An animation of a person shining a flashlight on an eye as it opens and closes.
Jocelyn Tsaih

The Dark Side of Light

Beneath the surface of one of Germany’s deepest lakes, researchers are studying the hidden effects of artificial light.

NEUGLOBSOW, GERMANY—Martina Bauchrowitz put her back into it, swinging the oars in a wide arc, and the small boat lurched away from the lakeshore. I gripped the hull, shivering in the early-spring air, and watched our progress toward the rose-shaped metal platform floating on the surface of Lake Stechlin, one of the deepest lakes in northern Germany.

After a few minutes of rowing, we bumped against the side of the rosette. Bauchrowitz and I secured the boat and climbed out. Beneath the blue sky and puffy clouds, beneath the shiny platform and the dark, choppy waves, is another world—invisible in daylight and, more important, in darkness.

We stood on a floating plastic pontoon anchored among 24 aluminum cylinders, each protruding a few inches above the surface so that they resembled connected rings, like the petals of a flower. Each is a miniature ecosystem. They are, essentially, giant test tubes, each the size of a grain silo, nestled in Lake Stechlin. In this system of artificial lakes, scientists at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries can test their hypotheses in a natural environment.

“Watch your step,” Bauchrowitz said. I thought she was worried that I would fall, or worse, drop my notebook into one of the ringed enclosures, but that wasn’t what she meant. She gestured toward browning spots of caked-on guano.

“The birds are a problem. When they shit here, we have extra nutrients we would not have without the platform,” she said.

Most biologists conduct their laboratory experiments inside well-appointed rooms within nondescript buildings at research centers or on university campuses. But here, the lake is the lab. Every fish is counted, the density of microscopic plants and animals is carefully calibrated, and any excess bird crap must be cleaned up. The cylinders inside the lake are kept as pristine as possible until an experiment begins, and the scientists tinker with them the way their counterparts might meddle with the contents of a petri dish.

The Lake Lab was built so that ecologists could study a form of environmental change that rivals climate change in its scale and reach: the spread of artificial light at night.

Since 2010, the scientific literature has exploded with research examining light’s effects on individual species, from birds to fish to trees to humans. The news, in general, isn’t good. Artificial light changes animal migration and reproduction, tree leaf growth, bird nesting and fledging, pollination, human sleep, and much more. It even affects the spread of diseases. In July, researchers reported that West Nile–virus–infected house sparrows that live in light- polluted conditions are infectious for two days longer than those that live in darkness are, increasing the risk of a West Nile outbreak by 41 percent.

But none of this happens in a vacuum. The Lake Lab allows researchers to study how the effects of artificial light cascade through entire ecosystems—in this case, through bacteria, plankton, algae, and fish.

As a research station studying broad environmental change, the Lake Lab is a microcosm of the larger, uncontrolled experiment humans have been conducting on the planet since the start of the industrial age. But the lab is a version that we can control. It is a place where scientists can determine just how bad things have become, and how we can make them better.

Light is the basis for all life, but it is more than just a source of energy. It is also a source of information, telling organisms when to sleep, hunt, hide, migrate, metabolize, and reproduce. Since the advent of incandescent light bulbs, humans have been interfering with those messages. And the interference is worsening with the spread of LEDs, which consume less electricity and so are often brighter and stay on longer and later than their predecessors.

Since 2012, when a satellite began taking detailed measurements, light emissions have been rising at a rate of 2.2 percent a year on average. Previous work showed that light emissions are growing by as much as 20 percent in some regions. This is faster than the average annual growth of the global economy, the global population, and emissions of carbon dioxide.

“Light is occurring at times, levels, and places where it should not be,” the researcher Franz Hölker told me over a twilight cup of coffee. “And now it is becoming more clear that it can impact whole populations and ecosystems.”

Hölker is tall and silver-haired, and speaks carefully yet emphatically. His blue eyes flash when he gets animated—which happens when he starts talking about moths’ response to light, or points out the omnidirectional downward glow of a streetlamp. A decade ago, he was one of the few scientists calling for more research on artificial light at night. Today he leads a program called the Loss of the Night Network, a multinational, transdisciplinary research group under the umbrella of the European Union. It began in Hölker’s lab at the Leibniz Association, a German consortium of independent research institutes named for the 17th-century Prussian philosopher and polymath Gottfried Wilhelm Leibniz. Hölker and his project, which is part of the Leibniz-Institute, are at the forefront of research on artificial light.

I’ve spent a lot of time reading the literature on this subject, and as we sipped our coffee, I told Hölker that I viewed him as leading the charge. “Ja. It was quite an exotic topic for a while,” he said, demurring.

Light pollution began as an astronomer’s lament. Professional and amateur sky watchers have complained for decades that city lights interfere with their observations, leaking into telescopes and obscuring their views of distant stars and galaxies. Beginning in the 1970s, astronomers began lobbying lawmakers to protect observatories, with mixed results. Southern California’s Palomar Mountain is home to the Hale Telescope, one of the most important observatories in 20th-century astronomy. In the early ’80s, astronomers persuaded lawmakers in Riverside County to ban upward-facing high-pressure sodium lamps within a 15-mile radius of Palomar, to keep the glare at bay. But as nearby San Diego grew, the horizon around the mountain filled with light anyway.

To promote model lighting rules and encourage their peers to speak as a group, the astronomers David Crawford and Tim Hunter founded the International Dark-Sky Association in 1988. Crawford was a professional astronomer, and Hunter an amateur enthusiast. The organization now certifies parks and protected areas as “dark-sky friendly,” and recommends outdoor light fixtures that reduce glare and upward light, among other activities. But the lights of the world have continued to brighten.

By the turn of the millennium, ecologists were taking notice too. In 2002, the American researchers Travis Longcore and Catherine Rich organized the first conference on the ecological consequences of artificial light at night. That meeting, and a research textbook that Longcore and Rich published afterward, are widely considered the genesis of interest in the environmental-science community. But ecological research on artificial light was still a niche subject.

Hölker is a fish ecologist by training, and as he studied changes in urban fish populations, he always noted light levels and temperatures, the two most important behavioral triggers for life in the water. He soon began to see a correlation between light levels and changes in activity in common types of freshwater fish such as bream, roach, and perch.

“We saw in our data that fish were behaving differently in an urban area compared to a rural area. Suddenly these diurnal fish were able to eat at night,” he said. “It was becoming more and more interesting, from a researcher’s perspective, and more and more surprising, to what extent artificial light at the wrong time can have an impact.”

In 2010, Hölker led the publication of two treatises on light pollution, one outlining the threat to biodiversity and the other calling for an urgent new research agenda on artificial light. Without new science across disciplines, Hölker and his co-authors wrote, “modern society may run into a global self-experiment with unpredictable outcomes.” Air, noise, and water pollution had been high-priority policy issues for decades, they pointed out, but “light pollution remains scientifically, culturally, and institutionally in the dark.”

This makes some cultural—and practical—sense. While most mammals are nocturnal, humans are not. The ability to illuminate the world at will is part of what makes us who we are, and has been for millennia, since long before streetlamps, long before cities, long before stories about evil darkness and virtuous light. As Hölker put it to me: “Humans tend not to believe that light can also have negative consequences.”

In the years following Hölker’s 2010 papers, ecologists started to study those consequences. The results emerged quickly and with certainty. In artificially lit environments, songbirds advance their egg-laying; wallabies delay their births; salmon change their migration; fish delay their spawning; and maple trees keep their leaves later in the fall, which can cause frost damage. The list goes on.

In one study, Hölker’s team showed that streetlights could draw in moths that passed within a radius of about 23 meters. European lampposts are generally about 20 to 45 meters apart, so the area they illuminate overlaps. This might be nice for pedestrians, but for insects it acts as a trap, preventing their dispersal through the environment. “It’s a vacuum-cleaner effect,” Hölker said. Other animals respond too, in a cascade that changes the makeup of entire communities.

In another study, he and his colleagues observed spiders, harvestmen (also called daddy longlegs), beetles, and other predators positioning themselves near streetlights like diners at a buffet. Hölker measured variants in single atoms in the guts of his specimens to quantify how the predators’ diets were changing to incorporate more aquatic insects, which were attracted to and mesmerized by high-pressure sodium lamps. (In an effort to safeguard dwindling insect populations, German authorities just announced a 100-million-euro initiative that will encourage the use of exterior lighting that is triggered by motion detectors rather than always on, and “insect friendly” lights of certain wavelengths.)

Jocelyn Tsaih

But as any ecologist will point out, artificial-light levels are just one of the many differences between urban and rural areas. Air pollution, noise pollution, paved streets and sidewalks, outdoor cats, and countless other stressors can also interfere with species’ reproduction and migration patterns. The Lake Lab is designed to eliminate those additional factors in order to isolate the effects of light.

The lab is the brainchild of Hölker and the ecologist Mark Gessner, who is married to Bauchrowitz. Gessner, a systems ecologist, was not initially drawn to studying light. Hölker and Chris Kyba, a physicist at the German Research Centre for Geosciences, in Potsdam, and a frequent collaborator, helped persuade him. Gessner, who now leads the Lake Lab with Hölker, says he’s remained agnostic about light’s ill effects, and wants to find out whether they’re severe enough to warrant pushing for broad policy changes. Maybe they are—but if they aren’t ecologists can use the Lake Lab to study other issues, such as nitrate pollution and microplastics, Gessner told me over lunch one day.

“I’m really a believer in experiments,” he said. “Let’s find out; let’s clear our minds and see if we need to make it an issue.”

As Hölker’s earlier work established, light streaming from streetlamps and buildings can stop invertebrates and other animals as effectively as a physical fence. But in urban areas, and even in many rural areas, a more insidious problem is skyglow, or the indirect illumination caused by light bouncing off the atmosphere—the same process that turns the daytime sky blue.

On a clear night, skyglow can render a city five times brighter than it would be under natural conditions. On an overcast night, when light reflects off both the ground and the clouds, the sky can be 1,000 times brighter than it would be otherwise. Skyglow covers so much of the Earth that one-third of humanity can no longer see the Milky Way. About 99 percent of the population in North America and Europe lives under light-polluted skies. And it is getting worse, according to research that Kyba, Hölker, and a team of their colleagues published in 2017. From 2012 to 2016, they found, Earth’s artificially lit outdoor area grew by 2.2 percent a year.

“Skyglow is no longer an urban phenomenon. In a densely populated industrial country, it’s basically the whole country,” Gessner said.

In a city, or even in the suburbs, you might not notice skyglow; the clouds just seem bright. But far away from development, the contrast is clearer. Clouds are not blankets of bright haze, but instead dark smudges covering the stars. Cities appear on the horizon not as points of light, but as domes, their light reaching far into the atmosphere and reflecting off clouds and water droplets.

The Lake Lab is skyglow-free: Surrounded by the Stechlin-Ruppiner Land Nature Park, it is located in one of the darkest areas in Germany, and arguably one of the darkest regions in western Europe.

To simulate varying levels of skyglow at the Lake Lab, Andreas Jechow, whose background is in experimental physics and photonics, devised light rings to produce an evenly distributed layer of light one meter below the lake surface. To study how this simulated skyglow affects aquatic life, automated pulleys sink into the depths at regular intervals, carrying cameras and water-sampling equipment to study everything in the water column.

Each spring, the test-tube lakes are flushed so that their waters match that of the larger Lake Stechlin. Each enclosure reaches several feet below the lake bed, ensuring that the lakes are separate from one another and their host.

During an experimental season, thousands of water samples are toted back to shore on a rowboat or a small skiff with an outboard motor. In a wood-and-glass building steps from the dock, teams of biologists work in a well-appointed lab that rivals any biology bench in the Western world. It also has what might be the best view, with ceiling-high windows overlooking the shore. When I visited in April, the microbiologist Thomas Gonsiorczyk took foil-wrapped sediment samples from a lidded glass jar to prepare them for study. When the sample pucks were later baked at 500 degrees, scientists measured the gases they emitted to study their chemical makeup.

During the Lake Lab’s first season, in fall 2016, researchers experienced some unforeseen problems, bird feces among them. In the absence of predators, which scientists initially excluded, the tiny algae-eating crustaceans called water fleas didn’t move around as expected, which meant any changes in the algae population couldn’t be attributed to just light. The team had to introduce tiny silvery fish called Stechlin Maräne, or Coregonus fontanae, which are found only in Lake Stechlin. Placing eight to 10 fish in each lake produced the right amount of pheromones to make the water fleas move.

Each artificial lake has a littoral zone (an area where water meets the shore), which means parts of each enclosure are permanently submerged and others are exposed to the air. During the first season, aquatic plants started growing on the sides, interrupting the typical water chemistry and turning each test-tube lake into a shoreline instead of a deep, stratified body of water. Experimenters started to clean the walls, but this proved difficult and time-consuming. Now they hang tarps along each lake that can be lifted out like a Crock-Pot liner, saving the trouble of scraping the sides. The mini lakes also took on their own characteristics, displaying a level of individualism that Gessner found frustrating.

“Every year, we had one that behaved in a very strange way. There are very chaotic dynamics, which we don’t like as experimentalists. Maybe it was a flap of a butterfly,” he said. “As much as we try to create natural conditions, it’s still an experimental system. It’s not perfect.”

During my daytime visits to the Lake Lab, my eyes were drawn to the high white clouds overhead, the deep blue of the lake water, the trees on shore, and the scientists readying buckets and sample containers. At night, vision is less useful, and other senses take over: it’s possible to smell the familiar, sharp, piney cleanliness of evergreens; feel the spongy texture of sodden wood; and hear the water lapping at the shore. It’s almost possible to taste the dense musk of last fall’s rotting leaves.

At night, the Lake Lab comes to life. The two dozen cylinders glow with a greenish, milky hue. LED lights mounted above the rings shine into the water and over it, casting their carefully calibrated versions of skyglow. The researchers can tweak each ring to mimic different cities, from Berlin to Hong Kong, the world’s brightest town: that ring shines with 10 times the strength of a full moon.

Normally, water fleas lurk in deep, dark water during the day, hiding from fish. At night, they drift to the surface to eat algae. But when the water is illuminated by skyglow, the water fleas might stay down later, leading to algae overgrowth; when they are finally forced to migrate upward in order to feed, they might be easier targets for fish. Early results show that the water fleas’ daily migration patterns are changing, but not as dramatically as Gessner and Hölker had expected: The fleas are avoiding the upper layers of water but still edging close enough to eat, so the algae is not growing out of control. The composition of the test-tube lakes’ bacterial communities is changing, which alters the chemistry of the water. And fish are affected, too. The team member Franziska Kupprat has demonstrated that even under low levels of skyglow, melatonin—an antioxidant related to sleep that may have multiple protective effects—crashes by 50 percent in perch.

The results from the Lake Lab’s first two seasons are still taking shape. “Because it’s subtle, it’s harder to develop a clear narrative,” Gessner said. “We’ve collected more data than we can handle.”

And yet the lab has already moved the field forward, enabling experiments that would be impossible on smaller spatial scales: It is the only place in Europe where hydrologists, chemists, microbiologists, limnologists, ecologists, and fish biologists are cooperating on the study of a single ecosystem.

This broad, all-inclusive nature characterizes light pollution, too. Studying an intrusive, rapidly spreading, impossible-to-contain phenomenon requires studying everything. So the researchers are expanding the scope and complexity of their interests and experiments. For his doctorate, Hölker studied the freshwater fish roach and bream in lakes throughout Germany. Now he studies how light affects microbes, spiders, and butterflies; how it spreads through the atmosphere; and how we can do a better job of quantifying it. His most recent paper, published in February, was co-written with Kyba and Jechow, and used methods borrowed from deep-space astronomy to quantify how clouds darken the sky above Lake Stechlin.

As a system built to study other systems, the lab is also a microcosm of modern science’s shift toward its past. People such as Leibniz and the 18th-century explorer-naturalist Alexander von Humboldt were students of everything. They studied plants, animals, how temperature and climate affected them, and how they interacted. Humboldt in particular understood that Earth is a system—that everything is connected, that no organism is an island.

But things changed in the decades after Humboldt, and then Charles Darwin, tromped through the tropics. During the 20th century, naturalists specialized, dividing into astronomers, ecologists, botanists, and molecular biologists. They trained their microscopes on single organisms, single cells, and single genomes. Modern scientific research is atomized, and generalists are harder to find, and fund. The Lake Lab is the opposite of narrowly specialized, and light pollution is, too.

Kyba, a Canadian who has lived in Germany since 2008, came to the field via particle physics, and started studying light pollution because he needed a job. But now—like a true physicist—he views his field as the study of everything. Finding out why things are how they are, and how they work, is worthwhile in its own right, he told me over dinner at his home in Potsdam one night.

“Light pollution is such a new phenomenon; simply describing the world we are in is the very first step,” he said. “Simply knowing something about the world is a kind of science.”

The Lake Lab began as a way to study light pollution, but in its third year, experiments will begin to address climate change, too. The lab’s current set of experiments focuses on how neighboring lakes affect one another. Scientists are creating different conditions in different enclosures and enabling various rates of water exchange. The lakes are monitored from the air, and ultimately the team wants to translate its results to water-quality monitoring projects that can be adopted throughout Brandenburg’s 3,000 lakes, and Germany as a whole, so that scientists can watch what happens as the waters warm.

Bauchrowitz, who took me through the lake, said she sees the Lake Lab as a way to test hypotheses that are too broad to measure in a petri dish.

“The Lake Lab for us is about global environmental change. And light pollution is one part of global environmental change,” she said.

Researchers who study artificial light at night tend to take the term light pollution literally. All of the light-pollution scientists I have ever met have expressed a preference for the darkness, even as they confess to reading in bed at night or watching TV into the wee hours.

“It’s like a red-pill moment,” Kyba told me. “Whenever people get educated about light pollution, they can’t help noticing it everywhere.”

It’s impossible not to think about it in a city, especially one as sprawling and ancient—and as modern and reinvented—as Berlin. Sidewalks and streets are lit from above and below so that cars, cyclists, and pedestrians are less likely to run into one another. Storefronts and most residences affect a warm and inviting glow, even when no one is home. The capital city’s steely, graffiti-limned architecture takes on an imposing gleam. Parks are often the darkest islands in an urban environment. But even parks can be illuminated by lights—a safety measure that Kyba considers counterproductive.

A couple of years ago, he attended a conference at Duke University. Suffering from jet lag, he decided to go for a midnight walk with his light meter and a fellow light researcher. The pair came across an emergency call station, one of those towers with a bright-blue light and a phone, where someone could ostensibly summon the police. Kyba found it blindingly dangerous.

“When your eyes are adapted and you are walking around on the path, you can see relatively well. But as you walk toward that lamp, you can’t even see. You have to walk with your hand screening your face, because you can’t see anything otherwise,” he said. When Kyba’s companion stood near the light, he was invisible to Kyba.

“We are so used to lighting being the way it is, it’s almost impossible to imagine there could be a problem, and that we can do better,” he said.

Doing better isn’t hard, though. Kyba told me about the amber lights he installed in his bathroom, for instance. Long-wavelength light, shifted to red or orange hues, does not suppress melatonin to nearly the same degree. Hölker told me that he uses warm light in the evening and low light levels in his garden. He turns the lights off when he’s inside, allowing pollinators, nocturnal mammals, and birds to experience a patch of night.

“Everybody should be able to justify why there has to be light in the environment,” Hölker said. “Sustainable lighting is quite easy. The problem is, so many people are not informed about the negative aspects.”

After dinner , Kyba led me to his back porch, overlooking Potsdam. The crescent moon hung in the west, barely visible behind his roof. The sky to the east was clear. Kyba pointed me toward the northeast, where the sky was full of a pearly haze, obscuring even the brightest stars. Berlin.

Hölker had told me that 30 percent of that light comes from the streets, including headlights and streetlamps. These are getting brighter in once-poor but rapidly gentrifying neighborhoods such as Kreuzberg and Neukölln, where authorities are installing LEDs that use less energy but are brighter in the wrong ways, full of easily scattered blue wavelengths. Tegel Airport, Berlin’s international hub, contributes 4 percent of Berlin’s upward light. The city’s famous TV Tower contributes a huge portion, too.

Afterward, Kyba took me back to the S-Bahn station in Potsdam to make sure that I got on the right train. I watched the lights of Berlin grow brighter through the train windows. Back in Hackescher Markt, I squinted at the beer garden near my hotel, and decided to make my way back to the water instead—not a deep lake this time, but Berlin’s Spree river.

Berlin Cathedral loomed across the black water. Museum Island, harboring much of Berlin’s cultural history, was lit by lamps. Trees obscured TV Tower, though I could see its light from blocks away. It was as dazzling as a stadium on a Friday night. I decided to stop when I found that I could no longer see into the distance, through the bright foreground. Kyba’s warning about the bright lights rang in my ears and suddenly, I felt afraid. I turned around, toward the shadows. Trying to get my bearings, I looked up, like I always do. The moon was there to guide me home.