keep a building around 10 degrees Fahrenheit cooler than the outdoor air on a summer day. Most Arizonans have air conditioning, and in a city where temperatures that regularly reach well over 100 degrees, the coating isn’t likely to replace it. But it could help buildings use less energy to stay cool. For the small fraction of residents there who don’t have air conditioning, the coating could potentially help protect their health on the hottest days; in 2017, more than 155 people in Phoenix died in heat waves.
In places where air conditioning is currently uncommon, such as Seattle, where there were a record number of hot days last July, but only one-third of residents have A/C, the coating might help stave off the need to install new systems. In the developing world, the coating could potentially help those who don’t yet have electricity or who can’t afford an air conditioner. But the biggest impact of the technology–which the researchers hope to commercialize–is likely to be its ability to help air conditioners use a little less electricity and help cut emissions. Without changes, the emissions from air conditioners are predicted to almost double by mid-century.