And of course, the more light pollution there is, the fewer stars we can see -- which makes it difficult for astronomers to study the heavens with ground-based telescopes.
To find out whether the human demand for light is still on the rise or leveling off, an international team of scientists used the Visible Infrared Imaging Radiometer Suite, also known as VIIRS, a satellite sensor that's a collaboration between NASA and the National Oceanic and Atmospheric Administration.
The researchers studied data from the month of October in each year from 2012 to 2016. They found that over that time, Earth's artificially lit outdoor surface grew by 2.2 percent each year, and the total radiance grew by 1.8 percent per year. On top of that, the outdoor areas that already had been lit when the study started in 2012 also brightened by 2.2 percent per year.
The fastest growth took place in countries in developing regions, such as Asia, Africa and South America. Countries that already were brightly lit, such as the United States and Spain, appeared stable. A small number of war-ravaged countries such as Yemen and Syria saw a drop in their artificially lit levels.
But even in those brightly lit countries that appeared stable, light levels are probably still on the rise, the scientists said.
That's because many well-lit cities have been replacing yellow-orange sodium lights with energy-saving LED lights. Those sodium lights put out a small amount of infrared radiation, which would have made them look brighter to VIIRS. The white-toned LED lights have more blue wavelengths, which VIIRS cannot see. The combination of losing the infrared signal and not being able to see the blue wavelengths means that VIIRS registers this areas as dimmer, even though they may be brighter than they were before.
"That means that this measurement that we're reporting is a lower bound on how Earth's light is increasing," said study lead Christopher Kyba of the German Research Center for Geosciences. "And in fact, the true increase that a human being would perceive is actually larger than what we report here."
Researchers had long suspected that the introduction of LEDs would mean less energy used for lighting, he said. The problem is this also made lighting cheaper.
"Whenever you make lights more efficient, you just don't save energy," Kyba said. "What happens instead is that people put more lights up."
The blue light in those LEDs also are particularly disruptive for many nocturnal animals, researchers said.
There are some possible solutions, Kyba said, such as using LEDs that don't have a blue component. There also are ways to position and manage existing light sources -- say, the lamps in a parking lot -- so that they are not as bright but still are effective.
Such solutions could save cities a lot of money, he added.
"We're very interested in working together with cities and with local governments, with industry partners, to try to find ways that we can eventually reverse this trend," he said.
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