The night sky is rapidly getting brighter, eroding star visibility worldwide

Observations from citizen scientists worldwide over the past 12 years reveal a worrisome trend – stars in the night sky are becoming more difficult to see due to rapidly growing light pollution. The change in their visibility as reported by citizen scientists is equivalent to a 9.6% per year annual increase in sky brightness, an increase faster than satellites have indicated, and which occurred despite policies to prevent increases in light pollution. To put this in perspective, the authors note that under such sky brightness change, a child born in an area where 250 stars were visible would likely see fewer than 100 stars in the same location 18 years later.

In many inhabited places on Earth, the night sky never fully grows dark. It instead glows with an artificial twilight caused by the scatter of anthropogenic light in the atmosphere. This type of light pollution, called skyglow, is responsible for the visible brightening of the night sky and the erosion of our ability to see stars. Although the ubiquity and luminance of skyglow have increased exponentially over much of the last century, its global change over time is not well understood. Satellites that can measure global skyglow are limited in resolution and sensitivity and are often blind to the wavelengths of light produced by modern LED lights that have come to dominate lighting over the past decade. To better understand how growing light pollution is affecting our view of the stars, Christopher Kyba and colleagues evaluated 51,351 citizen scientist observations of naked-eye stellar visibility from 2011-2022. To determine the background brightness of the night sky, Kyba et al. asked participants worldwide to compare star maps of the night sky at different levels of light pollution to what they could see with their own eyes through the online “Globe at Night” platform. According to the findings, the night sky has increased in brightness from artificial light by roughly 7 to 10% per year, which is equivalent to a doubling of the night sky’s brightness in less than eight years. This increase is much higher than estimates of the evolution of artificial light emissions (~2% yearly) based on radiance measurements taken by satellites.  “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,” write Fabio Falchi and Salvador Bará in a related Perspective. “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.”

For reporters interested in trends, several studies published in Science and Science Advances have reported the rapidly growing issue of light pollution and its environmental impact, including a Science study from March 1973 that discussed increases in outdoor illumination and the resultant impact on astronomy. A June 2016 Science Advances study by Fabio Falchi et al. presented a world atlas of artificial sky luminance that quantified the magnitude of artificial skyglow globally. In a November 2017 study also published in Science Advances, Kyba et al. used satellite data to show the expansion of Earth’s artificially lit outdoor surface area. Other recent studies have demonstrated the environmental risks associated with artificial lighting, including an August 2021 Science Advances study that demonstrated the detrimental impacts of street lighting on local insect populations and a September 2022 Science Advances study that highlighted the harmful effects of nighttime lighting to ecosystems across Europe.