Is light pollution a real problem?

[This is a transcript of the video embedded below. Some of the explanations may not make sense without the animations in the video.]


For much of human history, light meant safety. Electric lights are today one of the hallmarks of civilization. But in recent years, activists have begun to complain about “light pollution” caused by too much or the wrong type of artificial light. In the French city of Rennes some young guys are running around at night turning off shop lights.

Is light pollution a real problem or just something that first world people complain about because they have nothing else to complain about? What could we do about it in any case? And how much of a problem are Elon Musk’s Starlink satellites really? That’s what we’ll talk about today.

This is a photo of the night sky in Patagonia. You can clearly see the Milky Way. If you took a photo with the same exposure at night in Barcelona, it would look like this. If you adjust the exposure so that it matches what we see with our own eyes, you’d see this. Artificial lights make the sky glow, you can’t see the Milky Way, and you barely see any stars.

And that’s what the night sky look like now above most cities in the developed world. Many who live there have never seen the Milky Way. During a major black out in Los Angeles in 1994, the police received numerous calls because people worried about this weird cloud in the night sky.

Alright, you may say, but most people have seen the Milky Way at some point. Yet few have seen the Zodiacal light or even know what it is. The Zodiacal light comes from sunlight that reflects off dust which floats around in the Solar System. The dust is probably mostly from Mars. The Zodiacal light, which you see in this photo, is sometimes called the “false dawn” because it can be mistaken for an upcoming sunrise. Fun fact: Brian May, lead guitarist of the band Queen, did his PhD in Astronomy about the Zodiacal light.

Light pollution is now incredibly common in the developed world. According to a paper from the Light Pollution Science and Technology Institute in Italy, by 2020, 60% of the population in Europe can’t see the Milky Way. In the USA that percentage is now at 78%. Light pollution is also the reason that astronomical observatories now must be located in isolated deserts, like the ones in northern Chile, or on island in the middle of the ocean, like Hawaii or the Canary Islands.

Okay, you may say, too bad for astronomers, but who really cares. Most of us can live very well without seeing the Milky Way. Indeed, but light pollution doesn’t just obscure our view of the night sky. Too much light, or the wrong kind of light, affects our circadian rhythm, our inner clock that regulates biological functions.

The circadian rhythm strongly relies on the input we receive from a certain type of photoreceptor in our eyes. That’s not the cones and rods, which are responsible for day and night vision, respectively. It’s a third type of photoreceptor that was only discovered in the 1990’s, called the intrinsically photosensitive retinal ganglion cell (ipRGC). It’s not involved in vision itself. Instead, it allows the body to use light as input to set the clock for the circadian rhythm. It is particularly sensitive to light at the blue end of the spectrum.

And if you mess with the input on those photoreceptors, the whole system gets messed up. According to a 2016 report from the American Medical Association scientists have found links between altered circadian rhythms and insomnia, depression, dementia, diabetes, heart disease and even cancer.

Now, that scientists have found links between this and that doesn’t always mean much, but some of those studies leave little doubt that light pollution has a major impact on our quality of life, and our health. For example, in a 2016 paper, researchers from Stanford University and NASA interviewed over 15 thousand Americans about the amount and quality of their sleep. Then they looked for correlations with nighttime light levels, using the GPS coordinates of the respondent’s homes and light data from satellites.

They found that living in areas with greater outdoor lights at night was associated with delayed bedtime and wake-up time, and increased daytime sleepiness. It also increased the dissatisfaction with sleep quantity and quality, all with a p-value smaller than 0.0001. That still wouldn’t count as a discovery in particle physics, but for medicine that’s an amazingly strong correlation.

In 2017, another American team of researchers published the results from a study that tracked the health of 100 thousand nurses for 22 years. They found that those who lived in places with more light at night were at an increased risk to develop breast cancer, even after accounting for individual and area risk. It was not a tremendously big increase, just 5% at 95% confidence level, but it wasn’t the first such finding.

A 2008 study in Israel had also found a link between light pollution and breast cancer. And a 2019 study in Spain, which followed several thousand people over 5 years, found that exposure to light pollution in the blue part of the spectrum specifically was associated with an increased risk of breast (+47%) and prostate cancer (+100%). It’s not that the light itself causes cancer, but rather, scientists think it’s that light pollution, by affecting the circadian rhythm, alters hormone levels, and the link between hormone levels and cancer risk is well established.

So too much light at night isn’t good for us, but it’s even worse for animals, especially birds. According to the United States Fish and Wildlife Service, light pollution kills between 5 and 50 million birds each year in North America alone. The major problem is that the glow above cities makes the birds think that sunrise is near, so they don’t get enough sleep, are chronically exhausted, and fall victim to predators or illness more easily.

Another animal that suffers from light pollution are sea turtles. When the baby turtles hatch on illuminated beaches, they walk towards the lights rather than towards the sea. There are also animals which hunt at night, like owls, which starve simply because their prey sees them coming.

And light pollution is constantly getting worse. A 2017 paper from researchers in Europe and America found that the area lighted at night increases by 2.2% per year worldwide, and in places where light pollution was already present its brightness increases also by 2.2% per year.

Okay, so we have seen that light pollution negatively affects health and quality of life, it isn’t good for animals either, and it’s getting worse. Now, the world certainly has larger problems than that, but on balance light pollution is fairly easy to fix with a little advance planning. That basically means, cities must watch out what lights they install.

In many places the old, yellowish light bulbs are now being replaced with modern white LEDs. They are cheap, energy efficient, and last long. Their light emission is also much more focused, so they can be directed downward, which reduces the upward glow. That’s better for birds and star lovers. So far, so good.

But unfortunately, the new LEDs also emit much more light in the blue spectrum. And according to the report from the American Medical Association which I mentioned previously, these blue-rich LED street lights appear to be five times more disruptive to our sleep cycle than the old street lights.

It would be better to use LEDs that emit more on the yellow end, such as narrow band amber LED (NAB-LED). Their emission spectrum is also, as the name says, fairly narrow, so it can be more easily filtered out of images which makes astronomers happy. At present these LEDs are much more expensive than the blue-white ones, so this is not a common choice. As so often, we’ll have to decide whether the improvement in life quality is worth the money.

Now what’s with Elon Musk’s Starlink satellites? While those also cause light pollution in some sense, they are really an entirely different problem. Musk’s company now has about 1900 of those satellites in orbit. They are aiming for a licence to make that as much as 30,000. These satellites provide worldwide internet access which covers literally the entire globe. But astronomers hate them.

The Starlink satellites are fairly small but they fly low, at “just” 340 kilometers of height. They can be seen like the moon because they reflect sunlight. And because they fly so low they are brighter than the satellites higher up. The biggest problem with the Starlink satellites however is that they can autonomously change their orbits, so astronomers can’t schedule observations to avoid them.

For example, in November 2019 astronomers at the Cerro Tololo International Observatory in Chile were capturing an image of the night sky. Or at least they were trying. That’s what came out. The satellites are so bright that they even show up in the observatory webcam!

Elon Musk isn’t the only one who wants to occupy low orbits, he’s just the first one. There’s also Amazon Kuiper, Samsung, OneWeb, India Astrotech, and two dozen more companies. In the long run we might end up with as much as 100000 satellites. Connie Walker from the International Astronomical Union said in a recent interview with the BBC: "By the end of a decade, more than 5,000 satellites will be above the horizon at any given time at a typical dark-sky observatory location. A few 100 to several 1,000 of these satellites will be illuminated by the Sun.”

In 2020 researchers from the European Southern Observatory published a paper in which they estimate the impact of these new satellite fleets on their astronomical observations in the visible and infrared. The satellites are a real problem in the two hours before and after sunrise and, as you expect, are more of a problem for images that capture a large part of the sky than for small parts. ESO estimates that between 1 and 40% of images will be affected during the first and last hours of the night.

Another observatory, the Zwicky Transient Observatory in Southern California, reported that already in late 2020 about 6% of their images were affected. By August 2021 the share of affected images had increased to 18%, and they expect that by the time Starlink reaches 10,000 satellites all their images will contain trails from the satellites.

Last month, the International Astronomical Union announced they’d founded the Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference. Its purpose is to lobby for legislation that makes the low earth orbit satellites less disruptive for astronomy. 

I am really torn on the issue. On the one hand I think that the global internet coverage which the satellites may bring is a blessing for many poor and remote regions of the planet. On the other hand, private companies should be a little more respectful to the scientific cultural good of astronomy. What do you think? Let me know in the comments.