The earth is losing its nights as they are supposed to be. 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.
Nothing has captured the march of wealth and progress like a society's ability to light up the night - first with campfires and torches, then with gas lamps, finally with electric lights.
The modern night time image of the Korean peninsula as seen from space, with darkness north of the 38th parallel and brilliant light in the vibrant south, powerfully captures the connection between civilization and illumination.
Now, however, a new study of satellite images, published in Science Advances, suggests that we may have taken a good thing too far. The night time face of the planet is getting brighter and brighter, and that may be doing significant harm to the health of human beings, animals and the ecosystem as a whole.
''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 per cent to 6 per cent 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''.
Suburban sprawl in the US and other developed countries is gobbling up once dark, quiet expanses of land, while explosive growth in China has been producing entirely new cities in what was once empty countryside.
The switchover from traditional sodium vapour street lights to LEDs has brightened things further, with yellow-gold urban lighting giving way to a brighter blue-white. And since LEDs are more energy efficient and therefore cheaper to operate, places that could get by without much lighting before are now being fully illuminated.
''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 per cent of vertebrates and more than 60 per cent of invertebrates are nocturnal, he pointed out. It can affect plants and even microbes and could already 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 in the darkness of 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.
And of course, the more light pollution there is, the fewer stars we can see - a whole generation has grown up which has hardly ever seen a star in the sky.
To determine just how extensive the brightening of the night has become, a team of researchers led by physicist and ecologist Christopher Kyba of the German Research Center for Geosciences in Potsdam reserved time to use America's Suomi NPP satellite, jointly operated by NASA and the National Oceanic and Atmospheric Administration.
The goal was to track and map night illumination around the planet from 2012 to 2016 - a job was a lot more complicated than simply staring at the ground and noting which square miles are lit at night and which are dark.
For one thing, cloud cover meant some parts of the planet could be seen only intermittently. For another, transient, unintended light - such as wildfires in the western US and especially in Australia during the four-year observation period - could throw off the results, with some areas appearing as if they had actually gotten darker over time, but only because the blazes had at last been extinguished. Then too there is scattered light. One of the reasons light pollution makes stargazing difficult even miles from a city is that the atmosphere acts as a sort of deflector and distributor of urban lighting, faintly illuminating the otherwise dark countryside.
A lot of the researchers' work thus involved teasing this misleading data out of their overall findings. When they finally did, and analysed what they had left, they came up with some revealing numbers: Over the course of the survey period, the earth's artificially illuminated area had grown by 2.2 per cent per year, a rate that does not appear to be slowing.
The share of that lit land that is constantly illuminated - with the night lights never going off as they do, say, in shopping or dining districts, where businesses shut down only late at night - also increased 2.2 per cent per year. Overall radiance, or the brightness of all of the lighting combined, grew by 1.8 per cent annually.
Some comparatively small but brilliantly lit areas make disproportionate contributions to the night radiance. A single international airport, for example, can be 30 times brighter than an entire town in the American west.
And rich countries, no surprise, are bigger contributors to light pollution than poorer countries. A growth in a region's gross domestic product of 13 per cent over the course of the observation period was reflected in a nearly matching 15 per cent increase in night lighting. The researchers broke that finding down further, showing that levels of artificial lighting increase steadily as per capita income in a given region rises from the equivalent of $2,000 or less, to $6,000, then to $17,000, and finally to greater than $17,000.
And nature is paying a price. One 2017 study found that artificial lighting near waterways draws insects up from the water surface and toward the lighting source, disrupting food chains and weakening the local ecosystem. Another study this year found an equally direct cause and effect between increased lighting over beach areas and a dramatic decline in sea turtle populations, as hatchlings are lured away from the water and toward the light, where they are snapped up by predators. Migrating birds, which navigate partly by light from the moon and the stars, can be thrown off course when light pollution washes out the sky.
Vegetation is affected, too. A 2016 study showed that trees are increasingly blooming out of season, as lighting coaxes their buds to burst too soon, leaving them vulnerable to damage by cold temperatures before the true onset of spring. That could affect fruit orchards and crops as well.