Every year, the night sky grows brighter, and the stars look dimmer
Light pollution caused by the incessant nighttime glow of electric lights appears to be intensifying, according to research using observations from tens of thousands of people at various locations around the world. The new study analyzed data from more than 50,000 amateur stargazers and found out that artificial lighting is making the night sky about 10% brighter each year.
That’s a much faster rate of change than scientists had previously estimated looking at satellite data. The research, which includes data from 2011 to 2022, is published Thursday in the journal Science.
The areas best documented were 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 at more than 19,000 locations worldwide, the researchers said. The “citizen scientists” who provided the data made observations of naked-eye star visibility.
“We are losing, year by year, the possibility to see the stars,” said Fabio Falchi, a physicist at the University of Santiago de Compostela, who was not involved in the study.
“If you can still see the dimmest stars, you are in a very dark place. But if you see only the brightest ones, you are in a very light-polluted place,” he said.
The study focused on “skyglow,” the artificial brightening of the night sky due to human-created light scattering in the atmosphere and returning to Earth. The nighttime glow of the sky above a big city is a familiar sight but even locales with smaller populations experience it.
As cities expand and put up more lights, “skyglow” or “artificial twilight,” as the study authors call it, becomes more intense.
The 10% annual change “is a lot bigger than I expected — something you’ll notice clearly within a lifetime,” said Christopher Kyba, a study co-author and physicist at the German Research Centre for Geosciences in Potsdam.
Kyba and his colleagues gave this example: A child is born where 250 stars are visible on a clear night. By the time that child turns 18, only 100 stars are still visible.
“This is real pollution, affecting people and wildlife,” said Kyba, who said he hoped that policymakers would do more to curb light pollution. Some localities have set limits.
The study data from amateur stargazers in the nonprofit Globe at Night project was collected in a similar fashion. Volunteers look for the constellation Orion – remember the three stars of his belt – and match what they see in the night sky to a series of charts showing an increasing number of surrounding stars.
A 2017 study based on satellite observations found that Earth’s artificially-lit outdoor surface at night was growing by about 2% annually in brightness and area. This data might represent an underestimate because the global observing satellite currently used is not sensitive to the type of light emitted by modern LED lights, Kyba said. And satellites have a hard time detecting illuminated signs because these shine primarily sideways, not upward, Kyba added.
More than half of the new outdoor lights installed in the United States in the past decade have been LED lights, according to the researchers.
The satellites are also better at detecting light that scatters upward, like a spotlight, than light that scatters horizontally, like the glow of an illuminated billboard at night, said Kyba.
Skyglow disrupts human circadian rhythms, as well as other forms of life, said Georgetown biologist Emily Williams, who was not part of the study.
“Migratory songbirds normally use starlight to orient where they are in the sky at night,” she said. “And when sea turtle babies hatch, they use light to orient toward the ocean – light pollution is a huge deal for them.”
Part of what’s being lost is a universal human experience, said Falchi, the physicist at University of Santiago de Compostela.
“The night sky has been, for all the generations before ours, a source of inspiration for art, science, literature,” he said.
“It is true that we will never get to a situation where there is no sky brightness over cities. But it is entirely reasonable to imagine that with excellent design, the Milky Way could be visible over cities with even a few hundred thousand inhabitants,” Kyba said.
“On the best nights, I have been able to see faint traces of the Milky Way at my home near the center of Potsdam, Germany, with a population about 180,000. If all of the poorly directed lights in Potsdam were replaced by more effective ones, the level of skyglow would be drastically reduced, and the Milky Way would be more generally visible,” Kyba added.
“For nearly the entire evolutionary history of life on this planet, the night sky was lit by starlight, moonlight and natural airglow. Until about 150 years ago, to step outside at night was to be confronted with the cosmos,” Kyba said. “Who could say what the cultural and religious impact is when a formerly universal human experience becomes something that only extremely rich or extremely poor people experience?” (Reuters/AP)
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