A new report from the citizen science program Globe at Night reveals that 'skyglow' has stolen away the night sky from billions of people worldwide, and the problem is far worse than we previously thought.
Look up into the sky on a clear night. What do you see?
If you're in an area far from city light pollution, your eye will likely be able to pick out several thousand stars twinkling above. Also, for much of the year, the vast core of our Milky Way galaxy will be stretched across the sky.
This image of the night sky captures The Milky Way from Lower Five Islands, Nova Scotia, on May 8, 2019. Although the view above shows a pristine view of the cosmos, hints of urban light pollution are visible along the horizon. Credit: Barry Burgess/UGC
The closer you get to the core of a large urban centre, though, the less of this grand spectacle you will see. Bright urban environments interfere with the human eye's ability to see in the dark, and without engaging the eye's more light-sensitive receptors, dimmer stars are lost from view.
Additionally, the night sky above cities literally appears to glow as the light emitted from urban sources is scattered back toward the ground by the air itself. This phenomenon is called 'skyglow', which washes out the night sky, hiding all but the brightest stars and planets from sight.
Watch below: What light pollution is doing to city views of the Milky Way
This problem is nothing new. The impacts of city lights on the night sky have been observed for some time. Also, it is getting worse as urban centres expand, with satellite observations revealing a roughly 2 per cent increase in light pollution per year.
However, the results of a new study published this week in the journal Science show that the stars are disappearing from our view at a rate even faster than we thought.
"The rate at which stars are becoming invisible to people in urban environments is dramatic," Christopher Kyba, the lead author of the study from the German Research Centre for Geosciences said in a press release.
"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," Kyba explained.
This infographic reveals the impact of urban light pollution on the night sky by showing what is typically visible from Dark Sky sites on the left, and transitioning to denser and denser urban environments towards the right. Credit: NOIRLab/NSF/AURA, P. Marenfeld
Kyba, along with colleagues at the GFZ German Research Centre for Geosciences and the National Optical-Infrared Astronomy Research Laboratory (NOIRLab), examined over 50,000 reports submitted by citizen scientists around the world to the Globe at Night project. Their results show that, from 2011 to 2022, skyglow increased by about 6.5 per cent per year across Europe and by around 10.4 per cent per year in North America.
Averaged across the entire world, the rate of increase comes out to around 9.6 per cent per year. That's nearly five times greater than the rate observed by satellites!
"This shows that existing satellites aren't sufficient to study how Earth's night is changing," Kyba said in a NOIRLab statement. "We've developed a way to 'translate' Globe at Night observations of star visibility made at different locations from year to year into continent-wide trends of sky brightness change. That shows that Globe at Night isn't just an interesting outreach activity, it's an essential measurement of one of Earth's environmental variables."
Kyba believes the higher rates of increase found from human observations are likely due to two factors, the GFZ press release said. First, satellite observations record light that is emitted more or less straight up, but the increase in urban light pollution is due mainly to sources that shine horizontally, such as from building windows and electronic billboards.
Second, cities have been switching from orange sodium vapour lamps to white LEDs that emit more light in the blue end of the spectrum. Blue light has more of an impact on human night-vision and is scattered more by the atmosphere, thus contributing more to skyglow. However, satellites that observe the whole Earth at night are not typically sensitive to those blue wavelengths of light.
These two images of Calgary, from Dec. 24, 2010, on the left and Jan. 6, 2020, on the right, show how much the city's lighting has changed over the past decade. The orange sodium vapour lamps used in 2010 have been largely replaced by white LED lights in 2020. Credits: left - NASA (eol.jsc.nasa.gov), right - Jessica Meir, NASA
According to the GFZ website: Light pollution is a familiar problem that has many detrimental effects, not only on the practice of astronomy. It also has an impact on human health and wildlife, since it disrupts the cyclical transition from sunlight to starlight that biological systems have evolved alongside. Furthermore, the loss of visible stars is a poignant loss of human cultural heritage. Until relatively recently, humans throughout history had an impressive view of the starry night sky, and the effect of this nightly spectacle is evident in ancient cultures, from the myths it inspired to the structures that were built in alignment with celestial bodies.
"The increase in skyglow over the past decade underscores the importance of redoubling our efforts and developing new strategies to protect dark skies," said Dr. Connie Walker, a co-author of the study who developed the Globe at Night program at NOIRLab. "The Globe at Night dataset is indispensable in our ongoing evaluation of changes in skyglow, and we encourage everyone who can to get involved to help protect the starry night sky."