If, like me, you’re a stargazer in an urban area then you will already know exactly what this article is about. Light pollution and its trademark “skyglow” is getting worse. Much worse. It’s getting so serious that stars, beautiful open clusters and even distant galaxies in the night sky there were visible just a decade ago are now impossible to see.
All urban stargazers know this and have done since the switch from orange sodium vapor lamps to white LEDs, which emit much more blue light. Now we have a mass of observational evidence.
Published today in the journal Science, a new paper analysed 51,351 naked-eye observations by citizen scientists in 19,262 locations (including 3,699 in Europe and 9,488 in North America) from 2011 to 2022 as part of the “Globe at Night” Citizen Science Project from the National Science Foundation’s NOIRLab. Each stargazer looked at their night sky on cloud- and moon-free nights and reported which of a set of eight star charts (each showing the sky under different levels of light pollution) best matches what they see.
The main finding over the 12 years is incredibly depressing—the sky brightness increased by seven to 10% per year (and 9.6% on average).
In Europe, they found a 6.5 per cent increase in brightness per year matched the data; in North America, it’s 10.4 per cent.
The culprit, of course, is rapidly growing light pollution despite policies to prevent it.
“The rate at which stars are becoming invisible to people in urban environments is dramatic,” said Christopher Kyba, lead author and researchers at the GFZ German Research Centre for Geosciences and the Ruhr-Universität Bochum, Germany. “If the development were to continue at that rate, a child born in a place where 250 stars are visible will only be able to see 100 stars there on his 18th birthday.”
Light pollution is not just a threat to stargazing.“Skyglow affects both diurnal and nocturnal animals and also destroys an important part of our cultural heritage,” said Constance Walker, co-author of the study and head of the Globe at Night project of NSF’s NOIRLab since its inception.
Over a large part of the Earth’s land surface, the sky continues to glow with an artificial twilight long after sunset. This “skyglow” is a form of light pollution that has serious effects on the environment and should therefore be the focus of research, as Constance Walker, co-author of the study and head of the Globe at Night project. In August 2021 a study in Science Advances demonstrated the detrimental impacts of street lighting on local insect populations and in September 2022 another study highlighted the harmful effects of nighttime lighting to ecosystems across Europe.
Worryingly, the rate of change is much faster than satellite measurements of artificial light emissions on Earth suggest, the authors report. Satellite data for the locations of the observers in the study record that artificial brightness had slightly decreased—by 0.3 percent per year in Europe and by 0.8 percent in North America—over the same time-frame.
One reason for the discrepancy is likely changes in lighting practices. “Satellites are most sensitive to light that is directed upwards towards the sky,” said Kyba, who thinks that it is horizontally-emitted light that accounts for most of the skyglow. “If advertisements and facade lighting become more frequent, bigger or brighter, they could have a big impact on skyglow without making much of a difference on satellite imagery,” he said.
Another reason is the swap over the last decade from warm orange sodium vapor lamps to blue light-emitting white LEDs, which have been adopted en masse in street lighting, car headlights and security lighting at night. The problem LED lighting is that they emit light in the blue spectrum. “Our eyes are more sensitive to blue light at night, and blue light is more likely to be scattered in the atmosphere, so contributes more to skyglow,” said Kyba. “But the only satellites that can image the whole Earth at night are not sensitive in the wavelength range of blue light.”
It’s the main reason why the study used individual people working together as if they were a global sensor network, said Kyba.
“Perhaps the most important message that the scientific community should glean from the Kyba et al. study is that light pollution is increasing, notwithstanding the countermeasures purportedly put into operation to limit it,” writes Fabio Falchi and Salvador Bará in a related Perspective in Science. “Light pollution is an environmental problem and as such should be confronted and solved … awareness must greatly increase for artificial light at night to be perceived not as an always-positive thing, but as the pollutant it really is.”
The project is ongoing (anyone can contribute a report) and researchers want to identify trends for other continents, individual states and cities.
Wishing you clear skies and wide eyes.