The world is in a precarious position at the moment. Our planet is experiencing a climate crisis. Scientists expect that its effects will become more plain in coming decades. In previous posts here, we explored how outdoor lighting and light pollution interact with environmental concerns. That includes topics such as climate change, corporate social responsibility, and the "Rights Of Nature".

The laws in many countries mandate the use of outdoor lighting because it serves the public interest. Lighting has demonstrated safety benefits and saves lives. But when lighting becomes excessive, light pollution can cause its own problems. As the world transitions to renewable energy, this can create new challenges. This post explores concerns on both sides of the issue and asks whether competing interests can be balanced in reasonable and effective ways.

Artificial light at night (ALAN) has several connections to environmental concerns. For one, lighting consumes electrical power. That has a significant climate impact via the burning of fossil fuels. Also, ALAN exerts pressure on species already experiencing environmental stress. That stress comes not only from climate change, but also from sources like habitat and biodiversity loss. Energy-efficient lighting like LED seems at first like a solution the first problem. But it can also contribute to making the second problem worse.

Its lower cost of operation has led to rapid adoption of LED technology in the past two decades. With this has come overconsumption of outdoor ALAN. Outdoor lighting is now perceived to be cheap and free of environmental consequences. There is evidence to suggest that had led to the installation of a lot of new and unnecessary lighting. It may have also eliminated any overall environmental benefit from the transition to LED. And it means more light in the nighttime environment, yielding further ecological stress.

In concept, the simplest solution to the climate crisis is to reduce humanity's energy consumption. But our recent history suggests that we're not going to do that. Instead, economists expect energy demand to grow coming years and decades. If so, the next-best approach is to 'decarbonize' the global energy generation portfolio. That requires finding sources of energy that aren't limited to an exhaustible fuel supply.

"Renewable" energy sources like solar, wind and geothermal harness the power of forces of nature. These forces are at work around us all the time, their energy is "free" and it yields no polluting byproducts.  We have used some, like the wind, to power our transportation for thousand of years. Yet until recently, we couldn't move that energy from where we captured it to where we wanted to use it.
Making the best use of renewables now involves constructing both facilities to collect the energy and the infrastructure needed to move it. For this reason, people who live in areas near renewable projects often oppose their construction. Often there are real concerns about the environmental impact of these facilities. When faced with the task of evaluating applications to allow them, governments have to make difficult choices. Sometimes they conclude that the benefits involved outweigh any adverse local impact. Those impacts can include light pollution.

There are certain "speciality" lighting applications that must evaluated in their own contexts. Lighting for specific public safety applications is among those applications. Wind farms, whether built on land or in bodies of water, are a particular concern.  Wind turbines are obstacles for both pilots and sailors in both daytime and nighttime. That's true whether they're in active operation or not. Countries have set rules for lighting windmills at night to reduce the chance of accidents. These rules often supersede other concerns in environmental impact assessments.

The principle underlying these rules is that certain safety concerns cannot be mitigated. As a necessary condition of permitting wind farms, they are seen as acceptable risks. These considerations have led governments to deny permission to build some projects. In other cases, the perceived benefits are too important not to permit renewable energy projects.

Neighbors often oppose wind projects on aesthetic grounds. Their concerns about property values are often labeled NIMBYism ("Not In My Backyard"). Yet there are also real concerns about the environmental impact of this lighting. Renewables projects are often built in remote locations. These are often ecologically sensitive places. Windmills can be hazardous to birds, for example, and in particular at night for migratory species. Very large wind projects involving hundreds or thousands of turbines could light up the night sky. This may happen even when the lighting employed is the smallest amount allowed by law.

So what is the real impact, and should we worry about it? Until recently there were essentially no scientific results to inform the debate. Salvador Bará (Universidade de Santiago de Compostela, Spain) and Raul Lima (Politécnico do Porto and the University of Coimbra, Portugal) just published the first significant work on this topic. They made a simple model of the visual impact of individual wind turbine lights. They used this model to compare their expected brightness to that of astronomical sources like bright stars and planets. It takes into account real-world influences like attenuation of light by the Earth's atmosphere .

Bará and Lima found that windmill lights can be very bright at short distances. Up to 4 kilometers away, medium-intensity turbine lights can exceed the apparent brightness of the planet Venus. After the Sun and Moon, it is often the brightest natural object in the night sky. At 10 km away, the lights can still be brighter than the brightest stars. They can remain visible to the unaided eye to distances as far as almost 40 km. The authors concluded that "the visual range of wind farms at nighttime may be significantly larger than at daytime, a factor that should be taken into account in environmental impact assessments".

​So far this does not address the environmental effects of these lights. The researchers also looked at the amount of illumination on the ground that the lights can cause. Except at very close range, the intensity is low. As a benchmark, the authors used the expected light intensity on the ground from a starry night sky in the absence of the Moon. They found that even the brightest beacon lights on turbines yield these conditions out to distances of only a few hundred meters. Within 5 km the intensity is almost completely negligible. That said, it could still be harmful to wildlife very close to the windmills, whether on land or at sea.

What about the night sky? "Skyglow, in absolute terms, should not be very intense far away from the poles," Bará tells us. "The luminous intensity of the standard lamps used here at nighttime is 2000 candela, and this would make less than about 24,000 lumens per beacon, something like a couple of streetlights." Of course, if designers packed turbines into wind farms, the collective effect might be detrimental to the night sky.

​In a paper published last year, Bará and colleagues considered the effect of skyglow immediately near streetlights. They found that the contribution to skyglow from an isolated, single street light is small and localized close to the light. Small wind farms may thus have little impact on the night sky in their surroundings. But that's less likely as installations grow larger.

What can be done about all this? Clearly there is a climate need to shift energy production away from fossil fuels and toward renewables. Almost one-third of global electricity production now comes from renewables. That percentage is steadily increasing. Certain flavors of renewable energy, like solar, don't need facility lighting at night. But they're not suitable for every condition or situation. For example, electricity generated from solar cells drops to zero at night. To reduce uncertainties, experts recommend diversifying investments in types of renewables. This ensures that no one technology dominates in an area.

To the extent that wind energy is a good option in many parts of the world, there remains the question of where to put it. The social unacceptability of wind farms near populated places is unlikely to change. That leaves locations that may be especially sensitive from an ecological perspective. At the same time, transportation safety concerns will persist and even grow.

Technology may be able to help. For example, some wind farms are now equipped with Aircraft Detection Lighting Systems (ADLS). These systems sense the approach of aircraft within a predefined volume of space around and above wind farms by using low-powered radar. When an aircraft enters a predefined detection zone, the ADLS switches on bright lighting to make windmills visible to the pilot. Once the aircraft exits the detection zone, ADLS turns off the lighting automatically. ADLS can improve situational awareness of obstructions like windmills even in challenging weather. It is sometimes deployed in conjunction with Enhanced Vision Systems (EVS) equipment. EVS combines views from infrared and visible-light cameras to help pilots see through fog, snow, and other low-visibility conditions.

At the end of the day, we can't have it all. So we have to make accommodations. Situations arise where even our best technology can't fully mitigate dangers associated with wind energy projects. But where to locate these facilities remains a choice. Decisions should consider ecological sensitivity in relation to the amount of traffic and the need for lighting. In other words, it's a problem of many variables that has more than one solution. Sometimes the answer is to not build the project at all.

Dark-sky advocates have long wrestled with such tradeoffs. It is often the case that the most "wildlife friendly light" is the one that is never installed to begin with. And yet, better lighting practices exist in the real world alongside complex social and political influences. Lighting for safety remains among the most complex of them all. As the climate crisis becomes more acute, we are again confronted with often difficult choices. The right balance between competing risks serves both the interests of nature and society. That balance leads in a direction toward a world that is more sustainable in all its aspects.

The Sun is the nearest star, a slow-spinning globe of plasma that powers almost all life on Earth. But it's not only a passive radiator of energy that warms our planet. Rather, it's an active and dynamic system that reaches all the way out to us across almost 150 million kilometers. As its substance reaches the Earth, complex interactions yield various effects on the planetary environment. Among those is an influence on the brightness and appearance of the night sky we're still struggling to understand. The emerging view is that "space weather" determines what the night sky looks like in the absence of light pollution. And that turns out to be important in defining what a "dark sky" is. As we approach the peak of the current solar cycle, here we dig into the ways the Sun changes our experience of night.

Nuclear reactions taking place deep in its core power the Sun. Protons fuse to make helium nuclei at temperatures approaching 15 millions of degrees. These fusion reactions release light, which takes about a million years to work its way up to the thin edges of the Sun's atmosphere. Along the way there are a lot of electrically charged particles running around. The rotation of the Sun drags the particles along. In turn, that generates a magnetic field. And that's where things get interesting.

A simple view of this magnetic field is like a simple bar magnet, with a "north pole" and a "south pole". On large scales, the field is weak; a typical refrigerator magnet is about ten times more intense. If that were true everywhere on the Sun, not much in the way of interesting phenomena would ever happen. Yet the local strength of the magnetic field can be much higher.

​The lines of the global magnetic field twist as they wrap around the interior of the Sun. Much like winding up a rubber band, the field lines strain under the tension and begin to kink. Some of these contortions emerge from the visible surface of the Sun. We see these protuberances as sunspots. Eventually the contortions burst under pressure, producing what we see as solar "flares". Coronal mass ejections (CMEs) sometimes follow flares. These events release incredible amounts of energy and hot, charged particles into space.

A few years after this process starts up, it takes about as long to quiet down again. Strong magnetic fields almost disappear at the surface, sunspots disappear, and flares end. The Sun remains quiet for some time. Then the process starts all over again. A complete cycle takes about 11 years to repeat, and we have seen it repeat with near-perfect reliability for at least four centuries.

We are now near the maximum of this cycle, the 25th such event since astronomers began counting in the 1700s. This cycle has an intensity like those over much of the past 250 years. Notable outbursts associated with CMEs have occurred in recent months. In May 2024, millions of people around the world saw auroral displays during a strong solar "storm".  Such events are likely through at least 2025.

The world recently watched the spectacle of the 2024 Olympic games. Assembling the world's best athletes once every four years sets the Games apart from many other athletic competitions. The Olympic flame extinguished, another long period commences before they meet again in another world city.
​
The world astronomy community has its own version in the form of a similar gathering of greats with a long period between. The International Astronomical Union's General Assembly, held somewhere in the world every three years, is sometimes called the "Olympics of Astronomy". The events draw astronomers from all over the world for meetings where many would only ever encounter one another. Since dark skies were on the agenda at the latest edition, it's a good time to catch up here on efforts to protect astronomy for the benefit of future generations. 

The International Astronomical Union, or IAU, is the world's main professional body representing astronomers. Founded in the wake of the First World War, it now represents over 12,000 people in astronomy and related fields. Their professional predecessors were among the first to sound the alarm about light pollution. Astronomers made some of the first descriptions of its effects on the night sky. The earliest accounts come from the 19th century in the era of gas lighting. By the turn of the 20th century, electric light quickly became a new scourge. Astronomical observatories moved out of the capitals of Europe for more rural locations. Some viewed this as the price of progress while wondering whether the price was too high.
​
The scientific study of light pollution began in the 1960s and 1970s, and again astronomers led the way. One of the first scholarly mentions of light pollution was a little over 50 years ago in the pages of the journal Science. Astronomers worked to understand the influences that had been brightening skies over observatories for decades. And they put energy into changing public policies in and around observatory sites. For instance, the city of Flagstaff, Arizona, enacted what may be the world's first outdoor lighting law in 1958. Nearby Lowell Observatory played no small part in that effort.

IAU was a little late to the party in recognizing the seriousness of light pollution. It contributed to the the Starlight Declaration of 2007, which called access to dark skies “a fundamental socio-cultural and environmental right”. At its 27th General Assembly in 2009, the IAU adopted a resolution on light pollution. It urged its members to work to reduce light pollution from the local to international levels. And in 2020-21, it helped arrange two international workshops on the subject. The events addressed the connection between light pollution and cultural heritage, "dark sky oases", astrotourism, and the bio-environment.
​
The IAU has also built dark skies into its formal activities. In 1973 it set up Commission 50, dedicated to "Protection of Existing & Potential Observatory Sites". In 2018 it became Inter-Division B-C Commission B7, reflecting the duel influences of Commissions B ("Facilities, Technologies and Data Science") and C ("Education, Outreach and Heritage"). In the early 2020's IAU began to realize the threat posed to astronomy by large satellite constellations, which we previously wrote about here. In response it established the Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference (CPS).

As athletes were competing in Paris, astronomers from 107 countries gathered in Cape Town, South Africa, for the 2024 General Assembly. Besides dedicated sessions about light pollution and "Dark and Quiet Skies", these themes recurred in other programming during the two weeks of the General Assembly. Each of the IAU Offices (for Development, Outreach and Education) included aspects of these topics in their offerings.

CPS hosted a session early in the General Assembly dedicated to progress on the satellite issue. Richard Green, Interim CPS Director, opened the session with a summary of the status quo and developments since establishment of the Centre. Each of the four CPS "hubs", or sections, put on presentations during the daylong session. This was further supported by a poster session with a variety of results across CPS technical, policy and community engagement efforts. The session concluded with the perspectives of figures in the commercial space industry who described their actions taken by companies to reduce the impact of satellites on astronomy.

Some common themes emerged. CPS touted its achievements in encouraging the industry to find creative solutions to mitigate potential harms to the night sky. CPS leaders called for more dialog and engagement with industry as the way to get the best outcomes. Technical presentations evaluated mitigations attempted to date. And space policy experts examined possibilities for affecting change in a challenging global regulatory climate. In particular, they pointed to the gradual emergence of best management practices among industry participants. These informal or "soft" approaches may have the best chance of success.
​
At the same time, other presenters took a more skeptical point of view, criticizing elements of the engagement between astronomers and the industry. They pointed out inadequate attention to the issue among many astronomers. Lively exchanges between participants pulled at all the threads involved: legal, technical, and commercial. It was clear that the players are still far from agreement on some points. It's also the case that there is much we don't know about the broader problem. That includes the "carrying capacity" of orbital space and what effects re-entering satellites will have on Earth's upper atmosphere. Yet all agree that more satellites will be in orbit in the future, and we still lack the means of ensuring the sustainable development of space. ​

Later in the General Assembly dedicated sessions on light pollution took place. Inter-Division B-C Commission B7 held a business meeting to discuss strategy in the next 'triennium', or three year period of IAU activities. Its leadership acknowledged that much of the Commission's attention focused on the satellite problem in the previous triennium. It aimed to sketch out the major pieces of a strategic plan for 2024-27. Freeform discussion among the meeting attendees followed.

Many ideas came up: opportunities to inform the framing of national light pollution legislation; better quantifying the cost to astronomy from light pollution; and standardizing the ways we measure and report light pollution impacts on astronomy. Those involved are also struggling with the degree to which their advocacy should directly address environmental concerns.

Supporters see that strategy as one that may prompt people to care about light pollution in ways that astronomy alone won't. The IAU itself is changing. It now often looks outward and engages with society beyond the community of professional astronomers alone. To do so calls for them to avoid excluding audiences from certain spaces because astronomers are "the experts".

On the last day of the General Assembly, the IAU Executive Committee Working Group on Dark & Quiet Sky Protection convened another daylong session. The format was again a mix of presentations, posters and freely flowing discussion among participants. Nearly all agreed on the need to broaden the appeal of dark night skies and the protections from light pollution they need.

The development of astrotourism, a form of sustainable tourism oriented toward night-sky viewing, is seen as a key element in protecting more places in the world. The need for well-crafted and implemented outdoor lighting policies is acute. But several speakers also noted that people most affected by those policies should be consulted as a matter of basic democratic principles.

Lastly, the session looked beyond both our home planet and the wavelengths of light we can see with our eyes alone. The quiet part of "Dark and Quiet Skies" refers to radio frequency interference (RFI). This is the equal of light pollution in the radio part of the electromagnetic spectrum. Radio astronomers continue to suffer interference from diverse sources of artificial radio energy on the ground. But now they also find their observations under attack from above. Yet ways to focus the public's attention on RFI threats to radio astronomy remains elusive. Many people are simply unaware of the existence of radio astronomy, much less of RFI.
​
Many of these threats to astronomy may play out again as humanity establishes a permanent presence in the cosmos. Plans to commercially develop the Moon, for example, are ramping up fast. Astronomers have long prized access to the Moon as the site of future telescopes with exquisite sensitivity. Such facilities could revolutionize our understanding of the universe. But they now face prospects like RFI from satellites in orbit around the Moon and damage from lunar dust kicked up by various activities. There is still a window of opportunity to protect the Moon from interference that would impact astronomy. The time remaining to protect the most vulnerable sites is running out. The IAU has established a Working Group on Astronomy on the Moon whose work is just beginning. ​

Astronomers were the vanguard in sounding the alarm about light pollution. And for good reason: it threatened the success of their enterprise. Now of course we have powerful space telescopes situated far above our planet. But we still rely on their ground-based counterparts to be the workhorses of research and discovery. And even the most remote of those facilities is under assault by light pollution.

There are many takeaways on the subject of dark (and quiet) skies from two weeks in Cape Town for this year's IAU General Assembly. It seems that many astronomers don't pay a lot of attention to the issue, if they ever did. While the dark skies sessions were well-attended, they faced competition from science sessions held on the same days and times. The General Assembly offers a jam-packed schedule that inevitably involves conflicts among parallel sessions vying for participants' attendance.

​To some astronomers we talked to, putting their attention on dark and quiet skies feels like work that they often assume someone else is (or should be) doing. They face their own struggles with inadequate research funding and an unstable labor market. That was all the more evident among attendees from developing economies that don't enjoy the comparative luxuries of their North American and European colleagues. It's not that they don't care about light pollution. But they rarely have time to think about it, much less to take action.

The satellite problem has thrown a new complication into the mix. Despite the efforts of organizations like CPS, it has even less recognition among astronomers than ground-based light pollution. Some wondered aloud why the community failed to expect the threat, which forced it to play defense from the beginning. Yet there is still hope that the IAU will lead on all these issues and to come out more forcefully in support (and defense) of astronomy and astronomers everywhere.
​
We're entering an era unprecedented in the history of astronomy. The biggest telescopes ever built will soon see first light. Machines like the James Webb Space Telescope are pushing the frontiers of discovery nearly to the origin of the universe itself. Computing power is cheap, allowing us to make very sophisticated models to understand the physics of the cosmos. And yet it is all threatened by forces from beyond and above. Astronomers still hold a place of special fascination and even reverence among the public. Will they use their status to rally that public in support of their science? The outcome will determine the very future of astronomy itself.

Scientists have had access to images of the Earth at night as seen from space for decades. But regular views of ALAN on our planet with instruments optimized for the purpose are still unavailable. Noam Levin (The University of Queensland & Hebrew University of Jerusalem) gave a plenary talk in which he summarized current satellite capabilities in this area. He also identified five key challenges that are holding back advance of remote sensing of ALAN:
​

1. The need for strong and reliable identification of clouds in nighttime images
2. Monitoring hourly changes in nighttime lights over a single night
3. Different times of the night in which satellites pass over a given place on Earth
4. Better estimates of blue light emissions
5. Understanding how ALAN is distributed close to the ground to explain what we see from orbit

Of those, Levin identified the blue-light issue as the most pressing.

Researchers spent considerable effort in the last decade on adding complexity to their models of light pollution. Newer modeling software takes into account real-world weather conditions. It follows ALAN further on its journey from source to target, through many orders of light scattering. And it now adds elements once unavailable. Stefan Wallner (University of Vienna) presented a new model including the polarization state of light. His group tested its output using a custom all-sky scanner that efficiently detects sky light polarization. This helps "ground truth" such models.

Brian Espey (Trinity College Dublin) showed a modified light pollution model that incorporates information about the size and height of obstacles in urban environments. Dense clusters of tall buildings, for example, create a "canyon" effect in satellite images of cities at night. Different measurements of light result depending on the angle of observation from orbit. He found that high-resolution obstruction modeling provides a way to calculate lighting conditions and estimate energy usage. This, he argued, can be a lever on decisions about how cities use outdoor lighting.
​
Models are also beginning to inform other areas of investigation besides night-sky brightness. Alexandre Simoneau (Cégep de Sherbrooke) showed a new version of his group's ILLUMINA modeling software. Users can apply this version to problems involving light originating outdoors that enters indoor spaces through windows. Coupled with direct measurements of this light flux, it represents an important advance in understanding how much nighttime light exposure people receive indoors. That, in turn, addresses concerns about the extent to which that light may affect their health and wellbeing.

Global outdoor lighting technology and practices have experienced a sea change in the past decade. The arrival of white LED lighting products on the market led to its current dominance. With this has come a shift in the colors of light that cities emit at night. White LEDs, rich in blue light, have displaced earlier technologies that emitted much less of it. As a consequence, some measurement methods assuming the color of the "old" light may not work as well as before.
​
In past years at LPTMM and other conferences, we heard from researchers who urged the light pollution research community to take heed. Adapting sensors and cameras to this new reality became clear. And our colleagues in the biological sciences were quick to remind us that measurement devices designed for the human visual system do not work well when applied to other species. Our sensitivity to different colors of light is just not the same as that of other animals. Gradually, researchers have come to make measurements in many colors besides those relevant to human vision.

Camille Labrousse (The Hebrew University of Jerusalem) studied the ongoing transition from legacy lighting sources to white LED. For this she used a four-color version of the popular TESS photometer (“TESS-4C”). She compared the TESS data with measurements using the LANCube device made by the Cégep de Sherbrooke. In turn, she compared both sources with remote sensing data from the Chinese SDGSAT-1 satellite. This helped her determine which kinds of light sources contributed the most light pollution in her study areas.

Iván Kopaitic (Pontificia Universidad Católica de Valparaíso, Chile) introduced BLUBO, a new device for measurement of skyglow in three colors. Kopaitic's team designed BLUBO to help with enforcement of Chile's national light pollution laws. It uses a photodiode and custom electronics to measure light at night in the blue part of the spectrum. He reported that tests of BLUBO are just beginning, but the early results are promising.

And the need to distinguish colors is not limited to ground-based measurements of light pollution. Alejandro Sánchez de Miguel (Universidad Complutense Madrid and the University of Exeter) described a new method of categorizing light sources in satellite remote sensing data. The process involves making color ratios of different sources and applying land-use maps to the satellite images. The results again underscore the value of color information: there is “significance of spectral diversity and spatial information in enhancing our understanding of light pollution and its sources.”

Over two decades of experience with portable, inexpensive measurement devices like the Sky Quality Meter (SQM) have transformed our understanding of light pollution. The SQM, and devices like it, provide long time series that make it easier to spot trends. As more such data become available, we gradually get a better handle on how light pollution is evolving around the world.

Christoph Goldmann (Natural History Museum Vienna) shared data from 15 years’ worth of SQM data from Upper Austria. Even up to distances of hundreds of kilometers, Vienna is the main source of skyglow in the region. Long-term monitoring efforts like this help direct resources toward mitigations that may improve conditions over time.

Li-Wei Hung (U.S. National Park Service) presented early results from a new all-sky camera system. This camera enables in a single shot what before took over an hour to produce with a more narrow-angle system. She explained how her team overcame limitations like optical distortion at the edge of the camera field of view. And as with the group's earlier system, she found they could calibrate their images by observing certain stars whose brightness is very well known.
​
Lastly, Christof Reinarz (Pontificia Universidad Católica de Valparaíso) showed the design and early results of a new sky brightness measuring device that has only one sensing element. The design uses a micromirror array that scans the sky light across the single-channel sensor. From this they can reconstruct "images" of the night sky using computer algorithms. The benefits of their design includes low cost, few moving parts, light sensitivity in important color ranges, and the ability to operate even in high ambient light levels.

Studies of light pollution have advanced far beyond their initial interest only among astronomers. Research now more often influences policy making, lighting technology development, and decisions about where, when and how to use ALAN. This includes bringing ideas to market. Andreas Jechow (Brandenburg University of Applied Sciences, Germany) shared recently published results of an effort in which researchers partnered with a European lighting manufacturer. Their goal was to design and test prototype lighting products whose light distributions are chosen to reduce insect attraction at night. They found that while reducing the illumination level caused little change in attraction, controlling the light distribution made a big difference. They saw insect attraction drop by up to a factor of 6 at their dark test site compared to the attraction rate to more "standard" luminaires.
​
In separate presentations, Andreas Hänel (Dark Sky Germany) and János Sztakovics (Eszterházy Károly Catholic University, Hungary) argued for the importance of characterizing naturally dark "reference sites" in Europe. This is important in the context of European laws and initiatives such as the EU Biodiversity Strategy for 2030 and EU Nature Restoration Law of 2024. As laws and norms increasingly call for not only the slowing of environmental pollution but also its restoration to unpolluted conditions, tactics are shifting. Dark "references" in Europe serve as important benchmarks to gauge whether policies are working or not.

Image credit: NASA's Goddard Space Flight Center  
1135 words / 5-minute read

Summary: Knowledge is power, but only if it's accessible. The "Artificial Light At Night: State Of The Science" report improves access to the knowledge in thousands of papers about light pollution and its effects. This post explains how the report was written and is updated each year, and it presents some research highlights from 2023.

Accessible scientific information about light pollution and its consequences is key to combating this growing environmental problem. But until recently there were few accessible summaries about artificial light at night (ALAN).

This prompted DarkSky International to publish the first "Artificial Light At Night: State Of The Science" (SOTS) report in 2022. SOTS considers the contents of thousands of published papers, theses and reports. It summarizes the current scientific consensus view on various aspects of ALAN. Written in non-technical language, it includes a comprehensive bibliography to support its content. 

​Each year, DarkSky International updates this report. Such regular updates help keep track of the hundreds of new studies published each year. We are excited to share State Of The Science 2024, which includes the most recent studies and papers published in 2023. 

SOTS is in many respects a 'summary of summaries'. The report authors considered almost 5,000 pieces of scientific literature listed in publicly available Artificial Light At Night Research Literature Database, or ‘ALANDB’.

Scientific literature about ALAN is growing quickly. Rising attention to the problem of light pollution has led to a rapid increase in the number of papers published on this topic. The chart below shows the number of papers published each year for the past 20 years that were included in ALANDB. In recent years, the database contents have grown by an average rate of 15 percent per year.

ALANDB contains a variety of kinds of literature, from journal papers to government reports. SOTS prefers peer-reviewed studies as sources. The scientific community holds this kind of material in the highest academic regard. But where appropriate, SOTS includes references to white papers, student theses, and other sources.

Each source added to ALANDB receives one or more of 22 top-level keywords describing the contents of the paper. These keywords help group papers into a set of seven broad categories: the night sky; human health; wildlife and ecology; public safety and crime; energy and climate; and social and environmental justice. In recent years, SOTS included papers about “space light pollution” from swarms of satellites orbiting the Earth. As the database entries are sorted, quality checks weed out sources that have methodology problems . Sources with the most significant results are highlighted for inclusion in the resulting report.

Using ALANDB as source material, the process of composing SOTS is like completing a jigsaw puzzle. Relationships among the topics become clearer as one looks at the 'big picture'. Drawing connections among topics, and sources within a topic, helps reveal underlying trends. We then better understand the state of light pollution science. But we also begin to see where the missing pieces are. Updating the report each year requires only looking at the “new” pieces and seeing where they may fit.

SOTS has several target audiences. One of them is the scientific research community. New researches to this field, in particular, enjoy an accessible introduction. Issue advocates become more knowledgeable about the facts supporting their cause. Policy makers better understand the issues they must decide. And the public finds an entry point to learn about a major social and environmental challenge.

2023 was an eventful year for light pollution research. In July, the journal Science for the first timefeatured light pollution on its cover, exactly 50 years afterthe first mention in its pages. The special issue featured five comprehensive review papers. Also, the Royal Society's Philosophical Transactions B publisheda 'themed' issue on ecological light pollution. Its papers "investigate light pollution ecology at various environments and scales, from single processes to whole communities, to better understand the relationship between light pollution, ecological balance, and human influence."

​​The 504 ALANDB sources published in 2023 that were considered for SOTS report break down by major keyword as shown in the pie chart below.

Fully 75% of the papers are about wildlife, remote sensing (of ALAN), or human health. They have been the leading topics for the past few years.

Certain key points emerge from the survey of papers published in 2023:

• Few studies break completely new ground, but scientific conclusions about light pollution grew stronger. Most studies simply add weight to previous conclusions.
• New remote sensing tools and techniques give unprecedented views of the Earth at night. This provides more granular data about the distribution of ALAN on land and through time.
• Understanding how light pollution affects global biodiversity is a top priority. In particular, the role of ALAN as it interacts with influences like climate change is becoming clear.
• Health researchers sounded the alarm about ways in which ALAN exposure affects older people. In particular, they found connections to neurodegenerative diseases like dementia and Alzheimer's.
• The nature of the relationship between ALAN and crime remains very unsettled. Past research on ALAN and crime incidence yielded ambiguous results. But newer work focuses on users of outdoor spaces and how lighting influences their perceptions of safety.
• Interest in how light pollution may influence air quality is rising. Some studies found further evidence that ALAN alters the nighttime chemistry of air, linking to poor air quality over cities. In turn, certain kinds of air pollution affect the formation of skyglow.
• The impacts of satellite 'megaconstellations' on the night sky are still poorly understood. Researchers now look at factors beyond only the brightness of individual satellites.

Recent research achievements mark a clear advance in our knowledge about light pollution. But many questions remain. SOTS concludes with a list of some of these questions, such as:

• How does ALAN contribute to species population decline or extinction?
• To what extent is outdoor ALAN harming human health and wellbeing?
• Of known light pollution mitigations, especially policy solutions, which are most effective?
• Which kinds of legal mechanisms may help solve the satellite problem?

We expect that researchers will wrestle with these and many other questions in 2024 and beyond. New kinds of light-sensing devices are becoming available, as well as new platforms on which to place them. Computer models of skyglow grow more sophisticated and their outputs more realistic. Possibilities for engaging citizen scientists in research abound. And a new generation of young scientists is enriching the field.

Yet it remains difficult in most cases to get adequate support for light pollution studies. It's a multidisciplinary subject, and many funding agencies don't know where to place it. Full funding of large-scale, complex projects is still unusual. This often leads to 'siloed' knowledge that may hold major advances back. But as the field gains mainstream recognition, it becomes more attractive to potential funders. Expect new breakthroughs as access to resources improves.

Want to learn more about the SOTS 2024 report? Have a look at this presentation from our own John Barentine to the DarkSky International advocate community introducing the report and reviewing its highlights: