Long 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.

For centuries, desert dwellers in North Africa, India and Iran tapped into a law of physics called radiative cooling. All objects — people, plants, buildings, planets — give off heat in waves of invisible light. On a clear, starry night, that radiation can rise through the atmosphere until it escapes Earth entirely. Coldness, which is really the absence of heat, is created through this invisible connection to the cosmos.

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.

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.

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.

And in climate change, he saw evidence of what happens when radiative cooling is disrupted. Earth also sends heat into space — that’s how it balances incoming energy from the sun. But the greenhouse gases created by human activities block infrared radiation, trapping it in the atmosphere. The planet is more than 1 degree Celsius (1.8 degrees Fahrenheit) warmer than in the preindustrial era, a shift that has worsened wildfires, intensified hurricanes and altered ecosystems across the world. United Nations scientists say humanity must reduce emissions by 7 percent a year to avoid still more catastrophic effects.

Yet radiative cooling has rarely been discussed as a potential tool for climate action, Raman said. Most researchers saw the phenomenon as an interesting physical fact with few practical applications. The reason: It is only measurable at night, when objects are emitting heat but not receiving any in return. Come morning, energy from the sun cancels out any cooling effect.

“Every paper made some kind of statement to the effect of, ‘Well, it’s usefulness is kind of limited because … you most need cooling during the day,’ ” Raman said. “Then I thought, well, why can’t we make this work during the day?”

The trick was to develop a material so perfectly reflective it absorbed almost no energy, even when exposed to full sunlight. On top of that, Raman wanted to maximize the amount of radiation the film sent into space.

So he found a loophole in the greenhouse effect.

A brief physics lesson: Though we often think of them as separate phenomena, the light that we see and the radiant heat we feel are just different kinds of electromagnetic wave. Visible light comes in an array of wavelengths, from short violet to long red. Thermal radiation typically spans a range of longer wavelengths in the infrared part of the spectrum.

Earth’s atmosphere blocks some outgoing infrared radiation — and it’s blocking even more now that it’s chock full of carbon. But there are “windows” that electromagnetic waves of just the right length can slip through. Somehow, Raman would have to find a way to get objects to emit only radiation that fit through those windows.

With colleagues in Stanford’s engineering department, led by professor Shanhui Fan, he began crafting a film from many microscopic layers. The thickness and composition of these layers were designed to interfere with the way different wavelengths of light travel. Incoming solar radiation would rebound right back into space. Outgoing thermal radiation would bounce around between the layers, like a pinball in a machine; only the desired infrared wavelengths would be able to escape.

Chris Atkinson was program director for Advanced Research Projects Agency — Energy, a division within the U.S. Energy Department, which funded Raman’s early work. When he first heard about the experiment, “my initial response was, if this was so good, why hadn’t it been done before?” he recalled.

But Raman and his colleagues had something their predecessors lacked: modern nanotechnology. They could manipulate their materials, molecule by molecule, until it behaved exactly how they wanted.

“I was struck by the elegance and simplicity of it,” said Atkinson, now a professor of mechanical engineering at Ohio State University. “The fact that you can get something for nothing is remarkable, especially in the energy realm.”

In a few years the Stanford group had its first prototype. Placed outside in the hot California sun, it felt cold to the touch. It was a giddy, counterintuitive sensation, even to Raman.

Yet even after he convinced himself that daytime radiative cooling was possible, it wasn’t until a trip to visit his grandmother in Mumbai that Raman started to see how it could also be useful.

A growing number of homes in Mumbai had air conditioners in their windows, something he rarely saw during childhood visits. That’s an unqualified victory for people’s health, Raman said; exposure to extreme heat can lead to a range of illnesses, from respiratory illness to psychological distress.

But as demand for air conditioning grows, so too will its environmental impact. The hydrofluorocarbons used as coolants and the fossil fuels burned to power the appliances are major contributors to global climate change, associated with about 7 percent of all greenhouse gas emissions. By 2050, when the demand for air conditioning is expected to triple, cooling could become one of world’s top sources of planet-warming gases.

“We kind of realized there was a huge problem and a huge opportunity,” Raman said, “and that this technology, if we developed it correctly, could be a really meaningful solution.”

That realization gave him more parameters for his cooling material. It had to be cheap, so it would be accessible to people of all income levels. It had to be able to integrate into existing air-conditioning systems. As they continued to tinker with the technology, Raman and his collaborators set up a company, SkyCool Systems, to help bring it into the world.

The company produces SkyCool panels that can be incorporated into existing cooling systems. Water running through the panels is chilled by the film, then transported into the air-conditioning system, where it lowers the temperature of the refrigerant. This reduces the amount of electricity needed to turn hot air into cold.

It wasn’t difficult to convince Jesus Valenzuela, store manager at the Stockton, Calif., Grocery Outlet, to test the technology. Between the deli case, the dairy aisle, the freezer section and all the backroom storage, cooling alone cost him more than $100,000 a year. On top of that, the California native was worried by the disasters climate change had already wrought on his state.

An offer from Lime Energy to pay the installation fee sealed the deal. If the film worked, Valenzuela would only owe SkyCool Systems the savings from his electricity bill for the first two years.

The panels were installed this spring. Though Eli Goldstein, SkyCool’s co-founder and CEO, explained the technology to him, he didn’t quite get how the coldness of space could help chill chicken cutlets and freezer pizza.

“There’s a lot of technical things I don’t know about,” he said. But that didn’t matter: The SkyCool panels had lowered his electric bill by about $3,000 over the course of the summer, he said.

“All I know,” Valenzuela said, “is that it’s saving me money.”

The SkyCool technology still needs to be refined, Atkinson said, and it must become significantly cheaper before it can be deployed widely. But the big scientific hurdle has been surmounted, he said. The rest is mostly business.

Meanwhile, Goldstein, Raman and their colleagues are working on further applications of the film. With a grant from the California Energy Commission, they have contracted with the California State University System to replace all the air conditioners in a school building — hoping to cool the entire structure with just the sky. In May, Raman published a journal article on the possibility of modifying off-the-shelf paints to enhance radiative cooling; if it works, building owners could simply paint their roofs to make the structures significantly cooler.

Raman has even researched the possibility of using radiative cooling to create light. In a study last year in the journal Joule, he demonstrated how cooling one side of a thermoelectric generator while keeping the other at air temperature could create a temperature gradient that, when converted into electricity, could power a lightbulb.

Each demonstration of radiative cooling’s power fosters in Raman a sense of kinship with the ice makers of long ago. He imagines them experimenting night after night, using trial and error to perfect their technique — the same scientific process Raman uses today.

In a rapidly changing world, it’s a reminder of what remains the same, he said: The laws of physics. The needs of people. The power of science to explain the workings of the planet and improve the lives of everyone who lives on it.