Future humans may not be able to see the stars from Earth — study

In most of the world, we’re winning the battle against darkness — but not how we want to be doing so.

Light spills from our open windows, banishes the shadows from our streets and parks, and fights for our attention on storefronts and billboards. And it shines into the sky, where it reflects and scatters off air, water droplets, and dust. All that scattered light makes the whole sky seem to glow faintly, around the clock. It’s never really dark anymore.

“For nearly all of human history, on clear nights we were confronted with the cosmos,” Christopher Kyba of the German Research Center for Geosciences tells Inverse. “I think it's hard for us, now that we rarely see the Milky Way and starry skies to have a feel for what it was like to have the universe on full display several nights a month.”

That unceasing glow gets brighter every year, according to a recent study, and it’s swallowing up stars in the process. Starting with the faintest, the stars’ light fades into the background of skyglow. If a child born today can look up and see about 250 stars in the night sky, by their eighteenth birthday 150 of those stars will have faded from view, lost in the endless glow of our electric twilight.

Kyba and his colleagues published their results in the journal Science.

• What’s New — The night sky gets about 10 percent brighter every year, according to Kyba and his colleagues, who worked with thousands of citizen scientists around the world to measure changes in skyglow over the last 12 years.

Since 2011, the people who gathered Kyba and his colleagues’ data compared the night sky above them to pictures on a website called Globe at Night, whose images of the night sky simulated different levels of brightness from nearby city lights. The results gave Kyba and his colleagues something they could measure: the brightness (or apparent magnitude, to astronomers) of the faintest star that could be seen in the night sky.

The data suggest that overall, skyglow is getting brighter by about 10 percent each year, and it could double in less than eight years. That’s a global average; some places are lighting up the sky even faster, while others are fighting to preserve what’s left of the dark. It varies from city to city and country to country, and even from continent to continent.

“Based on our previous work with satellite data, the rates of growth are fastest in developing countries,” Kyba says. “But it's important to keep in mind that they are starting off from a much lower base, so (for example) a 1 percent increase in Germany would probably mean a lot more new total light than a 10 percent increase in Ghana.”

But the overall trend for most of the world is an empty, starless, glowing sky.

• Why It Matters — In another generation, people in most of the world may not know what it’s like to look up at night and see a sky full of stars — or even a handful of them.

But there are more concrete things at stake, as well, from entire ecosystems to simply getting a good night’s sleep. Life on Earth evolved with a day-night cycle, and abruptly changing that is causing serious problems. The unending glow of the night sky is an environmental problem, even if we don’t often think of it as one.

“Looking at the International Space Station images and videos of Earth’s night hemisphere, people generally are only struck by the ‘beauty’ of the city lights, as if they were lights on a Christmas tree. They do not perceive that these are images of pollution,” write light pollution researchers Fabio Falchi and Salvador Bara, both of the Universite de Santiago de Compostela in Spain, in a paper commenting on Kyba and his colleagues’ study. “It’s like admiring the beauty of the rainbow colors that gasoline produces in water and not recognizing that is chemical pollution.”

According to earlier studies, too much light at night is especially bad for insects — which are pretty close to the foundation of most food webs, and which we rely on to pollinate crops. It’s also harmful to plants and even fish. Brighter nights also give some predators an advantage, which puts some prey species — which evolved to rely on darkness for cover and safety — at risk of extinction.

Meanwhile, many of us may be too tired and grumpy to do much about it, since too much extra light at night can cause our bodies and those of other animals to produce less melatonin, the chemical that helps regulate our sleep.

In other words, we need the night, and we need the night to be dark.

• Here’s the Background — The Globe At Night data came as a surprise to Kyba and his colleagues, since satellites that measure how much light is shining up from the ground have reported just a 2 percent increase each year. But it turns out that satellites like the Defense Meteorological Satellite program and the Suomi National Polar-orbiting Partnership satellites have some huge blind spots when it comes to human lighting: like light-emitting diodes, or LEDs.

Most of the outdoor lighting installed in recent years uses LEDs, but the bulbs emit in shorter wavelengths that light-monitoring satellites can’t see, but which also scatter more in the atmosphere and have a bigger impact on skyglow. That’s why satellite data tends to underestimate the problem.

• What’s Next — Kyba and his colleagues say their data is a warning that most of our current efforts to reduce light pollution aren’t really working. Putting shields on streetlights to keep them from shining skyward, for example, sounds like a great solution, but Kyba says that light shining horizontally may actually be a bigger problem than light shining straight up. This is because light shining toward the horizon has more of a chance to scatter off material in the atmosphere and light up the sky.

Kyba has some ideas about what might actually help.

“In terms of policy, approaches like requiring buildings to turn interior and exterior lights off at a certain time or at a certain time after they are no longer occupied seems sensible, as does restricting the times at which advertising lighting is allowed to be on,” he says. “You can also do things like restrict the maximum luminance of signs, and the maximum illuminance under gas station canopies.”

Meanwhile, Kyba and his colleagues hope to get more data from Globe at Night, especially people in parts of the world that aren’t well-represented by the current group of volunteers — which means pretty much everywhere outside North America, Europe, and Japan.

“That would make it possible to have a much better understanding of the spatial variation of these changes,” he says. “I'm not sure how best to increase coverage in areas that don't have a lot of it at the moment. The International Dark-Sky Association is encouraging their advocates to raise awareness about Globe at Night, and I know that Astronomers Without Borders has also raised awareness.”

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