Earth is losing its darkness. A new study using satellite data finds that artificially lit surfaces around the world are spreading and growing brighter, producing more light pollution at night.
The findings, described in the journal Science Advances, track what researchers called a worrisome trend that has implications for the environment as well as human health.
“This is concerning, of course, because we are convinced that artificial light is an environmental pollutant with ecological and evolutionary implications for many organisms — from bacteria to mammals, including us humans — and may reshape entire social ecological systems,” said Franz Holker of the Leibniz-Institute of Freshwater Ecology and Inland Fisheries, one of the study’s authors.
Thanks to electric lights, outdoor lighting grew at a rate of 3 percent to 6 percent annually in the second half of the 20th century. While this has benefited human productivity and safety, it has come with a dark side: The night is no longer dark enough.
Half of Europe and a quarter of North America have experienced seriously modified light-dark cycles, the authors wrote, calling it a “widespread ‘loss of the night.’ ”
This light pollution can have serious consequences for living things, which have evolved in accordance with a natural day-night cycle, where the only major sources of light at night would have been the moon or more intermittent sources such as volcanoes, lightning, wildfires or auroras.
“From an evolutionary perspective, now, artificial light at night is a very new stressor,” Holker said. “The problem is that light has been introduced in places, times and intensities at which it does not naturally occur, and many organisms have had no chance to adapt to this new stressor.”
That’s a big problem, given that 30 percent of vertebrates and more than 60 percent of invertebrates are nocturnal, he pointed out. It can affect plants and even microbes. It also could be harming vital interactions between species, such as the pollination of plants and spreading of seeds by key nocturnal creatures.
Humans are impacted by artificial light too because there are certain physiological processes that happen during the day and certain ones that happen at night — and they often work against each other, Holker said. That’s why working against our biological day-night clocks can result in many kinds of issues, from depression-like symptoms to obesity and diabetes.
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.
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 October in each year from 2012 to 2016. 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. Outdoor areas that were 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. Countries that already were brightly lit, such as the U.S., seemed stable. A small number of war-ravaged countries saw a drop in levels.
But even in countries that appeared stable, light levels are probably still on the rise.
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.
“In fact, the true increase … is actually larger than what we report here,” said study lead Christopher Kyba of the German Research Center for Geosciences.