We're Losing Night, Thanks to a Spike in Artificial Light

Night lights across the continental United States. NASA Earth Observatory/NOAA NGDC

Around the globe, night is getting brighter, even as cities turn to energy-saving lightbulbs that ought to be reducing the amount of light that escapes from human settlements. And although there are stark differences in increased brightness among regions, the analysis presented in a new paper published today in the journal Science Advances is still only a conservative estimate of how bright the planet is becoming, the authors said.

"Earth's night is getting brighter," said first author Christopher Kyba, a remote sensing specialist at the GFZ German Research Centre for Geosciences, during a press conference held about the paper. "And in fact, that true increase that a human being would perceive is actually larger than what we report here."

Losing the night doesn't just mean we can no longer wonder at the twinkling stars—darkness plays a hugely important ecological role for many species. Darkness itself triggers daily cycles of waking and sleeping, but the increasing or decreasing length of darkness can also cue animals to adjust their behavior for the changing seasons.

Losing night can affect plants as well as animals, and can reduce the amount of valuable so-called ecosystem services, like pollination and seed dispersal, that a community can provide. "From an evolutionary perspective now, artificial light at night is a very new stressor," said co-author Franz Holker, an ecologist at the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Germany. We humans aren't immune either: Increased night light can mess with our sleep cycles and exacerbate other health issues.

And the problem isn't just lightbulbs: A phenomenon called "sky glow" also comes into play. That's when Earth's atmosphere scatters and reflects light back down to the surface, making it appear brighter than it would be otherwise. Although sky glow doesn't increase light a whole lot on its own, it's so common around the world that it adds up fast—and quickly outweighs natural light. "In the absence of moonlight there is actually very little naturally occurring light at night," co-author Kimberly Baugh, who studies satellite imagery at the University of Colorado Boulder, said. Those sources include auroras, volcanos, lightning, and wildfires.

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In the new paper, the team of scientists studied four years' worth of data from a satellite called Suomi, which orbits Earth in north-south loops between the poles about 14 times a day. Overall, they saw a 2.2 percent increase in lit surface each year, and a similar increase in brightness itself within lit areas.

The specific years they were looking at, between 2012 and 2016, were particularly intriguing, since that lines up well with the transition away from yellowish sodium lights toward whiter LED lights in street lamps and other locations. But those LED lights are actually harder for the satellite to see—which means that the new numbers are lowballing just how many more comforting nightlights humans have brought to Earth's surface.