If you look up on a pitch black, cloudless night, you might be able to see the rest of our galaxy, the Milky Way, stretching across the sky.
"It's like a jeweled carpet over us," says Christopher Elvidge, a scientist in the Earth Observation Group of the National Oceanic and Atmospheric Administration's National Centers for Environmental Information. "It's spectacular."
But you can't see it from just anywhere. For one-third of the world's population, the Milky Way is overpowered by the glow of street lamps, bright billboards, and other artificial light spilling out into the night.
More than 80 percent of the world's population experiences some level of light pollution, according to a paper published Friday in the journal Science Advances. Ninety-nine percent of populations in the United States and Europe live under such artificially glowing skies.
And this is not a new phenomenon, Dr. Elvidge, a co-author on the new paper, says. "We have multiple generations of people that have grown up and live in areas where they never see the Milky Way."
This doesn't mean that humanity's stargazing days are over, he tells The Christian Science Monitor. Elvidge and his colleagues used satellite data to put together an atlas of artificial sky brightness. Professional and amateur astronomers alike can use this map to identify where they might find the purest views of the night sky.
But the atlas won't just be useful for those who want to see the stars.
Light pollution has been linked to all kinds of disturbances in the natural world. It sends migratory birds off track – and on track to smack into buildings. Confused by the brightness, animals that hunt or forage at night are more cautious. And baby sea turtles that rely on moonlight to guide them to the ocean after they hatch may instead hustle toward the glowing strip mall on the other side of the beach.
The research team hopes that their atlas will help lawmakers assess the improvements needed in their region to reduce these problems.
"The new atlas provides a critical documentation of the state of the night environment as we stand on the cusp of a worldwide transition to LED technology," study lead author Fabio Falchi, from the Italian Light Pollution Science and Technology Institute, said in a press release. "Unless careful consideration is given to LED color and lighting levels, this transition could unfortunately lead to a 2-3 fold increase in skyglow on clear nights."
So what can be done?
Dr. Falchi alludes to a push to switch to Light-Emitting Diode (LED) lightbulbs. The US Department of Energy supports such a conversion. "If all U.S. lighting installations were replaced overnight with the best LED technologies available in 2014, our nation would save 4,896 trillion Btu of energy," the department says on its website. "That is MOST of the 7,000 trillion Btu we now use for lighting."
Not the perfect light source
Sounds great, right? That's not all good news, says Travis Longcore, assistant professor of architecture and spatial sciences at the University of Southern California and science director at the Urban Wildlands Group, who was not part of this project. The light from the heavily promoted LED bulbs are heavy in the blue part of the spectrum, he tells the Monitor.
And that is a problem because blue light has shorter wavelengths than in other parts of the spectrum, which means that it scatters more easily.
"Artificial sky brightness is actually scattered light that's coming back down towards the Earth," explains Elvidge. When the light reaches into the sky from its original source, such as a street lamp, "some part of that light goes to space, but another part gets scattered. And when it gets scattered, it gets scattered in all directions. One of those directions is back down towards Earth," he says. "That is the light that actually obscures the astronomical features."
"You can think of it this way: If the sky gets brighter, at some point the brightness of the sky exceeds the brightness of some of the stars."
And if the original source of light is emitting blue-heavy light, that process will happen more readily.
But it's not just about wavelength, Dr. Longcore says. "Blue light is right in the area of the spectrum where the circadian response is most sensitive," too. That means that sleep rhythms in animals, including humans, will be most disturbed by light in that part of the spectrum.
That doesn't mean we have to do away with LED lights, Longcore says. LEDs don't have to be blue-heavy, but it just so happens that the ones most often chosen for lighting up city streets are the type that scatter light best.
"We're not anti-LEDs at all," Cheryl Ann Bishop, communications and public affairs director for the International Dark-Sky Association (IDA) who was not part of this study, tells the Monitor. "But we're really concerned about the retrofit that is going to the higher color temperatures."
"The standard right now seems to be 4000 Kelvin," she says, which is in the blue-heavy spectrum. "That's why IDA recommends 3000K and below."
Other light pollution solutions
There are other ways to reduce artificial sky brightness, too.
Elvidge says some municipalities and newer developments are putting in "full cutoff" lighting. That means that there is no light pointing up into the sky.
"They usually look like a square box and only the lower surface is open for the light to come out," he describes.
Streetlights or any other lights pointing up into the sky are usually wasting light, Longcore says. "You're just lighting the undersides of birds and airliners."
It could be as simple as changing our priorities for how much we need to light up the night. Longcore points to Germany and Slovenia as having policies and norms that reduce light pollution and, as a result, have cities that glow less on the new atlas. "They don't follow the same standards, and culturally, they don't demand that places are lit up like Walmart parking lots," he says.
It's not just about street lights. Think about places like Times Square in New York City. There, flashy billboards make the square almost as bright as day at all hours of the night.
"The great thing about this is there are ways we can do far better than we are doing right now and not sacrifice any of the needs we have in terms of lighting at night for human safety and well-being," Longcore says.