A citizen science initiative called Globe at Night has some sobering news for humanity. Our artificial light is drowning out the night sky for more and more people. And it’s happening more rapidly than thought.
Globe at Night is an international citizen science project run by the National Science Foundation’s NOIRLab. It raises awareness about light pollution and how it’s impacting our view of the night sky. A new research article based on observational data from Globe at Night shows that the average night sky grew 10% brighter each year in artificial light for the past eleven years. And as our artificial light becomes brighter, more stars are becoming obscured.
The paper is “Citizen scientists report global rapid reductions in the visibility of stars from 2011 to 2022,” and it was published in the journal Science. The lead author is Christopher Kyba, a researcher at the German Research Centre for Geosciences.
“The introduction of artificial light probably represents the most drastic change human beings have made to their environment.”
We should be able to see several thousand stars on a clear night. We should be able to see the grand arc of the Milky Way, too. But that’s becoming increasingly difficult for more and more people. Our growing artificial lighting is responsible, but it’s difficult to measure with satellites. A lot of our artificial light spreads horizontally, which satellites can’t measure effectively.
Standing beside the Milky Way. Drowning out the night sky with artificial light blocks us off from nature, and that’s not good for humans. Credit: P. Horálek/ESOThe authors of this paper got their data from citizen scientists taking part in Globe at Night. Participants rated their view of the night sky by comparing it to a standard set of images. The images were calibrated to show the night sky as seen by each participant in their location on Earth. The images are on a gradient so that each one shows progressively fewer stars to mimic increasing light pollution. Participants then selected the image that best matched their real view. The data gives an estimate of what’s called the naked eye limiting magnitude. It tells how bright a sky object must be to be seen.
These are example star charts used in the Globe at Night citizen science project. These are the ones from 30 o north showing the Orion constellation. Participants matched their naked eye view with these charts. The upper left is representative of the view from a city centre, and the bottom right is representative of the view from a remote location. Image Credit: Kyba et al. 2023.The study shows that our growing light pollution is having a profound effect. About 30% of people around the globe and approximately 80% of people in the United States can no longer see the Milky Way. This is changing faster than scientists thought. But they’ve based their conclusions on satellite data, which struggles to measure the glow from surface light. The Globe at Night collects data from the ground, and this study is a wake-up call.
This is what sky-glow looks like in Mexico City. By Fernando Tomás from Zaragoza, Spain – Flickr, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=348732At a time when war rages in Europe and humanity is confronting climate change, changes in the night sky might not seem very impactful. But there’s something poignant and sad about it. The night sky is part of humanity’s natural and cultural heritage. And it’s fading before our eyes.
Yet this is about more than humanity’s contemplative relationship with the night sky, something that reaches well back into pre-history. The increasing artificial lighting clashes with our natural day/night cycles and can be detrimental to human sleep cycles. It’s also disruptive to wildlife since their natural cycles have evolved alongside the natural day/night cycle, too. Not only can our artificial lighting disrupt habitat the same way a bulldozer can, but it disrupts predator/prey relationships since predators use light to hunt and prey uses dark to hide. It’s also driving nocturnal animals away from our lighted areas.
In an interview at Darksky.org, study lead author Christopher Kyba said, “The introduction of artificial light probably represents the most drastic change human beings have made to their environment. Predators use light to hunt, and prey species use darkness as cover,” Kyba explained. “Near cities, cloudy skies are now hundreds or even thousands of times brighter than they were 200 years ago. We are only beginning to learn what a drastic effect this has had on nocturnal ecology.”
We intuitively know that we’re lighting up the night sky as more and more of us live in larger and larger cities. But data has been difficult to gather. That’s where Globe at Night comes in. Since 2006, they’ve been gathering data on stellar visibility. In their paper, the authors analyzed more than 50,000 observations submitted to Globe at Night between 2011 and 2022.
This figure from the study shows where citizen scientists’ observations were distributed globally. A shows the number of observations by year and continent, and B shows the spatial distribution of all years combined. Image Credit: Kyba et al. 2023.The data plots the growing price we pay for our light pollution. It shows that for the average Globe at Night participant, the sky is becoming 9.6% brighter each year. During an average person’s childhood, which spans 18 years, the night sky brightness increases by a factor of four. This isn’t true in every location on Earth, so it wouldn’t be so bad for people in rural or remote locations. But for people in cities or in suburbs close to cities, it’s likely even more pronounced.
“At this rate of change, a child born in a location where 250 stars were visible would be able to see only around 100 by the time they turned 18,” said lead author Kyba.
This figure from the study plots the Sky Brightness Factor (SBF) with the Naked Eye Limiting Magnitude (NELM) to illustrate how we’re losing visibility of the night sky. An SBF of 1 indicates starlight, and SBF 10 indicates that the sky is 10 times brighter than starlight. (Since it’s a logarithmic scale, these are plotted as log10SBF = 0 and 1, respectively.) Smaller NELM values on the y-axis show that fewer stars are visible. The graph plots 2011 data, 2021 data, and all-years data. Image Credit: Kyba et al. 2023.Kyba was also the lead author of a 2017 research article that looked at artificial lighting. It’s titled “Artificially lit surface of Earth at night increasing in radiance and extent,” and it was published in the journal Science Advances.
This figure from the 2017 study shows the rate of change globally for both Area and Radiance. Changes are shown as an annual rate for both lit area (A) and the radiance of stably lit areas (B). Annual rates are calculated based on changes over a four-year period from 2012 to 2016. Since this is based on satellite data, some of what it shows is related to warfare (Syria) and forest fires (Australia.) Image Credit: Kyba et al. 2017.Some of our lighting problem is related to technology. The development of LED streetlights created a more energy-efficient type of lighting that’s lowered the cost of lighting outdoor areas. Lower energy lighting technology hasn’t reduced light, though, because it’s cheaper to use. People can use more of it without paying more. “Regardless of historical or geographical context, humans tend to use as much artificial light as they can buy for ~0.7% of GDP,” the authors of the 2017 study point out.
A glittering night-time map of Europe. Most of the lights in this image are streetlights, which make up the majority of light pollution. The atmosphere scatters the light, creating sky-glow. NASA Earth Observatory images by Joshua Stevens, using Suomi NPP VIIRS data from Miguel Román, NASA’s Goddard Space Flight CenterHumanity grew up under the night sky, and the evidence from ancient cultures makes that clear. Numerous ancient structures still standing today were oriented toward celestial objects in the night sky and the changing of the seasons. These are called archaeoastronomical structures and archaeoastronomical observatories. From Machu Picchu in Peru to Gaocheng in China, these structures show how strong the relationship between people and the sky was. (There’s even some evidence that the Incans used the observatory at Machu Picchu to observe the Pleiades and time the planting of corn.)
For most of humanity, our connection to nature via the night sky is being severed. But all is not lost. Some of it comes down to how much we value the night sky and what policies we’re willing to put into place. The Globe at Night project is important because it illustrates what’s at stake.
From the paper: “We draw two conclusions from these results. First, the visibility of stars is deteriorating rapidly, despite (or perhaps because of) the introduction of LEDs in outdoor lighting applications. Existing lighting policies are not preventing increases in skyglow, at least on continental and global scales. Second, the use of naked-eye observations by citizen scientists provides complementary information to the satellite datasets.”
“The increase in skyglow over the past decade underscores the importance of redoubling our efforts and developing new strategies to protect dark skies,” said study co-author Constance Walker from the National Optical Astronomy Observatory. “The Globe at Night dataset is indispensable in our ongoing evaluation of changes in skyglow, and we encourage everyone who can to get involved to help protect the starry night sky.”
