When nighttime data is adjusted for population, certain areas outside cities stand out. Tim Wallace

Artificial light that floods the night sky is thought to be only an urban phenomenon. But when you adjust for population, the picture is dramatically different.

The rule of thumb is that if you want to see the Milky Way, you have to venture out to the countryside. That’s where the illumination from streetlights and brightly lit offices that floods cities hasn’t obscured the night sky. But a recent map tracking the artificial lighting seen at night through satellite imagery paints a very different picture. In the map of the United States by Tim Wallace, a cartographer at Descartes Labs, metropolitan areas like New York City, Los Angeles, and Chicago are dark, while rural areas in the Dakotas and sparsely populated towns right outside major urban centers stand out as bright spots.

It’s not that cities have dramatically reduced light pollution (they haven’t). Most of the light is coming from places where there are lots of people,” Wallace says of traditional nighttime maps, which look almost identical to population density maps. Instead, Wallace’s map is the result of taking 2015 nighttime data from the National Oceanic and Atmospheric Administration and (roughly) normalizing it for population so that it shows the amount of light emitted per person in an area.

What’s left is a cartographic look at a problem that’s often overshadowed: Light pollution is also a threat to rural areas, and it’s disrupting ecosystems that rely on natural darkness. Not only does rural lighting make star-gazing harder for enthusiasts and space researchers, it also seeps into nearby habitats, changing the resting and feeding behavior of wildlife. For nocturnal migratory birds, for example, these sources of illumination can be confusing, and even deadly. It’s a problem that the International Dark-Sky Association has been calling attention to.

“We’ve always had this two-pronged approach: addressing the problem largely where it exists, which is in the cities, while recognizing the rural areas where darkness still exists [as] a form of natural resource,” says John Barentine, the organization’s policy director. “So we are concerned about some of these places that are lighting on Tim’s map.”

Where is the light coming from? Aside from Disneyland in Florida, and airports, Wallace and Barentine point to three particular kinds of economic activities in rural areas: oil and gas extraction, the expansion of warehouse hubs, and increasingly, greenhouses.

Oil and gas production

Oil and gas production sites like the Bakken Shale can produce as much light pollution as cities. (Tim Wallace)

Looking at Wallace’s map, it’s hard to miss the burst of light in western North Dakota. That’s the location of the Bakken Shale, one of the U.S.’s largest oil producers. Production peaked in spring 2015 at 1.33 million barrels a day. Barentine points to two other spots that light up for the same reason: the Eagle Ford Shale in South Texas, and the Permian Basin, located in western Texas and southeastern New Mexico. The pollution comes from a mix of on-site lighting for worker safety and the flaring of excess natural gas.

”If you draw a box around the bright blob in western North Dakota and add up all that light, there’s approximately as much light being emitted by that operation as there is by the incorporated boundaries of Chicago,” he says. That’s not unique to that the Bakken Shale. A 2018 investigation from The Revelator found that, based on 2013 data from NOAA, the brightest sections of the Eagle Ford Shale are as bright as Reno, Nevada.

Between 2010 and 2013, when the U.S. was in the midst of a significant oil boom, measurements from the National Park Service’s Natural Sounds and Night Skies Division found that man-made light visible in the north unit of Theodore Roosevelt National Park, near the Bakken oilfield, increased by 500 percent—faster than at any other national park in the country, according to Inside Energy. The University of Texas’s McDonald Observatory, which is a member of the International Dark-Sky Association, has been working with oil companies to find ways to mitigate light pollution. In 2018, the observatory laid out a set of guidelines for improving lighting practices at rig sites. It recommended readjusting lighting fixtures at sites so they don’t point up at the sky and redesigning blueprints to minimize the amount of lighting needed.

The expansion of warehouse hubs

Warehouses and and nearby parking lots often remain lit at night. (Tim Wallace)

While much of downtown Chicago is blacked out on Wallace’s map, the town of Joliet just southeast of it shines brightly. Similarly in California, Los Angeles remains dim but the cities that sit on the edge—Riverside and San Bernardino—stand out in stark contrast.

These three areas are just a handful of the places that have become dominated by warehouses, data centers, and fulfillment and distribution hubs. As e-commerce took off over the last decade, once-cheap farmland became a popular site for tech and retail companies. “One of the things we kept seeing over and over again were these these little exurbs that 10 years ago might have had one DHL [warehouse], but now they have an Amazon fulfillment center and a DHL and something for Target,” says Wallace, who worked with the New York Times last year tracking the transformation of America’s landscape.

The real estate firm Cushman & Wakefield calculated that developers added nearly 850 million square feet of warehouse space between 2013 and 2017, more than double the amount built over the previous five years. And the buildings keep getting bigger. Amazon alone, for example, has nearly 350 facilities across the U.S.; several are more than 1 million square feet. “These buildings are massive,” says Wallace, “and they pop in this light map because the parking lots are lit up and the buildings often have lights on [at night] as well.”

According to Barentine, the rise of these industrial hubs aligns with the rise in sales of exterior lighting fixtures, which have seen some of the most dramatic spikes in popularity in recent years.

Greenhouses and the cannabis industry

A 42-acre tomato greenhouse in Madison, Maine, stands out as a bright spot. (Tim Wallace)

When asked if any places stood out to him, Wallace pointed to Madison, Maine. There, surrounded by acres of empty fields and forest, is a 42-acre indoor farm—about the size of 32 football fields—operated by the tomato grower Backyard Farms. On Wallace’s map, the farm is one of two spots that glows prominently. “That blew my mind. You can see this tomato greenhouse in this super-remote area in Maine,” he says. “It’s a massive complex where their lights must be on at night.”

As it turns out, the legal cannabis and horticulture industries are on the Dark-Sky Association’s radar as light pollution sources, and not just in rural areas, says Barentine. “Greenhouses are now built increasingly in suburban and even urban areas. [Companies] run those operations 24 hours a day, and for some good part of the night, they’re running lighting at full blast.” It’s not just the duration of the light that concerns conservationists, he adds, but new LED technology that allows growers to change the color of the light to what their specific crops need.

Anecdotally, they’ve become a nuisance to nearby residents. In Snowflake, Arizona, last month, residents complained of a mysterious purple glow. The source? A 1.7 million-square-foot (40-acre) marijuana farm just a few miles away.

Early research also suggests that the unnaturally colored lighting can disrupt the plant cycle in wildlife corridors, which in turn, could affect the food supply. Barentine acknowledges that more research on the impact of greenhouse lighting is needed, but says the lack of regulation is a concern. “In isolation, light may not be a really serious hazard to species,” he says. “But if they are already being stressed for other reasons, and you add that on top, it can be very significant.”

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