To help cool a hot planet, the whitest of white coats

Xiulin Ruan, a professor of mechanical engineering at Purdue University, didn’t set out to make it into the Guinness World Records when he began trying to make a new type of paint. He had a loftier goal: to cool down buildings without torching the Earth.

In 2020, Ruan and his team unveiled their creation: a type of white paint that can act as a reflector, bouncing 95% 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 potent formulation that increased sunlight reflection to 98%.

The paint’s properties are almost superheroic. It can make surfaces as much as 8 degrees Fahrenheit cooler than ambient air temperatures at midday, and up to 19 degrees cooler at night, reducing temperatures inside buildings and decreasing air-conditioning needs by as much as 40%. It is cool to the touch, even under a blazing sun, Ruan said. Unlike air conditioners, the paint doesn’t need any energy to work, and it doesn’t warm the outside air.

In 2021, Guinness declared it the whitest paint ever, and it has since collected several awards. Although the paint was originally envisioned for rooftops, manufacturers of clothes, shoes, cars, trucks and even spacecraft have come clamoring. Last year, Ruan and his team announced that they’d come up with a more lightweight version that could reflect heat from vehicles.

“We weren’t really trying to develop the world’s whitest paint,” 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 down the Earth.”

While the paint is officially the world’s whitest, it isn’t blindingly so because it scatters light, Ruan said. It doesn’t look all that different from white paint from the hardware store.

The paint is at least a year from being ready for commercial use, and work is underway to increase its durability and dirt resistance. Ruan said the Purdue team has partnered with a company but can’t yet name it. The team is also developing colored paints that use the 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 been urgently working to develop reflective materials, including different types of coatings and films, that could passively cool the Earth. The materials rely on principles of physics that allow thermal energy to travel from Earth along specific wavelengths through what’s known as the transparency or sky window 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 would barely affect space. The sun already emits more than 1 billion times more heat than the Earth, he said, and this method merely reflects heat already generated by the sun. “It’d be like pouring a cup of regular water into the ocean,” Munday said.

He calculated that if materials such as Purdue’s ultra-white paint were to coat between 1% and 2% of the Earth’s surface, slightly more than half the size of the Sahara, the planet would no longer absorb more heat than it was emitting, and global temperatures would stop rising.

Munday noted that covering half the Sahara, or any contiguous surface, with that much radiative material shouldn’t 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 spots 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 woodlands, water and plants.

While humans in such hot and picturesque places as the Greek island of Santorini and the aptly named Casablanca, Morocco, have long used white paint to cool dwellings, and municipalities are increasingly looking to paint rooftops white, Ruan said commercial white paints generally reflect 80% to 90% of sunlight. This means they still absorb 10% to 20% of the heat, which in turn warms surfaces and the ambient air. The Purdue paint, by comparison, absorbs so much less solar heat and radiates so much more heat into deep space that it cools surfaces to below-ambient temperatures.

Still, there are concerns. The standard version of Purdue’s ultrawhite paint uses barium sulfate, which has to be mined, driving up its carbon footprint, although Ruan noted that titanium dioxide, which is used in the vast majority of commercial paints, also has to be mined.

Geoengineering — manipulating different processes to control the Earth’s climate — has also been criticized for distracting from the root problem: Humans must stop burning fossil fuels to avoid more catastrophic effects of climate change. But even if all fossil fuel use stopped immediately, climate disasters would continue to unfold because of the amount of greenhouse gases that are trapped in the atmosphere. Large-scale radiative cooling, Munday said, would be akin to a life raft.

“This is definitely not a long-term solution to the climate problem,” Munday said. “This is something you can do short term to mitigate worse problems while trying to get everything under control.”