Light pollution is increasing rapidly, and the number of naked-eye stars in the night sky in some places has decreased by more than half in less than 20 years, according to a study published Thursday.
The researchers, whose findings are published in the journal Science, said the increase in light pollution (sky glow) they found was much greater than measured by nighttime satellite observations of Earth.
To study the change in global sky brightness from artificial light, the researchers used star observations from 2011 to 2022 submitted by more than 51,000 “citizen scientists” around the world.
Participants of the “Night Globe” project run by the US National Optical-Infrared Astronomy Research Laboratory were given star maps and asked to compare them to the night sky where they were located.
The change in the number of visible stars reported was equivalent to a 9.6 percent annual increase in sky brightness, on average, based on participants’ positions, the researchers said.
Given such a change in stellar brightness over an 18-year period, a location with 250 visible stars would see that number drop to 100.
Christopher Kyba, one of the authors of the study, said that most of the stargazing with the naked eye came from Europe and the USA, but there was also good participation from Uruguay, South Africa and Japan.
“The global trend we measured in Skyglow probably underestimates the trend in countries with the fastest increases in economic development, because the rate of change in light emission is highest there,” the researchers said.
The study coincided with replacing many outdoor lights with light-emitting diodes (LEDs), but the effect on sky glow from switching to LEDs is unclear, the researchers said.
“Some researchers speculated that this would be beneficial; others speculated that the high luminous efficacy of LEDs could be detrimental due to spectral changes or rebound effect, which leads to the installation of more or brighter lights or longer operating hours.” they said.
According to research, the global LED market share for new general lighting has increased from less than one percent in 2011 to 47 percent in 2019.
“Despite (or perhaps because of) the use of LEDs in outdoor lighting applications, the visibility of stars is rapidly deteriorating,” the researchers said.
“Current lighting policies are not preventing increases in sky glare, at least on continental and global scales.”
-‘Facing the Cosmos’-
Kyba, a physicist at the German Research Center for Earth Sciences, told AFP that while the team was able to assess the erosion of star visibility due to sky shine, not much research has been done on its ecological impact.
“There’s tons of research on light shining directly on animals and plants,” he said. “But it’s really hard to do experiments on the effect of sky-flare.
“You’re not going to do anything like shut down New York and see what happens in the East River.”
Science aside, light pollution has changed the character of the night sky.
“Throughout all of human history, humans have faced the cosmos when they went out at night, at least on clear nights without the moon,” said Kyba.
He said, “You walk out and there are stars, there is the Milky Way. It is there and it shines on you.”
“And now it’s like a truly extraordinary experience,” he said. “It definitely makes a difference for us as humans that we don’t have this experience, which used to be a very universal experience.”
The Globe at Night campaign hosts an interactive data map at globeatnight.org and is seeking volunteers to collect more observations in 2023.