Xiulin Ruan, a professor of mechanical engineering at Purdue University, didn’t start out making it into the Guinness World Records when he started trying to make a new type of paint. He had a higher goal: to cool buildings without burning the Earth.
In 2020, Dr. Ruan and his team unveiled their creation: a type of white paint that can act as a reflector, bouncing 95 percent of the sun’s rays away from the Earth’s surface, up through the atmosphere and into deep space. A few months later, they announced an even more powerful formula that increased sunlight reflection to 98 percent.
The properties of the paint are almost superhero. It can make surfaces as much as eight degrees Fahrenheit cooler than ambient air temperatures at midday, and up to 19 degrees cooler at night, reducing temperatures inside buildings and reducing air conditioning needs by as much as 40 percent. It’s cool to the touch, even under a blazing sun, Dr. Ruan said. Unlike air conditioners, the paint does not need energy to operate, and it does not heat the outside air.
In 2021, Guinness declared it the whitest paint ever, and since then it has collected several awards. While the paint was originally envisioned for roofs, manufacturers of clothing, shoes, cars, trucks and even spaceships have clamored. Last year, Dr. Ruan and his team announced that they would come up with a lighter version which could reflect heat from vehicles.
“We didn’t really try to develop the world’s whitest paint,” Dr. Ruan said in an interview. “We wanted to help with climate change, and now it’s more of a crisis, and it’s getting worse. We wanted to see if it was possible to help save energy while cooling the Earth.”
While the paint is officially the world’s whitest, it’s not blinding because it scatters light, Dr. Ruan said. It looks no different than white paint from the hardware store.
The paint is at least a year after it is ready for commercial use, and work is ongoing to increase its durability and dirt resistance. Dr. 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 the white, but better than some of the other commercial colors,” he said.
As the climate crisis worsens, scientists have worked urgently to develop reflective materials, including different types of coatings and movies that could passively cool the Earth. The materials rely on principles of physics that allow thermal energy to travel from the Earth along specific wavelengths through what is known as the transparency or skylight in the atmosphere, and out into deep space.
Jeremy Munday, a professor of electrical and computer engineering at the University of California, Davis, who researches clean technology, said this redirection will hardly affect space. The sun already emits more than a billion times more heat than the Earth, he said, and this method only reflects heat already generated by the sun. “It would be like pouring a cup of regular water into the ocean,” Dr. Munday said.
He calculated that if materials like Purdue’s ultra-white paint covered between 1 percent and 2 percent of the Earth’s surface, slightly more than half the size of the Sahara, the planet would no longer absorb more heat than it emitted, and global temperatures. will stop rising.
Dr. Munday noted that covering half of the Sahara, or any adjacent surface, with so much radioactive material should not happen for a number of reasons, among them practicality, wildlife concerns and weather disruptions caused by one region suddenly becoming much cooler.
But spreading radiative cooling points around the world could have global and local benefits, such as offsetting the urban heat island effect, which occurs because most buildings absorb and trap much more heat than natural surfaces like forests, water and plants.
While people in such warm and scenic places as Santorini and the aptly named Casablanca have long used white paint to cool homes, and municipalities are increasingly looking to paint roofs white, Dr. Ruan said commercial white paints generally reflect 80 percent to 90 percent of sunlight . This means they still absorb 10 percent to 20 percent of the heat, which in turn warms surfaces and the surrounding air. The Purdue paint, by contrast, absorbs so much less solar heat and radiates so much more heat into deep space that it cools surfaces to sub-ambient temperatures.
However, there are concerns. The standard version of Purdue’s ultra white paint uses barium sulfate, which must be mined, drive up its carbon footprint, although Dr Ruan noted that titanium dioxide, which is used in the vast majority of commercial paints, also needs to be mined.
Geoengineering – manipulation of different processes to control the Earth’s climate – has also been criticized for ignoring the root problem: Humans must stop burning fossil fuels to avoid more catastrophic effects of climate change. But even if all fossil fuels were to stop immediately, climate disasters will continue to unfold due to the amount of greenhouse gases trapped in the atmosphere. Large-scale radiative cooling, Dr. Munday said, would be akin to a life raft.
“This is certainly not a long-term solution to the climate problem,” Dr Munday said. “This is something you can do in the short term to mitigate worse problems while you’re trying to get everything under control.”