(p. A1) 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 potent formulation that increased sunlight reflection to 98 percent.
The paint’s properties are almost superheroic. 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 build-(p. A12)ings and decreasing air-conditioning needs by as much as 40 percent. It is cool to the touch, even under a blazing sun, Dr. Ruan said. Unlike air-conditioners, the paint doesn’t need any energy to work, and it doesn’t warm the outside air.
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. . ., 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 a 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,” Dr. Munday said.
He calculated that if materials such as Purdue’s ultra-white paint were to coat 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 was emitting, and global temperatures would stop rising.
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While humans in such hot and picturesque places as Santorini and the aptly named Casablanca have long used white paint to cool dwellings, and municipalities are increasingly looking to paint rooftops 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 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.
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(Note: the online version of the story has the date July 12, 2023, and has the title “To Help Cool a Hot Planet, the Whitest of White Coats.”)