Thanks to the use of artificial lighting at night, the night sky brightened faster than researchers imagined. A study of more than 50,000 star observations by local scientists shows that the night sky became about 10 percent brighter on average each year from 2011 to 2022.
In other words, a child born in a region where about 250 stars were visible each night would see only 100 stars on their 18th birthday, researchers report in the Jan. 20 issue of Science.
The dangers of light pollution go far beyond not being able to see as many stars. Excessive brightness at night can harm human health, cause migratory birds to fly into buildings, disrupt food webs by attracting pollinating insects to lights instead of plants, and can even interfere with fireflies trying to have sex.
“In a way, it’s a call to action,” says astronomer Connie Walker of the National Optical and Infrared Astronomy Research Laboratory in Tucson. “People have to consider that it affects our lives. It’s not just astronomy. It affects our health. It affects other animals that can’t speak for themselves.”
Walker is involved with the Globe at Night campaign, which began in the mid-2000s as an outreach project to unite students in Arizona and Chile and now has thousands of participants around the world. The co-authors compare the stars they see with maps of what stars would be visible under different levels of light pollution and report the results in an appendix.
“I was quite skeptical about the Night Globe as a tool for precise research,” admits physicist Christopher Kiba of the German research center GFZ in Potsdam. But there’s power in numbers: Kiba and his colleagues analyzed 51,351 individual data points collected between 2011 and 2022.
“Individual data is not exact, but there is a lot of it,” he says. “This Globe at Night project is not just a game; this is really useful data. And the more people participate, the more powerful it becomes.”
This data, combined with the Global Sky Brightness Atlas, published in 2016, led the team to conclude that the brightness of the night sky increased by an average of 9.6 percent per year from 2011 to 2022.
Much of this increase was not accounted for by satellites that collect brightness data around the world. These measurements showed only a 2 percent increase in brightness per year over the past decade.
There are several reasons for this, says Kiba. Since the early 2010s, many outdoor lights have switched from high-pressure sodium lamps to LEDs. LEDs are more energy efficient, which has environmental benefits and cost savings.
But LEDs also emit more short-wavelength blue light, which scatters particles in the atmosphere more than the orange light of sodium lamps, creating more sky glow. Existing satellites are not sensitive to blue radiation, so they underestimate light pollution from LEDs. And satellites can transmit light that shines toward the horizon, such as light from a sign or window, rather than straight up or down.
Astronomer and light pollution researcher John Barentine was not surprised that satellites underestimated the problem. But “I was still surprised by how much of an understatement it was,” he says. “This paper confirms that we have underestimated light pollution in the world.”
The good news is that solving the problem does not require major technological breakthroughs. Scientists and politicians simply need to convince people to change the way they use light at night – easier said than done.
“Sometimes people say light pollution is the easiest problem to deal with because you just flip a switch and it goes away,” says Kiba. “It’s true. But that’s ignoring the social problem — that this overall problem of light pollution is created by billions of individual decisions.”
Some simple solutions include dimming or turning off lights at night, especially floodlights or lights in empty parking lots.
Kiba shared a story about a church in Slovenia that switched from four 400W floodlights to one 58W LED that shines behind the cutout of the church to focus light on its facade. The result was a 96 percent reduction in energy consumption and much less wasted light, Kiba said in International Journal of Sustainable Lighting in 2018. The church was still there illuminated, but the grass, trees and sky around it remained dark.
“If this story could be repeated over and over again in our society, it would lead to the conclusion that you could really dramatically reduce the light in the sky, still have a lit environment, have better vision, and use a lot less energy,” he says. . . “It’s kind of a dream come true.”
Barentine, who heads a private dark-sky consulting firm, believes widespread awareness of the problem — and action — may be inevitable. By way of comparison, he points to the highly publicized 1969 oil spill on the Cuyahoga River near Cleveland, which fueled the environmental movement in the 1960s and 1970s and prompted Congress to pass the Clean Water Act.
“I think we’re on the cusp of light pollution catching fire,” he says.
