When we look at the night sky, we can see fewer and fewer stars. This is due to light pollution, which has caused the night sky brightness to increase rapidly again in the last decade, as a citizen science project shows. To do this, people from almost 20,000 locations assessed the visibility of stars in the sky and thus provided data on sky brightness. The evaluation of this data showed that the brightening of the night sky in Europe and North America has increased by around 6.9 to 10.4 percent per year since 2011 - this is significantly more than previously assumed on the basis of satellite data. One reason for this could be the increasing conversion of lighting to LEDs.
While our ancestors could see thousands of stars and the glowing band of the Milky Way in the night sky with the naked eye, this is only possible in a few places on earth today. Because our artificial lighting is increasingly displacing the nocturnal darkness. Especially in densely populated regions, this light pollution means that it no longer gets really dark, even at night. The main reason for this is stray light from street lamps, illuminated industrial plants, neon signs and other man-made light sources. As early as 2016, a study found that around 80 percent of the world's population is affected by light pollution and experiences abnormally bright nights. This affects our health because it can disrupt the day-night rhythm and the release of the sleep hormone melatonin, for example. The consequences for many animals are even more serious, because artificial lighting disrupts their prey capture, migration, reproduction and many other behaviors.
Volunteers as sky gazers
The problem of light pollution has been known for a long time and measures to reduce stray light have already been introduced in many countries. For example, street lamps were equipped with screens to prevent them from emitting upwards, and many of the orange sodium vapor lamps were replaced by more economical LED lamps. However, whether this reduces light pollution from the diffuse "skyglow" is unclear - and has so far been difficult to measure. "The only satellites that can currently monitor the entire earth for scattered light have limited resolution and sensitivity and cannot detect light with wavelengths below 500 nanometers," explain Christopher Kyba from the German Research Center for Geosciences Potsdam (GFZ) and his colleagues . As a result, they miss the short-wave light of the white LEDs.
The citizen science project “Globe at Night”, which has been running since 2006, offers an alternative. In this project, volunteers observe the night sky and then use an app or online form to indicate which of eight star maps for their location best matches what they see in the sky. Each of these maps shows the sky at different levels of light pollution and with correspondingly more or less visible stars. "The contributions of the individual people work together like a global sensor network that enables us to take a completely new approach to research," says Kyba. For their study, he and his team have now evaluated the data from more than 51,000 such observations made under cloudless skies from 2011 to 2022. They represent 19,262 locations worldwide, most of them in North America and Europe. From this data and a global model, the researchers determined the brightness of the sky and how it has changed in recent years.
Greater increase in brightness than expected
The evaluations showed that the nocturnal scattered light has increased dramatically in the last ten years: In Europe, the diffuse sky brightness at night has increased by around 6.5 percent per year, in North America it is as much as 10.4 percent. The global average is around 9.6 percent annually. "This means that over the course of an 18-year childhood, the night sky brightens by a factor of four," explain Kyba and his colleagues. "If development continues in this way, a child born in a place with 250 visible stars will only be able to see 100 stars there by the time they are 18." Light pollution seen from the ground has increased significantly more than satellite data suggested. Because they had only determined an increase in brightness of around two percent.
The scientists suspect that the difference between human observation and satellite measurements is due to several factors. One is the direction of propagation of the scattered light: "Satellites are most sensitive to light that is directed upwards towards the sky," explains Kyba. "But it's horizontally emitted light that accounts for most of the sky's glow. So if advertisements and streetlights become more frequent, bigger or brighter, they could have a big impact without being adequately reflected in the satellite imagery.” A second factor is the switch from sodium vapor lamps to LEDs in street lighting. "Because human eyes are more sensitive to short-wavelength light at night, LEDs have a major impact on our perception of sky brightness," says Kyba. "Therefore, this could also be a reason for the discrepancy between satellite measurements and the Globe at Night observations."
The research team draws two main conclusions from their results: On the one hand, light pollution in the night sky has continued to deteriorate rapidly despite – or perhaps because of – the switch to LEDs. "On the other hand, we were able to demonstrate that the Citizen Science data represent an important addition to the previous measurement methods," says Kyba. However, the “Globe at Night” project still contains too little data from other regions of the world and above all from poorer countries. "If we had broader participation, we could identify trends for other continents and possibly even for individual states and cities," says co-author Constance Walker of the National Optical-Infrared Astronomy Research Laboratory in Arizona. "The project isn't finished yet, so be sure to check it out tonight and tell us what you see!"
Source: Christopher Kyba (German Research Center for Geosciences Potsdam (GFZ), Ruhr University Bochum) et al., Science, doi: 10.1126/science.abq7781