Cooling the Earth one roof at a time: how an ultra-white paint reflects sunlight
Purdue scientists have created a white paint that, when applied, can reduce the surface temperature of a roof and cool the building below.
Xiulin Ruan, a professor of mechanical engineering at Purdue University, didn’t set out to make a Guinness World Record when he began trying to make a new type of paint. His goal was more ambitious: to cool buildings without making the Earth warmer.
In 2020, Ruan and his team unveiled their creation: a type of white paint that can act as a reflector, causing 95% of the sun’s rays to bounce away from the earth’s surface, through the atmosphere and into deep space. A few months later, they announced an even more powerful formula that increased the reflection of sunlight to 98%.
The properties of the paint are almost superheroic. It can make surfaces up to five degrees Celsius cooler than ambient temperatures at noon, and up to 10.5 degrees cooler at night, reducing indoor temperatures and air conditioning needs by up to 40 percent. According to Ruan, it is cool to the touch, even under a scorching sun. Unlike air conditioners, the paint does not need power to work and does not heat the outside air.
In 2021, Guinness declared it the whitest painting in history, and since then he has received several awards. Although the paint was originally conceived for roofs, manufacturers of clothing, shoes, cars, trucks, and even spacecraft have turned to it with enthusiasm. Last year, Ruan and her team announced that they had created a lighter version that could reflect heat from vehicles.
“Actually, we didn’t intend to develop the whitest paint in the world,” Ruan said in an interview. “We wanted to help with climate change, and now it’s more of a crisis, and getting worse. We wanted to see if it was possible to help save energy while cooling the Earth.”
Although the paint is officially the whitest in the world, it’s not blinding because it scatters light, Ruan explains. Its appearance is not much different from the white paint you buy at the hardware store.
It takes at least a year for the paint to be ready for commercial use, and work is underway to increase its durability and resistance to dirt. Ruan said the Purdue team has partnered with a company, but can’t name it yet. The team is also developing colored paints that use ultrawhite as a base. “They will work less ideally than white, but better than some of the other commercial colors,” he said.
As the climate crisis worsens, scientists are working urgently to develop reflective materials, including different types of coatings and films, that could cool the Earth passively. These materials are based on physical principles that allow heat energy to travel from Earth along specific wavelengths through what is known as a transparency or celestial window in the atmosphere, into deep space.
Jeremy Munday, a professor of electrical and computer engineering at the University of California at Davis who researches cleantech, said this redirect would hardly affect space. The sun already emits more than a billion times more heat than the Earth, he said, and this method is limited to reflecting the heat already generated by the sun. “It would be like pouring a normal glass of water into the ocean,” Munday said.
He calculated that if materials like Purdue’s ultra-white paint covered 1 to 2 percent of Earth’s surface, just over half the size of the Sahara, the planet would stop absorbing more heat than it emits and the global temperature would stop rising.
Munday noted that covering half of the Sahara, or any contiguous area, with that much radioactive material should not happen for a number of reasons, including practicality, concern for wildlife, and the weather disruptions caused by a region that otherwise it suddenly becomes much colder.
But spreading radiative cooling spots around the world could have global and local benefits, such as counteracting the urban heat island effect, which occurs because most buildings absorb and trap much more heat than natural surfaces such as forests, water, and plants.
Although the inhabitants of places as hot and picturesque as Santorini and Casablanca, as its name aptly suggests, have long used white paint to refresh their homes, and municipalities increasingly want to paint roofs whiter, Ruan explains that commercial white paints typically reflect 80 to 90 percent of sunlight. This means that they continue to absorb 10 to 20 percent of the heat, which in turn warms surfaces and ambient air. By comparison, Purdue’s paint absorbs much less solar heat and radiates much more heat into deep space, cooling surfaces to below ambient temperatures.
Still, there are reasons for concern. The standard version of Purdue’s ultra-white paint uses barium sulfate, which has to be mined, increasing its carbon footprint, though Ruan noted that titanium dioxide, which is used in the vast majority of commercial paints, also has to be extracted from the mines.
Geoengineering — manipulating various processes to control Earth’s climate — has also been criticized for distracting from the underlying problem: humans must stop burning fossil fuels to avoid more catastrophic effects of climate change. But even if all fossil fuels were immediately stopped from being used, climate catastrophes would still occur due to the amount of greenhouse gases that are trapped in the atmosphere. Large-scale radiative cooling, according to Munday, would be like a lifeboat.
“Definitely not a long-term solution to the climate problem,” Munday said. “This is something that can be done in the short term to mitigate worse issues while trying to get things under control.”
Cara Buckley is a climate reporter who focuses on the people working on solutions and stories of unusual responses to the crisis. She joined The Times in 2006 and was part of a team that won a Pulitzer Prize in 2018 for reporting on workplace sexual harassment.
This article was originally published on The New York Times website in 2023.