When I was a kid, my family lived in suburban Washington, D.C. This location made being a budding amateur astronomer tough; most stars were invisible against the overhead glare from city lights. At best there was only a hint of the diffuse Milky Way to see: the combined radiance of 100 billion stars was dimmed to near nothingness by bright streetlamps and storefronts.
This effect is light pollution—human-generated illumination cast up into the heavens—causing the sky itself to glow and washing out the stars. Astronomers have known for years that the situation is bad for stargazing, but it also has real and negative consequences for the well-being of many living things—plants, animals and even humans. More than 80 percent of humanity is affected by light pollution, our view of the skies being stolen away.
For most of us the stars are, in essence, going out.
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And each year it gets worse. How much worse, exactly, has been hard to say. Light pollution has been measured from space, but orbiting satellites don't detect light the same way the human eye does, so they may not yield results that match what we see from the ground. When people look at the sky, what is the change in sky brightness they perceive over time?
To find out, a team of scientists led by light-pollution researcher Christopher Kyba of the GFZ German Research Center for Geosciences in Potsdam turned to what may seem like a weirdly obvious detection method: human beings.
They used data from Globe at Night, a project run by the U.S. National Science Foundation's National Optical-Infrared Astronomy Research Laboratory, which uses citizen science to measure light pollution. The process is brilliantly simple. Volunteer participants are given a set of star charts (created by Jan Hollan of the Global Change Research Institute of the Czech Academy of Sciences) that show the sky with a range of stars visible: one chart shows only the brightest stars, the next includes somewhat fainter stars, and so on down to the faintest stars visible to the naked eye under ideal conditions. Participants then look at the sky and compare the faintest stars they can see with the ones on the charts, choosing those that best match what they observe.
Kyba and his team examined an astonishing amount of data from more than 50,000 citizen scientists around the world who sampled their local sky brightness between 2011 and 2022. Although there was considerable place-to-place variability—for example, on average, Europe saw a 6.5 percent increase in light pollution annually, whereas North America saw a 10.4 percent increase—the researchers found that globally, light pollution increased by 9.6 percent a year over the study period.
This might not sound like much, but it reflects an exponential growth rate, similar to how compound interest accrues on a debt. A year-over-year growth of about 10 percent means sky brightness is doubling roughly every seven or eight years. A moment's thought should make clear why that is deeply troubling. As Kyba and his co-authors explain in their paper, published in January in the journal Science, if there are 250 visible stars in the sky when someone is born, by the time they're 18 they'll see only 100, and over that same period the sky will have increased in brightness by more than a factor of four.
This result is all the more alarming because of its potential implications for satellite-based measurements, which have recorded only an approximately 2 percent annual increase. Based on their work, Kyba and his team argue that the satellites are severely underestimating the effects, obscuring the possibility of a looming future in which almost everyone loses sight of the stars.
Technological changes can account for much of this discrepancy. For example, Kyba and his colleagues point out that over recent years many older outdoor lightbulbs that emitted redder light have been replaced by LEDs that shine more brightly in blue—a color that scatters more easily in the atmosphere and to which many Earth-observing satellites' detectors are less sensitive. Moreover, satellites mostly see light that shines straight upward, such as from cities and poorly constructed streetlights, rather than horizontally cast rays from windows or billboards, which can greatly affect observers on the ground.
All this extra light at night has a large effect on the life under it. Researchers have shown that it negatively impacts many animals and plants: light pollution disrupts the migrations of birds, the delicate blossoming of flowers and even the luminous courtship of fireflies, to name just a few examples. It affects humans as well, possibly triggering insomnia, among many other health problems.
In some ways, this is reminiscent of the climate crisis: global in nature, difficult to notice day by day, and hard for individuals to grasp and mitigate on their own. I suspect, however, that if global warming increased by some 10 percent a year, we would have long ago tackled the issue head-on.
Worse, light pollution puts on a friendly face for many people who think increased light at night automatically means increased safety. Although more light does help in some cases—for example, illuminated roads make it easier for drivers to see at night—it isn't guaranteed to protect us as much as people might think. And on average, this increased illumination just throws even more unwanted light upward.
So what can we do about our brightening skies?
There is a lot already happening. Groups such as the International Dark-Sky Association, or IDA, advocate not for more lighting but for more intelligent lighting—smarter streetlights that concentrate their light downward are one example. Because these lights offer more efficient illumination, they save energy, too, eventually paying for themselves. IDA offers advice on how to contact local authorities about installing better fixtures and creating ordinances to reduce pollution. A number of cities in the U.S. and other countries are designated Dark Sky Communities, meaning they have shown “exceptional dedication to the preservation of the night sky” by discouraging wasteful lighting practices.
At the moment, simple awareness is one of our greatest benefits. Turning off your own outdoor lighting at night might not seem like a big deal, but if you tell others, that helps. Awareness grows. Any cause like this one needs a critical mass to get widespread notice, so everyone who participates can add to the solution.
Still, local solutions, such as the recent successes in Pittsburgh and Fort Collins, Colo., don't readily translate to global progress. This kind of change isn't easy for everyone; many areas in developing nations have dangerously insufficient lighting at night and use wasteful greenhouse gas–emitting fuels to power the meager light sources they have. More lighting could help people living there out of poverty but at the cost of a larger increase in sky glow. The research by Kyba and his team didn't cover developing nations well, so it's not clear at what rate their light pollution is increasing. But it's obvious enough that more efficient lighting would benefit these regions, too, if for no other reason than it would keep their costs down in the medium to long run.
In an epic thread on Twitter, Kyba goes over the methodology and results of his group's work and includes some advice on what individuals can do. He suggests using targeted illumination rather than flood lighting, deploying outdoor lighting only when needed, and opting for lightbulbs and LEDs that shine more red than blue to reduce how much light scatters across the sky.
We do need bigger and smarter solutions. Certainly the physical and biological effects of light pollution are a big concern, but more is at stake here: the loss of beauty and our connection to nature. The night sky is, quite simply, gorgeous, with treasures scattered among the stars. Going out under that velvet vault and watching a meteor shower or a lunar eclipse is a wonderful way to spend time with family and friends or to simply decompress. To see the stars is to nourish the soul. I've seen (and heard) countless owls and coyotes and other wildlife while outside at night, and observing the heavens gives me a profound appreciation of the natural world around me. The awe of the night sky is very real.
This isn't just a matter of a few inconvenienced astronomers. It's the equivalent of shuttering the Louvre, of closing concert halls, of mowing down vast fields of wildflowers. I wonder how deep my own love of astronomy would have become had I stayed in the suburbs of D.C., where the starlight would have diminished, not so gradually, as I grew up. I struggled to see the skies through that miasma as it was; only a profound love of astronomy kept me going. Many people don't even know that they—and their descendants—are losing this cosmic experience just over their heads.
We need the dark night sky, and it's up to all of us to make sure it's still there every time the sun goes down.
This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.