Light pollution is drowning the starry night sky faster than thought

Stargazing citizen scientists show how constellations are dimming at an alarming rate

19 Jan 2023. 2:20 PM

By Joshua Sokol

The star pattern Western astronomers call Orion has long drawn our eyes to the night skies. Some Ki’che’ Maya people see it as a cosmic hearth. For the Khoisan people of southern Africa, it’s an archer confronting zebras and a lion. Meanwhile the Carib people of South America see a one-legged hunter named Epietembo. But future generations might wonder what all the fuss was about. According to a new study analyzing citizen scientist reports, global light pollution is drowning out Orion and other constellations in many parts of the world—and it’s getting worse every year.

“The problem is worse than we previously believed,” says John Barentine, an independent dark sky researcher based in Arizona who didn’t participate in the study.

The most pervasive form of light pollution is skyglow, the background glimmer of photons emitted by streetlights, billboards, storefronts, and countless other human sources. Although handheld detectors can measure skyglow above any one location, scientists have so far struggled to quantify its full extent.!

Most regional or global measurements rely on satellite instruments such as the Visible Infrared Imagining Radiometer Suite (VIIRS), which takes daytime and nighttime images of Earth’s surface. VIIRS was used in 2017 to suggest the world’s light pollution output is increasing by about 2% per year. Yet the instrument is blind to blue wavelengths of light, which are emitted by increasingly common LED fixtures. Also, its eye-in-the-sky perspective mostly sees the light that escapes straight up into space, not the light that gets scattered in the atmosphere. As a result, many researchers believed that 2% estimate was an undercount.

To come up with a more accurate figure, Christopher Kyba, a physicist at the GFZ German Research Centre for Geosciences, turned to a trick older than the oldest book: stargazing. He teamed up with Globe at Night, a citizen science project run by the National Optical-Infrared Astronomy Research Laboratory. For more than a decade, Globe at Night has asked participants around the world to describe the stars and constellations they can see with their naked eyes. These measurements record the faintest stars visible from location to location on any given night.

Kyba and colleagues analyzed 51,351 observations collected by Globe at Night volunteers from 2011 to 2022 and cross-checked them against satellite-based maps of light pollution over the same time period. The results shed light on how much skyglow has obscured the night sky in different parts of the world over time. It’s a clever approach, Barentine says. “There is no way a research team with unlimited dollars could have put enough sensors out there in the world to get an equivalent result.”

In Europe and North America, where most of the participating citizen scientists live, skyglow has been increasing by 6.5% and 10.4% per year, respectively, the researchers report today in Science. In developing countries, citizen science data points are currently too scarce to draw conclusions. But based on previous satellite measurements, the team suspects skyglow in those places is increasing at even higher rates.

“It’s pretty shocking,” Kyba says. “Even worse than I had worried.” If the trend continues, he says, children born today in a light-polluted area who can see about 250 stars would see skyglow quadruple by their 18th birthday—leaving only 100 stars to wish on.

The findings add to mounting concerns about how artificial light harms wildlife, says Eva Knop, an ecologist at the University of Zürich. Previous research suggests even the faint shine of skyglow from cities hundreds of kilometers away can lure migratory songbirds to their deaths, distort predator-prey interactions, disrupt animal hormones, and interfere with a web of other biological processes.

By framing the problem of light pollution in relatable terms—the number of visible stars—the new results could aid conservation efforts, says Ashley Wilson, a biologist at the International Dark-Sky Association. “That is going to change our management strategies, and relationships with neighboring communities to protected areas.”

The findings also suggest existing dark sky ordinances haven’t accomplished much, Barentine adds. Despite warnings about light pollution, communities have continued to add artificial lighting, he says. “You put together cheap lighting and fear of the dark … and people are not choosing preservation of darkness.”

Better data, in the form of more Globe at Night users or a dedicated light pollution–monitoring satellite, could draw attention to the problem and help determine whether dark sky preservation efforts—such as retrofitting streetlights to direct their light downward and ordinances that set late-light curfews—are working, Barentine says. “The hope is that this is what sets the world’s hair on fire.”