Night Skies Are Even Brighter Than We Thought — and Getting Brighter

If you’re one for wishing on a lucky star, you may have noticed your options are dwindling. While on a very dark, clear night, most people could expect to see at least a couple thousand stars, artificial light is making it harder to see the stars from nearly everywhere on Earth, with potential consequences for birds, humans and many other species. According to a study published in Science in January, light pollution is having an even bigger effect on night skies than previously measured. 

“I was very shocked,” says Christopher Kyba, a Canadian physicist at the German Research Center for Geosciences and first author of the paper. “I really did not expect that the results were going to be as bad as we saw.”

Making use of a massive dataset of community scientist observations of visible stars, Kyba’s analysis suggests that between 2011 and 2022, night skies in North America and Europe brightened by almost 10 percent each year. That means an overall doubling of sky brightness every seven years. Put another way, a child born tonight under a sky with 250 visible stars would only be able to see about 100 by their 18th birthday.

The findings paint a much starker picture of light pollution than many earlier estimates, including Kyba’s own, which relied on satellite measurements. Satellites are likely to underestimate the true extent of artificial light at night for at least two reasons. First, earth-orbiting satellites are best at capturing lights that either point straight up or straight down—and reflect back up—through the atmosphere. That leaves out many sources of illumination like signs and house lights that cast their glow to the side. Because of the way light scatters through the atmosphere, Kyba says, lights that shine sideways contribute the most to overall sky brightness.

Satellites currently in orbit also do a poor job capturing blue-hued light. As more lights around the world are replaced by blue-skewed LEDs, the gap widens between what satellites perceive and the reality of sky brightness. “The super interesting thing about this paper is that it shows how much the satellites are missing,” says Travis Longcore, a light pollution researcher at UCLA who was not involved in the study.

Humans may miss seeing constellations, and there is strong evidence that light pollution negatively impacts sleep and overall health, but for other animals the impact can be even more dire. Longcore, who has studied the ecological impacts of artificial lighting for more than 20 years, says sky brightness can affect avians and other animals in a number of ways, from drawing migrating birds off-course and into dangerous cityscapes to changing when and where they lay their eggs. And it doesn’t take much: In one recent study of Western Snowy Plovers, Longcore and his colleagues found that even a small amount of artificial light—about the extra brightness of a half-moon in the sky—was enough to discourage the threatened shorebirds from roosting at a given site.

Light pollution has even been found to affect species that migrate during the day, like Purple Martins, and a study from last fall suggested birds attracted to artificial light end up with more exposure to toxic air pollution.

For more than half a century, scientists have hypothesized that at least some species of migratory birds use the night sky for navigation, which would make the loss of visible stars especially detrimental. Planetarium experiments in the 1950s and 1960s with warblers and Indigo Buntings provided compelling evidence that some birds use the night sky to chart their course. More recently, scientists in Russia and Germany have continued investigating birds’ “star compass,” but it’s difficult to to design a study that shows a bird is relying on stars, rather than the magnetic field, smells, or other cues. Regardless, it’s clear that light pollution is bad for birds in several ways.

“The night sky is a natural resource that should be preserved,” says Carolyn Burt, a light pollution researcher at Colorado State University who was not involved in Kyba’s study.

Compared to air or water pollution, at first glance light pollution seems like it has an easy fix, says Burt. “There’s a simple solution, right? You can just turn the lights off and then those consequences go away.”  But the reality of artificial light, she says, is more complicated, and tied up with people’s ideas of aesthetics and perceived safety. That’s something Burt, Longcore, and Kyba all hope to change.

“Sky brightness is a visual manifestation of waste,” Kyba says. Most important, he says, is to make sure you use artificial light only when and where you really need it. Instead of floodlights on your whole backyard, for example, try a small light right at your door, set with a motion sensor.

Want to do even more? You, too, can contribute to the global database of night sky observations Kyba analyzed. Anyone with a portable computer or smartphone can participate by going outside after dark and choosing a star chart that most closely matches the sky they see, wherever they are.

Despite his alarming findings, Kyba has reason to hope. He has seen a growing awareness of the problem, and he’s encouraged by the progress of German cities, which emit three to five times less light than American cities of similar size. While living in Potsdam, a German city of around 200,000 people, Kyba could see hundreds of stars. On occasion, he’d even catch a glimpse of the dense band of our galaxy.

“What that tells me is there’s no reason, actually, that you couldn’t have relatively large cities where, from a park in the center of the city, you’d be able to see the Milky Way,” Kyba says, acknowledging that it sounds far-fetched. But he insists, “It’s possible to imagine a future in which a lot of stars come back.”