The Problems of Light Pollution

Brian Handwerk

Science Correspondent

Gazing upward into the night sky, the Greeks recognized a human figure in the stars. They dubbed him Orion, and the hunter's story has persisted for thousands of years. The Egyptians regarded the same group of stars as the place where the soul of a god—Osiris—found eternal rest. The Arabs interpreted the starry figure as a giant. In India, the constellation was seen as a king who’d been shot by an arrow. To America's Ojibwe people, the celestial figure is Biboonkeonini the Wintermaker, who, they know by tracking movements of the stars, becomes most prominent during the coldest season of the year.

"The night sky belongs to everybody—this is humanity's common heritage," says Stephen Loring, an archaeologist with the Smithsonian's National Museum of Natural History. "Everybody all around the world has some kind of story about the night sky."

Celestial beauty is something sacred that is shared across human cultures around the globe, he says.

But today the connections that have always tethered humanity to the same night sky are disappearing behind the ubiquitous brightness of artificial lights. Glow from buildings, parking lots, roadways, billboards, landscape lighting and countless other sources blots out the stars over a surprisingly large part of the planet and disrupts the lifecycles of birds, insects and other creatures.

One-third of the world's population can't see the starry band of light in the night sky that makes up the Milky Way—the vast spiraled arms of our own galaxy. The stunning array is now rendered invisible for some 60 percent of Europeans and almost 80 percent of North Americans. And as light pollution literally makes the darkness disappear, all types of plants and animals, accustomed to cycles of night and day for millions of years, are being dramatically affected in ways both obvious and unknown. Just as a bright light coming into a bedroom window might interrupt the sleep of any one of us, darkness is a foundational need of untold floral and animal species; artificial light impedes their ability to rest, feed and reproduce.

Our night sky and the global impacts of its disappearance due to light pollution are now the focus of a new 4,340-square-foot exhibition, "Lights Out: Recovering Our Night Sky," on view at the Natural History Museum through December 2025. Fortunately, as the show makes clear, the solutions to recovering our night sky are known, are cost-effective and can often be implemented quickly—almost like flipping a switch.

Artificial light has been viewed as an example of progress in an increasingly anthropogenic world. Lights have allowed us to capture many more hours for work and play. They’ve driven economic growth and social connections. They’ve made us feel safe. Turning out the lights seems unthinkable; yet a lack of thought has produced enormous amounts of unnecessary and misdirected lighting.

Examine a satellite image of Earth from space and the amount of nighttime light that humans produce is obvious and astonishing. Glowing lights beam out from across the globe, wherever people congregate. Dark pockets where natural night is experienced are becoming the exception, found in the far north, Antarctica, and the deep interiors of the Amazon and Sahara.

Yet on the ground the problem often goes unnoticed; the change is a relatively recent phenomenon.

Cities began to adopt electric lighting in the 1870s, but only in recent decades has the proliferation of very cheap artificial lighting, particularly LED bulbs, truly set the skies aglow. In 2016, researchers produced a "World Atlas of Artificial Night Sky Brightness" that showed a staggering 80 percent of the world's population lived under skyglow. In Europe and the United States, fully 99 percent of the public isn't able to experience the pleasure of gazing at the stars under natural nighttime conditions.

A study published in 2021 found that light pollution has increased by at least half over just the past 25 years. The areas living under domes of skyglow, created by artificial lights and reflected off clouds and atmospheric molecules or aerosols, are growing by 2 percent each year, compounding at a rate that is outpacing population growth, meaning we’re also using more light per person.

John Barentine, an astronomer with Dark Sky Consulting, previously managed the International Dark Sky Places Program, where he spoke with many city dwellers who got the chance to observe the Milky Way for the first time. The experience for many of them was extremely emotional. "It was rare that I’d talk to somebody who kind of shrugged their shoulders," he says. "Many described it as a deeply spiritual, even [a] religious experience that I think taps into something that's deep in our core as human beings."

For the millions who have never had such an experience because they live in and near cities where they’ve been disconnected from the night sky for generations, Barentine suggests that they may not even realize what they’ve lost. "How do you get people to care about something that they don't have access to, and they may never have had access to?" he asks.

But the loss of the night sky goes far beyond human enjoyment of a starry sky. Most species are guided by a circadian rhythm, an internal clock attuned to light and dark that governs cycles of activity and rest.

Scientists are working to uncover the many ways in which artificial light and a lack of darkness can harm the planet's living things. Birds, for example, are paying a heavy price. Each year millions of migrating birds, traveling through the cool, less turbulent night air, are killed when artificial lights in urban buildings throw them off a course that is steered by the moon and stars. Attracted by the lights, confused birds crash into buildings or exhaust themselves trying to find their way.

"We’re talking about billions of birds dying," says Loring. The easy solution, he adds, is to just turn off those lights. "They don't have to be on all night long."

Some cities in migratory pathways are doing just that; Philadelphia, Chicago, San Francisco and other areas are periodically reducing lighting during key migration periods—and these "Lights Out" initiatives have caused bird deaths to drop dramatically.

Unnatural light can also hinder reproductive cycles, which have evolved to coincide with seasonal light changes. For example, tammar wallabies living near urban areas in Western Australia reproduce a month later than others, and these kinds of changes might threaten species when the timings of births no longer match the supplies of food.

Marine species, too, can suffer from bright skies. Studies show that light pollution throws off coral reproduction, because the natural light cycles of day and night along with the moon phases are necessary for the synchronized spawning process that leads to the release of eggs and sperm.

While we tend to think of plants as light-lovers, the biological need for darkness is crucial to their existence. Artificial light stresses plants, including trees, by driving excess photosynthesis, and altering the timing of budding and leaf dropping. Some flowering plants only open at night, when they attract nocturnal pollinators like bats and insects—if they aren't lured away by lights. More than 500 species, including bananas and mangoes, are pollinated by bats, and these critical plants are at risk when light disrupts nocturnal movement of their pollinators.

Insects are famously attracted to light bulbs, where they may die from exhausted flittering or be picked off by predators. Worldwide, insects are being decimated in diversity and in numbers. This stems from many factors, but light pollution is likely among them, as more than half of all insects are nocturnal. Some insects, like the cabbage moth, not only live at night but also reproduce exclusively in the dark. Studies show that artificial light interrupts the female moth's ability to produce pheromones that attract mates, thus stymying the insect's once-in-a-lifetime shot at reproduction.

Humans, too, can suffer from the proliferation of artificial light. Darkness triggers the production of the hormone melatonin, which helps us sleep, and exposure to even dim artificial lights can block melatonin production. This can cost us restful sleep, which is bad enough, but studies also show that these disruptions of our own circadian rhythms can be linked to a host of health problems, including depression, obesity, heart conditions and even cancer. Blue light wavelengths produced by today's ubiquitous LED bulbs, which are increasingly used in outdoor lighting, are especially disruptive to our natural rhythms of rest and activity.

Artificial light is also dimming our ability to see beyond our own planet into the vast, limitless possibilities beyond. Astronomers, peering deep into the darkened skies, were among the very first to raise an alarm that our own artificial lights were beginning to blot out the nighttime views.

The Harvard College Observatory, now part of Harvard and the Smithsonian's Center for Astrophysics in Cambridge, Massachusetts, once housed the largest telescope in North America. As lights multiplied in the surrounding city, the instrument—like other urban observatories—became obsolete.

As early as 1958, Lowell Observatory experts became vexed by local lights—and their concerns prompted nearby Flagstaff, Arizona, to enact the world's first light-pollution ordinance on April 15 of that year. Ordinance No. 440 prohibited the use of certain searchlights within the city limits.

Observatories have since been pushed to remote, dark, high-elevation sites, yet their ability to observe invisible light is still a major concern. "Astronomers are losing important access to our understanding of what's beyond us," says Kim Arcand, the Chandra visualization scientist and emerging technology lead with the Center for Astrophysics, who co-curated the new show.

"That ability to go outside and look up," she says, "means we can see a part of where we come from, quite literally the iron in our blood and the calcium in our bones, is coming from the previous generation of stars that have exploded long ago."

"There are very good reasons to protect dark skies for astronomy's sake," Arcand says, "but it all comes back to those same questions humans have been asking across the millennia: Where do we come from? Why are we here? And where are we going?"

There are still quiet, rural spots where the night sky is dark—and these regions are being celebrated. The International Dark-Sky Association recognizes more than 200 Dark Sky Places. Escaping urban areas for these starry refuges inspires people with the wonders of the night sky, but it doesn't solve the problem. Urban sprawl keeps growing, and light pollution from a bright city impairs views of the sky even to those more than 40 miles away.

Experts argue that the night sky does not need to be a tourist destination. Practical solutions for local communities not only save money, but they also recognize that many lights are simply unnecessary. According to the museum, about 30 percent of all the country's outdoor lighting goes to waste, squandering around $3.5 billion. Unshaded fixtures shine light needlessly into the sky, while others are wasted on low-trafficked roadways and at shopping centers and office parks that don't need to be continuously lit up all night long.

For safety and just for getting around, stark overhead lights can be effectively replaced with less bright lights or more reflective surfaces. Shades can redirect light where it's needed on the ground and reduce the ubiquitous skyward glow.

LED blue lights should be replaced with warmer-colored lights, which are typically strong enough to do the job. Timers and motion-detection lights can limit the lengthy amounts of time that unneeded lights are left to simply blaze away and illuminate nothing at all.

"For me there's something eminently fixable about an issue that we can improve if we just direct our lights the right way, if we just turn off lights when we’re not using them," Arcand says, noting that real change can occur at the community and even personal levels. "None of these things are hard or trying to take the fun out of life. They are such small things that the individual has some ability to enact change and just fix a very human-made problem."

There's plenty of proof that these ideas work. In 2017, the U.S. National Park Service used cameras to compare lights in Flagstaff, a leading dark sky city, with comparably sized Cheyenne, Wyoming.

Cheyenne, with no dark sky regulations, was 14 times as bright. "Flagstaff has chosen that path and said, ‘We’re going to go dark,’ so it can be done," Loring says. "And maybe it grows. Maybe cities and towns say, ‘The next time we need to fix the lights, let's do this.’"

"We know what policies are demonstrated to work, there is no technical obstacle, we’re not trying to devise a solution—we know what it is," Barentine stresses. "Our only barrier to implementing it is convincing enough people that there are good reasons to do this."

Barentine says, in his experience, not everyone is easily convinced to embrace those changes. Some people just cannot fathom how light, which they enjoy for practical, aesthetic and safety reasons, can be a bad thing.

"There's a very deep-seated human psychology that simply makes many of us afraid of the dark," he says. That's why efforts to restore the night sky stress using better lighting, not living in the dark.

And unlike air or water pollution efforts, which can take many years, cleaning up light pollution can have immediate results. Doing so can help restore the natural darkness to which so many species are attuned—and keep humanity's connection with the night sky strong.

"We have been looking up for millennia," says Arcand, "and it's given us an amazing amount of art, cultural stories and ideas." Among those inspired was Dante, whose Inferno ended its epic tour of hell when the author and Virgil escape from the tortures of the underworld to be greeted by a wondrous sight.

"Him first, then me—until we came to a round opening

Through which I saw some of the beautiful things

That come with Heaven. And we walked out

To once again catch sight of the stars."

Nobody would suggest that artificial light is making a kind of hell on Earth. But the unnecessary skyglow we create is definitely preventing all too many from enjoying the heavens above.

"Lights Out: Recovering Our Night Sky" is on view through December 2025 at the Smithsonian's National Museum of Natural History.

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Brian Handwerk | READ MORE

Brian Handwerk is a science correspondent based in Amherst, New Hampshire.