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Light Pollution Is Outshining Stars Faster Than Thought

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A twilight sky full of stars above some trees
The twilight sky as seen in Coonabarabran, Australia Alan Dyer / VW Pics / UIG via Getty Images

With street lamps, billboards and lights in homes, humans illuminate the night sky. This artificial glow disorients migrating birds, impacts human health and reduces the visibility of stars.

Now, a new study of night sky observations by citizen scientists suggests light pollution is obscuring the cosmos even more quickly than satellite data had previously suggested.

With more artificial lights, the brightness of the night sky increased by an average of 9.6 percent per year between 2011 and 2022, according to the research published Thursday in the journal Science. That’s equivalent to the brightness doubling every eight years.

“This study shows the stark reality of the problem and highlights how vulnerable unobstructed views of the cosmos really are in modern society,” Greg Brown, an astronomer at the Royal Observatory Greenwich in England who did not contribute to the research, tells the Guardian’s Nicola Davis.

The researchers collected 51,351 naked-eye observations of the night sky made by people around the world as part of the Globe at Night project. The citizen scientists were given a set of sky maps that depicted which stars could be seen at varying levels of light pollution at their location. They picked the one that best matched what they observed, providing the researchers with an estimate of the star visibility in a certain place at a certain time, according to the paper.

John Barentine, an astronomer and light pollution researcher who heads a dark-sky conservation consulting firm, tells Science’s Joshua Sokol that the researchers took a clever approach.

“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,” Barentine, who did not contribute to the new paper, tells the publication.

At the measured rate, sky brightness would increase by a factor of four in an 18-year period—a location with 250 visible stars might only have 100 visible stars 18 years later, the authors write.

“It’s pretty shocking,” Christopher Kyba, a co-author of the new paper and a physicist at the GFZ German Research Centre for Geosciences, tells Science. “Even worse than I had worried.”

In the past, satellites have measured a slower rate of light pollution increase: just 2.2 percent per year globally between 2012 and 2016, and 1.6 percent per year between 1992 and 2017, according to the paper.

One possible reason for this difference is that satellites are unable to detect blue light emitted by LEDs, which have increased in prevalence in recent years, physicists Fabio Falchi and Salvador Bará write in a perspective accompanying the paper.

Additionally, satellites might not capture decorative lighting on the sides of buildings that isn’t emitted upward, Kyba theorizes to CBC News’ Nicole Mortillaro.

While Barentine wasn’t surprised that satellites had underestimated the increase in light pollution, “I was still surprised by how much of an underestimate it was,” he tells Science News’ Lisa Grossman. “This paper is confirming that we’ve been undercounting light pollution in the world.”

Light pollution doesn’t just affect stargazers. The new findings add to concerns about the harm artificial light can cause wildlife, Eva Knop, an ecologist at the University of Zürich, tells Science. Artificial lights contribute to the deaths of millions of migratory birds each year. They can also affect human health and mess with our sleep, per BBC News’ Victoria Gill.

“People should consider that this does have an impact on our lives. It’s not just astronomy,” Connie Walker, a co-author of the paper and an astronomer at the National Optical-Infrared Astronomy Research Laboratory, tells Science News. “It impacts our health. It impacts other animals who cannot speak for themselves.”

Most of the observations in the study came from people in North America and Europe. And they tended to come from more densely populated areas, where there is more light pollution, the authors write. Kyba notes to the Guardian that light pollution could be increasing at a faster rate in developing countries.

“If these trends continue, eventually it will be very difficult to see anything at all in the sky, even the brightest constellations,” he tells the publication. “Orion’s belt will start to disappear at some point.”

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