By Will Dunham
WASHINGTON — Light pollution caused by the continuous nighttime glow of electric lights appears to be intensifying, according to a study using observations from tens of thousands of people around the world.
The study, published Thursday, found that the number of visible stars reported at observing sites from 2011 to 2022 decreased by an amount that indicated an increase in the brightness of the night sky of 7 to 10 percent per year, more higher than previously measured using satellite data.
The best-documented areas are North America—particularly the United States—and Europe, with minimal data for parts of Asia, Africa, and South America. More than 29,000 individual reports were made in more than 19,000 locations around the world, the researchers said. The “citizen scientists” who provided the data made observations of the visibility of the stars with the naked eye.
The study focused on “sky aurora,” the artificial illumination of the night sky due to man-made light scattering in the atmosphere and returning to Earth. The nighttime glow of the sky over a big city is a familiar sight, but even less populated areas feel it.
“The study is important for two reasons. First, this is the first time that sky brightness has been studied on a continental scale. Second, it shows that existing norms and legislation are failing to reduce sky brightness or even stop it from increasing,” said physicist Christopher Kiba of the German Research Center for Geosciences GFZ and Ruhr University Bochum, lead author of the study published in the journal . Science.
“Over the past decade, our understanding of the ecological consequences of light has grown dramatically, as has our understanding and modeling of light in the atmosphere,” Kiba said.
A 2017 study based on satellite observations found that Earth’s artificially illuminated outer surface at night is growing by about 2% per year in brightness and area. That data may be an underestimate because the global monitoring satellite currently in use is not sensitive to the type of light emitted by modern LED lights, Kiba said. And satellites have a hard time detecting illuminated signs because they shine mostly sideways, not up, Kiba added.
Light pollution raises concerns about its ecological effects on humans and animals. For example, research shows how light pollution threatens fireflies, which have specialized light-emitting organs and blink to communicate as part of courtship and reproduction.
The researchers said more could be done to mitigate light pollution, including better design of the direction, degree and type of lighting used.
“It is true that we will never reach a situation where there is no brightness in the sky above the cities. But it’s perfectly reasonable to imagine that with excellent design, the Milky Way could be visible even over cities with a few hundred thousand inhabitants,” Kiba said.
“On the best nights I could see faint traces of the Milky Way at my home near the center of Potsdam, Germany, population about 180,000. If all the poorly aimed lights in Potsdam were replaced with more efficient ones, the level of skyglow would be drastically reduced and the Milky Way would be more generally visible,” Kiba added.
Bright starlight now appears to be limited on Earth’s surfaces to remote locations where tourists can venture and less developed regions.
“Throughout almost the entire evolutionary history of life on this planet, the night sky has been lit by starlight, moonlight, and natural air light. Up until about 150 years ago, going outside at night meant facing the cosmos,” Kiba said. “Who can say what the cultural and religious impact is when a previously universal human experience becomes something experienced only by the extremely rich or the extremely poor?”
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