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," Franz Holker of the Leibniz-Institute of Freshwater Ecology and Inland Fisheries, one of the study's authors, said in a briefing.
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 study 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.
"It threatens biodiversity through changed night habits, such as reproduction or migration patterns, of many different species: insects, amphibians, fish, birds, bats and other animals," he said.
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 (for example, as night-shift workers must) can result in many kinds of issues, from depression-like symptoms to obesity and diabetes.