When Rebecca Sunenshine moved to Phoenix, Arizona, her first electric bill shocked her. “I called the utility and said, ‘You must have made a mistake.’ Because I think it was a $400 or $500 bill,” says Sunenshine, who is the medical director for Disease Control with the Maricopa County Health Department. “And they said, ‘Did you just move here?’”
The utility hadn’t erred. Air-conditioning accounts for about a quarter of Arizonans’ energy use, more than four times the national average. And it’s not a luxury. Without air conditioning, summer heat in Phoenix can be fatal. Last year, Maricopa County recorded 323 heat-related deaths—a record for the county. “We've had the highest number we've ever seen several years in a row,” says Sunenshine. The county is currently on track to exceed last year’s toll.
Extreme temperatures haven’t been confined to the Southwest. June was the hottest month ever recorded in North America. Early in July, an unprecedented heat wave claimed nearly 200 lives in Oregon and Washington, and another 600 people or more are thought to have died in British Columbia from the heat.
As global temperatures continue to climb, demand for air conditioning will soar. Within 20 years the electricity needed to power the world’s air conditioners is expected to triple. The fossil fuels burned to run them already add about 117 million metric tons of carbon dioxide to the atmosphere each year in the United States alone. The very devices that keep us cool are helping to bake the planet.
Even aside from their carbon footprint, air conditioners have a more direct impact on their surroundings, says Aaswath Raman, a professor of engineering at the University of California Los Angeles. “When you think about what an air conditioner does, it dumps hot air back into its environment. So every air conditioner is actually making its environment slightly hotter.”
Finding a more environmentally friendly alternative to conventional air conditioning systems has been the focus of Raman’s work for nearly a decade. He is at the forefront of a small global community of researchers who have been developing a variety of materials, including paints, thin films, and wood, all with a singular property: Using wavelengths of light, they can cool the surrounding air without any external power source, in some cases by 10 degrees Fahrenheit or more. These new materials could help mitigate some of the effects of the climate crisis, especially in cities, where the urban heat island effect can raise temperatures by more than 17 degrees.
“The neighborhoods that suffer most from the urban heat island effect tend to be lower income neighborhoods,” says Raman. “So it’s an issue of equity.”
A better way to cool
Raman is generally acknowledged to be the founder of this nascent field of research. The idea that there might be better ways to keep things cool came to him in 2012, when he was finishing his Ph.D. at Stanford University. While digging through some old scientific articles, he became intrigued by a concept that a few scientists had toyed with decades before, but then almost immediately dismissed as unworkable.
The idea was to harness a natural phenomenon known as radiative cooling to lower the temperature of objects. Radiative cooling is physics-speak for a process that happens all around us: Anything that has been warmed by some heat source eventually cools down when the heat source is removed. The most familiar example is the rather large object beneath our feet: the Earth itself, which warms during the day and cools after sunset, shedding heat out into space.
A few researchers had wondered if there might be some way to tinker with materials to make them radiate heat even in broad daylight, but the effort seemed futile. As long as the sun is out, objects don’t become any cooler than the ambient air temperature. “We came across references saying that this is impossible to do during the daytime,” says Raman.
For a young postdoc, an impossible project offered two possibilities: running into a dead end at the start of one’s career or discovering something that others had missed.
“It's really hard to find topics that no one is working on,” says Raman. “And usually there's a good reason—because it's completely pointless.”
Raman had trained in the physics of optics, studying how light of different wavelengths interacts with various materials. He had tools and technology at his disposal that weren’t available to the researchers who had abandoned their radiative cooling work years earlier. So in 2012 he submitted a proposal to the Advanced Research Projects Agency-Energy, or ARPA-E, a branch of the Department of Energy.
“Every three years they have this open funding call where any crazy idea can be submitted,” says Raman. “And I think they end up picking like one percent of submissions for funding. I was told that Steve Chu, who was the energy secretary at the time, said that this doesn't sound plausible.” Nevertheless, the agency gave Raman $400,000 and a one-year deadline to develop a material that would stay cool even on the hottest days. “It was probably one of the smallest grants they made,” he says.
A specific wavelength
Raman teamed up with Shanhui Fan, his mentor at Stanford. They planned to construct a thin but multilayered film that would take advantage of the way Earth’s atmosphere allows heat to escape into space. All the solar energy absorbed by the Earth’s surface is constantly being reemitted as infrared radiation, a form of light with a longer wavelength than visible light. Some of that infrared radiation is absorbed by water vapor, carbon dioxide, and other greenhouse gases, warming the atmosphere. That process had kept the world’s climate relatively stable and livable—until humans started burning fossil fuels and loading the atmosphere with billions of tons of carbon dioxide.
Not all infrared radiation, however, is absorbed by the atmosphere; some of it escapes into space. Earth’s atmosphere, it turns out, is transparent to certain infrared wavelengths—specifically, wavelengths between 8 and 13 micrometers. Think of the atmosphere as a blanket, with a few holes in it. Raman and Fan realized that if they could engineer their film to emit infrared radiation within that range, the radiation would flow through the atmosphere’s holes and leak out into space; the film would naturally cool down, dropping below ambient temperatures even during the daytime.
Their film consists of alternating layers of silica—glass—and hafnium dioxide, a compound used in the optics industry to coat lenses and mirrors. By fine-tuning the thickness of the individual layers, Raman and Fan created a film that was both highly reflective of visible light—so it wouldn’t warm up in the sun—and an excellent emitter of infrared radiation at just the right wavelengths to pass through the atmosphere unimpeded. If the film covered, say, the hood of a car, it would conduct heat away from the hood, cooling it without using any electricity.
Raman and Fan knew their experiment was working within six or seven months. They were on the roof of an engineering building on the Stanford campus with a sample of their film exposed to the sun. Rooftops can get exceedingly hot in the summer, reaching temperatures as high as 140°F. As a spot check, they tried a simple test: They shaded the film. Normally, when something is shaded it will cool off. But the film became warm, because the infrared radiation was no longer escaping into the atmosphere—it was hitting the shading material and warming it, which in turn heated the air around the film.
“It's super counterintuitive,” says Raman. “It's warmer in the shade because you're blocking its view of the sky.” Placed back in the sunlight, the film became noticeably cool to the touch, about 10° below the air’s temperature.
Following that early success, Raman, Fan, and their Stanford colleague Eli Goldstein cofounded a company called SkyCool and worked with 3M to further develop and commercialize the technology. In the spring of 2020, SkyCool installed film-coated panels on the roof of a California supermarket. Water flowing through the panels is cooled by the film and then pumped into the building’s conventional air conditioners and refrigerators, cooling their components and lowering the amount of electricity used to power them. Says Raman, “That adds up to around 15 to 20 percent in energy savings.”
A question of durability
Since Raman and Fan published the results of their rooftop experiment in 2014, a dozen or so research groups have designed paints, gels, and even blocks of wood that can remain cool in broad daylight. Many of the materials are so new that their durability remains a question, especially considering the places where most of them would be used: on rooftops, exposed to the elements and grime that would inhibit the infrared radiation.
“We've evaluated some of them,” says Tim Hebrink, a staff scientist with 3M, “and they can degrade or get soiled quickly.” But Raman and Fan’s film seems easier to maintain and clean than a coat of bright white paint, and the technology is ready to be scaled up. “We can make this film in rolls that are a mile long,” says Hebrink, “and a meter or two wide.”
For now, the film will most likely be used as a supplement to conventional cooling technologies, as it is at the California supermarket. Paradoxically, most buildings are so well insulated that heat from within can’t pass up into the film to be radiated away. But the film could help cool other sorts of structures. The city of Tempe, Arizona, is now field-testing 3M’s film on the roofs of a few of its bus shelters. Some preliminary results show that the roofs can be 30° cooler than the surrounding air.
And the technology could help reduce heat-related deaths. Raman is involved in a UCLA project called Heat Resistant Los Angeles. “The idea is, can we go beyond shade?” he says. Historically, cities have focused on providing shade trees, parks, and green belts to help cool urban environments, but such projects often bypass low-income communities and take years to establish. Raman envisions using canopies coated with his film to cool large outdoor spaces; these could be set up quickly at relatively low cost.
“It's very early days for the project,” he says, “so it's still kind of speculative. But I'm hoping in a year or two we'll have some cool results and demos to share.”
Cooling the planet?
At least one scientist imagines an even more ambitious scheme: erecting large-scale arrays of panels coated with a film like Raman and Fan’s to cool the entire planet and maybe slow or reverse global warming. Jeremy Munday, an electrical engineer at the University of California Davis, estimates that covering one to 2 percent of Earth’s surface with the panels would offset the warming caused by greenhouse gases. The required area would be a bit more than half the size of the Sahara Desert.
His rough calculation of the cost: $2.5 trillion, or about 10 percent of the gross domestic product of the United States. But when weighed against the calamitous effects of the climate crisis, it would be money well spent, he says.
“You have to be thinking about stuff that's kind of outside the box,” Munday says. “It's a cliché, I know, but we've been kind of marching along the same route for a long time. And I think sometimes you need to punctuate things by going to big changes.”
It’s an appealing idea—a solution to a crisis that threatens every nation on Earth. But is it remotely practical? “Radiative cooling may indeed help significantly with the urban heat island effect, but I think it’s very, very doubtful that [it] would have a significant role in global cooling,” says Mark Lawrence, a climate scientist at the Institute for Advanced Sustainability Studies in Potsdam, Germany.
A large-scale project of the sort imagined by Munday, he says, would take decades to construct—and arrive far too late to help us avoid the most catastrophic effects of climate change. In addition, Lawrence says, artificial cooling on such a scale might disrupt the world’s precipitation patterns, since rainfall and atmospheric circulation are driven by temperature differences between the land and sea. Some climate models, for example, show that artificial cooling schemes might weaken the monsoon rains that sustain India and Africa.
The World Health Organization estimates that between 1998 and 2017, heat waves killed at least 166,000 people around the world. If we continue to emit greenhouse gases at the current rate, deadly heat will put more than a billion people at risk by the end of the century.
The technology, then, may ultimately help cool our cities, and it may be able to prevent tens or hundreds of thousands of deaths from the brutal heat waves to come, which would be no small feat. But to cool the whole world, we’ve known for decades what needs to be done: Leave fossil fuels in the ground.