ong ago, in lands that were always warm, people got ice from the heavens. At sunset, they poured water into shallow earthen pits or ceramic trays insulated with reeds. All through the night the water would radiate its heat into the chilly void of space. By morning, it turned to ice — even though the air temperature never dropped below freezing. This wasn’t magic; it was science.

The world now cools off with the help of more than 3.5 billion refrigerators and air conditioners, a number that is quickly growing. But those appliances are also a major source of greenhouse gas emissions. In seeking relief from the heat, humans are making the globe even hotter, compounding the demand for cooling.

To break that cycle, University of California at Los Angeles materials scientist Aaswath Raman wants to turn ancient technology into a 21st-century tool.

Justin Andres, left, and Danny Laporte apply a protective layer of film containing copper and silver on SkyCool panels at Grocery Outlet in Stockton, Calif., on Oct. 5. (Sarahbeth Maney for The Washington Post)

Working with colleagues, he has developed a thin, mirror-like film engineered to maximize radiative cooling on a molecular level. The film sends heat into space while absorbing almost no radiation, lowering the temperature of objects by more than 10 degrees, even in the midday sun. It can help cool pipes and panels — like a booster rocket for refrigerators and cooling systems. Incorporated into buildings, it may even replace air conditioning. And it requires no electricity, no special fuel — just a clear day and a view of the sky.

“It sounds improbable,” Raman acknowledged. “But the science is real.”

Generations after people learned to make ice in the desert, he hopes that same science can help us survive in a rapidly warming world.

SkyCool panels send heat to the sky and pull down cooling from space. They’re used to help keep refrigerators cool, reducing the amount of electricity they need, at Grocery Outlet in Stockton, Calif. (Sarahbeth Maney for The Washington Post)

Growing up in Alberta, Canada, where his father worked in the oil industry, Raman had an up-close view of the problem confronting the planet. Though the burning of fossil fuels is driving dangerous changes in the global climate, it also powers most of modern society. “I had no illusions about being able to solve it immediately,” Raman said. “I understood how huge the energy industry is, and if you want to really displace it, anything that came after it would have to be just as big.”

He went college to study astronomy, but an interest in solar panels led him to photonics, the study of light . Much like astronomy, photonics allowed him to explore the fundamental workings of the universe. At the same time, he hoped it might lead to discoveries that improved conditions on Earth.

In 2012, as he neared the end of his doctoral studies at Stanford University, he stumbled upon a reference to radiative cooling in an academic journal. Intrigued, he dug up whatever research on the phenomenon he could find.

Examples of radiative cooling after dark, also called night sky cooling, were everywhere. Raman uncovered century-old descriptions of the ancient ice-making practice and case studies from the 1970s describing efforts to cool buildings with rooftop pools (most efforts were abandoned when the pools became too difficult to maintain). He witnessed the phenomenon in his own life; it’s the reason frost can form on clear nights when the temperature stays above 32 degrees Fahrenheit.