“To light a candle is to cast a shadow.” So wrote Ursula K. Le Guin in her 1968 young-adult fantasy, A Wizard of Earthsea. Le Guin’s poetic observation concerned the moral ambiguities of magic but it also eloquently describes the centuries-old technological quest to banish darkness and one of its greatest advances: the invention of the electric lightbulb.

There are undeniable bright sides to electric lighting. It is hard to work, travel or read in the dark, but with the flip of a switch the problem disappears. The economic boons are so profound that you can see their effects from deep space, in the shining networks of nighttime lights that outline our flourishing global civilization. But our modern love of light-filled nights casts a long shadow—wasting energy, disrupting ecosystems, and in some cases harming human health. And, as detailed in The New World Atlas of Artificial Night Sky Brightness released Friday, the same lights that lace our planet and reveal our presence to the outside universe are also smothering our views of the stars. The atlas appears in Science Advances.

According to the atlas, which was produced by analyzing tens of thousands of high-resolution images of nighttime lights on Earth from the NOAANASA Suomi National Polar-orbiting Partnership satellite, roughly one in three humans on Earth cannot see the Milky Way when they look up into the night sky. For Americans, the percentage is higher: Light pollution prevents four out of five from seeing our home galaxy as a gauzy river of light arcing overhead. Residents of particularly light-polluted countries such as Singapore, South Korea and Qatar can scarcely see any stars at all, and instead spend their nights in perpetual “artificial twilight.” Outside of the relatively barren open ocean and polar wastelands, the fraction of the populated planet with naturally dark skies is limited to off-the-beaten-path places like Chad, Papua New Guinea and Madagascar—and is steadily shrinking.

“Twenty years ago, light pollution could be considered only a problem for astronomers,” says lead author Fabio Falchi, a high school physics teacher in Thiene, Italy, who became concerned about the growing threat of light pollution in the 1990s after it began interfering with his hobby of amateur astronomy. “But fundamentally, life has evolved over millions of years with half the time dark and half the time light, and we have now enveloped our planet in a luminous fog of light. Light pollution has become a real environmental problem on a global scale.”

“A starry sky is something that touches your soul,” Falchi adds. “Our civilization’s religion, philosophy, science, art and literature all have roots with our views of the heavens, and we are now losing this with consequences we cannot fully know. What happens when we cannot be inspired by the night sky?”


Based on more than 35,000 satellite images and detailed ground-based measurements of sky brightness, this interactive visualization of a New World Atlas of Artificial Sky Brightness shows where the skyglow from artificial lighting blots out stars and constellations. Brighter areas correspond to higher levels of skyglow from light pollution. Credit: Falchi et al., Science Advances; Jakob Grothe/National Park Service, Matthew Price/CIRES and CU-Boulder.

Beyond the obvious downsides of light pollution—such as grumbling astronomers or sea turtle hatchlings and migrating birds drawn off course by bright lights—most of its related effects are surprisingly hard to quantify. Researchers still lack deep understanding of the complex relationships between many nocturnal predators and prey, let alone how varying levels of artificial light changes them. For humans the effects are similarly muddled. Based on multiple epidemiological studies, the World Health Organization in 2007 and the American Medical Association in 2012 each issued statements warning that extended exposure to light at night increases the risk of certain cancers, probably via alterations to circadian rhythms and associated hormone levels. Even so, most of these studies did not distinguish between exposure to outdoor sources, like streetlights, and indoor ones, like television and smartphone screens.

The most fundamental difficulty, however, has been that no one knows exactly how severe the problem is. Total levels of nighttime lighting are straightforward to calculate from satellite images—scientists have done this for decades—but determining how much “skyglow” all that light causes as it scatters through Earth’s atmosphere is a thornier task. Falchi and colleagues obtained their light pollution estimates by running the Suomi satellite data through an atmospheric model that calculates skyglow at zenith (directly overhead) in a cloud-free sky. They then compared and further calibrated these results with real measurements of sky brightness taken from various clear-skied sites on Earth. The model’s threshold for significant light pollution was when the night sky directly overhead was calculated to become just 1 percent brighter than its ink-black natural state.

According to study co-author Dan Duriscoe, a scientist with the U.S. National Park Service who supplied many of the sky-brightness measurements, a 1 percent change in overhead brightness is far more significant than it might seem, partly because well over half of Earth’s species are thought to be nocturnal. “A site with sky brightness just 1 percent above natural zenith probably has much more skyglow near the horizon, because it is probably in a situation where it lies within the light dome of something else hundreds of kilometers away,” Duriscoe says. “This is where the model is powerful for predicting light pollution threats from distant cities now and in the future. As populations grow and spread, areas with no evidence of artificial light are going to get much harder to find.”

Furthermore, the effects of clouds can greatly amplify even slight levels of light pollution. For that reason, says Travis Longcore, an urban ecologist at the University of Southern California Dornsife who was not involved in the study, the new atlas only provides a minimal baseline for what are likely much larger levels of light pollution. “You can easily have 10 times as much light coming down from an overcast sky…. Sites where the atlas shows a baseline clear-sky brightness equivalent to twilight or a half-illuminated moon are probably excluded as habitats for specialist species that need the true cover of darkness to survive and thrive.”

For humans, the most worrisome insights revealed by the atlas are the effects of an ongoing shift to light-emitting diodes—LEDs—as sources of outdoor light over older incandescent illumination. LEDs are far more energy efficient, durable and dynamically adjustable than incandescent bulbs, and the U.S. and several other nations are aggressively incentivizing their use. The most economical LEDs, however, shine brightest in harsh blue-white hues. Because Earth’s atmosphere preferentially scatters blue light (for proof, just look up into a clear sunlit sky), Falchi’s model suggests that a large-scale conversion to cheap blue-white LEDs for outdoor lighting could substantially increase skyglow even if the total amount of emitted light remains constant. Besides being aesthetically unappealing to many people, there is concern that blue-white LED light may also be dangerous. Humans have peak visual sensitivity to the yellow and green parts of the visible spectrum, says George Brainard, a photobiologist at Thomas Jefferson University who was not involve with Falchi’s study. But it is blue-white light—exactly of the sort most produced by cheap LEDs—that dominates the regulation of human circadian rhythms and other important biological cycles, Brainard says. “The wide-scale adoption of LEDs will reap huge energy savings, which is a good thing,” Brainard says. “The question is, are the great energy savings compromising human health and ecosystems?”

The greatest value of this study from Falchi and his colleagues may be the baselines it sets for policy debates about light pollution and any associated changes in the level of nighttime illumination. The atlas’s finding about the pernicious skyglow effects of blue-white LEDs, Longcore says, “is a message that incentivizing conversion to cheap blue-white LEDs is a colossal policy mistake.”

“If I as a university professor approached my institutional review board and said I want to experiment on entire populations by introducing wavelengths of light most associated with harmful biological and ecological effects, they would laugh me out the door, down the street and out of my office,” Longcore says. “Fortunately, we can now use much warmer-colored LEDs that eliminate the blue spike in spectrum that is so damaging because it scatters so much in Earth’s atmosphere.”

In addition to preferentially using warmer-colored LEDs, the study’s authors say, the loss of starry skies to light pollution could be forestalled or reversed with stricter regulations on artificial lights as well as the advent of driverless cars, which require less street illumination. There is, however, an even simpler solution.

“I like to tell people that the nice thing about outdoor lighting is its environmental impacts can be instantly removed,” Duriscoe says. “All you have to do is turn the lights off.”