The Milky Way’s luminous glow has inspired stories, paintings, songs, and poems for centuries: Japanese and Chinese folklore describe it as a river separating two lovers; in Greek legend, it is the spilled breast milk of the goddess Hera. Now, however, one-third of people cannot see Earth’s galaxy at night because of artificial lighting, which affects nearly 80% of the globe. The findings, part of a new atlas of worldwide light pollution, suggest that the problem is poised to get worse without regulatory action.
“This atlas is really a useful communications tool to open everybody’s eyes,” says Travis Longcore, a spatial scientist at the University of Southern California in Los Angeles. Longcore studies urban ecology, and was not involved in the study. “What a horrible thing to do to us as a species, to live in permanent twilight and never be able to see the stars.”
Light pollution has intensified in the past half-century, increasing about 6% each year in North America and Europe, according to research published using a previous atlas created 15 years ago by the same researchers. That atlas, and the new study, define “light-polluted skies” as having a luminance of 14 or more microcandelas per square meter—about 10% higher than normal night sky brightness levels.
The new atlas shows that now, more than 80% of humanity experiences light-polluted night skies, which includes roughly 83% of Earth’s population, and more than 99% of Europeans and Americans. By population, Singapore has the world’s most light-polluted skies, followed by Kuwait, Qatar, and the United Arab Emirates—all densely populated countries. Africa has the dimmest skies; the top 10 least polluted countries are on the continent.
Such pollution affects more than just our view of the Milky Way. Strong artificial lighting at night can cause birds to migrate at the wrong time of season, deter nighttime pollinators like bats, disrupt underwater ecosystems, and even decrease melatonin production in humans, leading to disrupted sleep cycles and increased risk of some cancers. And these detrimental effects can persist even after the lights have been dimmed or removed.
“Light pollution is usually seen as a problem that can be solved immediately by turning off the lights,” says Light Pollution Science and Technology Institute in Thiene, Italy, researcher Fabio Falchi, co-creator of the atlas, described in today’s issue of Science Advances. “We can surely lower the levels of light pollution by turning off the lights, but we cannot reverse the damage we have already done.”
In 2001, Falchi and his collaborators produced the first-ever light pollution atlas from data collected by a U.S. Air Force satellite. The time lag before the creation of the follow-up atlas was in part due to day jobs held by the Italian co-creators, says Falchi, who is himself a high school physics teacher. But this lag proved fortunate, as it allowed the team to draw data from the Visible Infrared Imaging Radiometer Suite instrument on the Suomi National Polar-orbiting Partnership satellite launched in 2011. Using 36 computers and 30,000 sky brightness measurements by citizen scientists to calibrate their calculations, the team created a model of light pollution around the globe. The model was based on the altitude of a given site, the angle at which light emitted upward from cities hit the atmosphere, and the reflection of that light back to Earth by the atmosphere. Overall, more than 300 regional maps were stitched together to form the global map.
The atlas also projects what would happen if all outdoor lighting in Europe switched from common high-pressure sodium lights to energy efficient 4000-K white light-emitting dioed (LED) lights. LED lights release more light in the blue part of the spectrum than sodium lights. Those blue wavelengths are more easily scattered by Earth’s atmosphere than other colors, which would considerably increase the light pollution they ultimately contribute, bulb for bulb. Blue light is also more easily picked up by the human eye, which means that people would perceive even brighter skies. The atlas team predicts a two- to threefold increase in worldwide light pollution if they continue to be adopted globally. Falchi says that, considering this, choosing LED lights is akin to stripping an automobile of its emissions control devices to increase engine efficiency. “We are searching only for lighting efficiency, and not paying attention to the quality of the lights that we are installing,” he says.
Poetry and mythology aside, this atlas is also a “first step” toward understanding light pollution’s impacts on wildlife, Longcore says. Light pollution comes from more than just lightbulbs, he notes: The key for many ecological researchers will be developing tools to calculate “horizontal illuminance,” the glare and reflection of artificial lighting from clouds and ground surfaces under different types of weather conditions. However, Longcore sees great potential in the atlas’s ability to raise awareness of a relatively underappreciated issue—and the dangers to both humans and wildlife from putting Earth into a state of “permanent twilight.”
*Update, 13 June, 10:25 a.m.: An earlier version of this story incorrectly stated that artificial lighting at night could lead to an increase of melatonin production. The story has been updated to reflect that it could lead, instead, to a decrease in melatonin.