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.
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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.
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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.
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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.
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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.
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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.
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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:
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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.
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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.
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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.
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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.
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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  
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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:

"Water is life." Anyone who lives in an arid climate has heard this phrase. As the global climate crisis intensifies, more people are hearing it. It signals the value of a crucial natural resource threatened by overconsumption and pollution. Many years of water conservation efforts have shown the need for a unified approach. And careful collective management of water resources is a proven way of sustaining access to clean water supplies.

​What lessons can we learn from this kind of resource management that we may apply to conserving nighttime darkness? In this post, we dig into the idea of light pollution reduction through the management of "lightsheds". This approach has great untapped potential to surpass current shortcomings in the way we protect the night around the world.

We have previous written here about aspects of outdoor lighting pollution. In "Good/Better/Best Outdoor Lighting Policies" we examined the main elements of such policies. And in "Toward a 'Clean Night Skies Act': a roadmap for a U.S. national light pollution policy​" we presented a vision of a national outdoor lighting management strategy.

We find there are two main regulatory approaches: "supply side" and "demand side". Supply side limits the kinds of lighting that can be sold, installed, and/or operated in a jurisdiction. (Make bad lighting difficult to get and people mostly won't use it.) The demand side aims to change behavior by incentivizing the use of some kinds of lighting  through, e.g., more allowances. (Encourage the development of a market for better lighting products catering to consumer preferences.) Some policy approaches are a combination of these two that apply in "carrot-and-stick" fashion. A hybrid policy might restrict some popular kinds of lighting applications to reduce light pollution while permitting others. 

Another dichotomy in lighting policy involves when enforcement happens: "before the fact" and "after the fact". The former takes place before operation of the lighting. Examples include planning permission, comparison with standards, obtaining building permits, and post-construction inspection. Policies are enforced after the fact through citations issued to property owners whose lighting does not follow the law. 

Most lighting policies thus tend to prescriptive and/or proscriptive: they say what is and isn't allowed, and if lighting is allowed, how it is to be designed, installed and operated. But in many countries, as a practical matter, this is all still matter of local enforcement. Local authorities make decisions about how to interpret lighting policies and how (or even whether) to enforce the law. 

But often that doesn't happen. The public doesn't like being told what to do. It also dislikes state regulation of individual behaviors. The result is that property owners may simply disregard the law. Jurisdictions with laws on the books may be loathe to enforce the law, citing factors like inadequate staffing.

From time to time, other ideas about how to regulate outdoor lighting are tried. Some countries have very strong top-down policies: national legislation decrees artificial light at night (ALAN) to be environmental pollution subject to regulation and tasking lower jurisdictions with enforcement. 

An example of this is Mexico, which recently amended its General Law of Ecological Balance and Environmental Protection of 1988, also known as LGEEPA. The move attached light pollution to the existing LGEEPA, which makes its control obligatory to municipalities.

Where there is strong public support, this method can work well. But it is difficult to enact this kind of legislation in contentious political environments. And implementing jurisdictions may well ignore the central government's mandate. As a result, these efforts often produce only symbolic outcomes. 
Another idea stems from the fact that light pollution isn't restricted to any particular jurisdiction. It does not respect boundaries between municipalities, regions or nations. As a global problem, it might benefit from a global solution. This could involve a new international treaty that binds signatories to meet pollution reduction targets. 

Such was the goal of the spectacularly successful Montreal Protocol on Substances that Deplete the Ozone Layer (1987). 198 countries that ratified it agreed to phase out the production of industrial chemicals that historically caused a 'hole' in the Earth's ozone layer. Since it took legal effect, compliance with the Montreal Protocol has been high, and the ozone layer is repairing itself. 

This example proves that treaties can achieve important and meaningful results. But such solutions need buy-in from many countries. Developing economies may balk, making it difficult to achieve consensus. And the process of drafting and bringing a new treaty into force is often painfully slow.
These challenges prompt some to ask: what if we tried something new and thus far untested? One such new way of looking at outdoor lighting regulation turns many of the existing ideas on their heads. 

A lightshed is the territory around a given point containing all the sources that send light at night to that place. Some of that light is directly emitted from sources on the ground near the point, while other sources contribute indirect light scattered in the atmosphere.  Managing a lightshed to reduce light pollution in that place targets the source of the pollution in an outcome-based fashion.

Lightsheds are analogous to watersheds. The U.S. National Oceanic and Atmospheric Administration (NOAA) defines a watershed as "a land area that channels rainfall and snowmelt to creeks, streams, and rivers, and eventually to outflow points such as reservoirs, bays, and the ocean". It's built on the water cycle that carries water through the terrestrial environment from and back to large accumulation basins through an interface with the land. 

The management of watersheds identifies water as a shared resource that all entities and jurisdictions in the watershed must protect in order that they themselves — as well as others — can continue to enjoy it. The intent is to make the resource indefinitely renewable by ensuring that the "cash flow" in the system is always greater than zero. It attempts to avoid a so-called "tragedy of the commons" in which groups within the watershed consume all of the resource.

The analogy between water and light isn't perfect, of course. There are plenty of sources of light in the nighttime environment but also plenty of sinks. There isn't a "light cycle" that works like the more or less closed loop of the water cycle. And while there are both natural and artificial sources of LAN, but there's nothing like "artificial water". (An imperfect comparison is how humans draw water out of the watershed, depleting it.) In a watershed the conservation object is the resource (water). But in a lightshed the object is natural nighttime darkness. 

Lightshed management focuses on places where nighttime darkness still exists (at least to begin with). Managers determine the sources of ALAN arriving at a given location through computer simulations that use satellite data as model inputs. Associations of governments at different jurisdictional levels pledge to protect darkness in the target location through policy changes. The policies aim to limit and ultimately reduce light emissions in their territories. This could involve a strategy like "cap-and-trade": the influence of newly installed lighting is offset by the removal of unnecessary lighting elsewhere. Over time, the rate at which ALAN reaches the target area slows to zero and then turns negative.  

Another way to look at this concept is to think of light pollution is the byproduct of consumption of light at night for useful purposes. Dividing the light emissions in a region by its population yields a metric like "lumens per capita". The number in a certain area depends on many social variables. But in that area it provides a point of comparison as time passes under a lightshed management plan. 

​By reducing waste and improving outdoor lighting, an area's lumens per capita consumption can drop. Even as its population grows, light pollution can continue decreasing. Acting on the entire system rather than its components, as lighting policy typically does now, might turn out to be the winning strategy. Instead of blanket regulations and ill-defined targets, lightshed management targets the underlying sources of the problem for remediation. 

To be clear, there are many ways that lightshed management may fail. No one has (yet) tried lightshed management as a policy lever on the problem of light pollution, at least not at any meaningful scale. As with new international treaties, it may be very difficult to sign up all the actors in a lightshed to take part in the project. And if even one major light polluter doesn't sign up, the integrity of the project may be compromised. 

Still, conditions are ripe for a small-scale experiment. We have glimpses of what this could look like already. Large parks and protected areas participating in the International Dark Sky Places Program are trying something a lot like lightshed management. Large and complex land arrangements accredited by the program must devise Lighting Management Plans. This often involves obtaining participation from many stakeholders. When it works, we see real reductions in light pollution; for examples, see here and here.

The next big test is to identify a lightshed and set a quantitative goal for light pollution reduction. For the test to be realistic, it should encompass a large landscape with diverse land uses, including urban centers. Both public and private landowners should be involved in the experiment. 

If it is successful, the same stakeholders should take part in planning for how to maintain the improved conditions. They have a strong interest in doing do, whether for conserving local nocturnal wildlife, furthering astrotourism development, or for many other reasons.

Big, challenging problems call for bold new solutions. Light pollution is readily reversible, and when steps are taken to reduce it we see clear social and economic benefits. Now is the time for society to take a more measured and deliberate approach to preserving nighttime darkness. Lightshed management may be the solution that brings the meaningful global change that the problem deserves.

When people hear the word "pollution," they may think of many different things. One that comes to mind immediately is air pollution, which is easy to perceive because it is often visible to the eye. The familiar brown cloud over some cities where air pollution is prevalent is its major manifestation. But much of the problem of air pollution is invisible, as it constituents don't interact with light in ways that the human eye can see them.

Despite its name, light pollution has some similarities with air pollution in that sense. One of them is that many of its hazards are not immediately obvious to people. Not all its affects are visible to the eye in a direct way. And many people don't notice it at all, given that it is most evident when few people are awake to see it.

We often frame light pollution as an environmental problem in that it is a problem of the environment. In other words, it is a way in which human activity interacts with the environment to cause harm. But it is also a problem for the environment, in that it negatively affects the environment in particular ways.

We have previously written here about how light pollution relates to climate change. It does so through electricity consumption and carbon emission. But artificial light at night (ALAN) also interacts with the air itself, changing its chemistry. Those changes can reduce air quality and impact public health.

​In this post, we dissect the two ways in which light pollution and air pollution interact. And we close with an argument for why dealing with one problem can help solve the other (and vice versa).

Scientists realized many years ago that poor air quality, especially in cities, can lead to adverse health outcomes. In particular, small particles suspended in the air, called aerosols, have harmful effects on human health.

Very fine particles known as "PM2.5" are so small that they can travel deep into the human respiratory tract. There they reach the lungs, embedding themselves in tissue and causing both short- and long-term health effects. The dominant source of PM2.5 in cities is exhaust from the internal combustion engines in motor vehicles. But they also come from burning wood, natural gas and other carbon fuels, and fires.

We also know that aerosol air pollution has a strong interaction with local weather conditions. When air over cities becomes stagnant and does not move, the pollution can settle in for many days at a time. This is especially true in winter when temperature inversions in the air may occur. Only when the winds change and blow the pollution away does the air quality tend to improve.

Efforts to write laws intended to reduce air pollution began in the 1950s. Where governments implemented such policies, they have been spectacularly successful. Despite these outcomes, air quality measures still have many opponents. The main objection they raise has to do with the cost of the regulations. As a result, one-third of the world's countries still have no laws regulating air quality. Over eight million people worldwide die each year from complications due to air pollution exposure. [1]

​Beyond reducing this public health burden, there are other reasons to reduce air pollution. One of them is that doing so would also reduce light pollution.

Skyglow is a phenomenon involving ALAN emitted on the ground. It travels upward into the sky, where it interacts with molecules and aerosols in the atmosphere. These interactions direct some light rays back down toward the ground. Observers looking toward the sky see some of this light. It competes with the light of the stars beyond, making it difficult to see them. This is the familiar experience for anyone who has lived in or visited a city where there are a few stars in the night sky.

The same aerosols responsible for air pollution also affect this light. The smaller particles tend to interact with light more strongly than larger particles. The effect is stronger for light that is more blue compared to other colors. This is the same reason why the daytime sky is blue and not some other color.

Scientific research has shown that the interaction between ground light and aerosols can make the night sky brighter. In places where air pollution is prevalent, this effect is stronger. [2] The connection between them is solid. Researchers now use nighttime light measurements from space to estimate how much PM2.5 is in the air. [3]

In 2021, we hypothesized that reducing air pollution over cities would also reduce night-sky brightness. We expected that the effect would persist even when all other influences on skyglow were fixed. To test this idea, we measured the sky brightness over a city before and after the passage of a weather front. These moving airmasses tend to clear out air pollutants where they collect under usual conditions.

​The experiment validated our hypothesis. [4] We saw reductions in the brightness of the night sky by up to a factor of 2.5 as the frontal boundary passed. No other explanation best accounts for the measurements than clearing the air of aerosol pollutants. The result also matches observations from other measurement sites where air pollution is significant. From this result we argued that, after improving outdoor lighting, reducing air pollution could be the next-best way to lower skyglow. 

That would be the end of the story if we were only concerned about skyglow. But there is another way in which air and light pollution are intimately related. If on the one hand we talk about tackling light pollution by clearing the air, on the other it may be that reducing light pollution can itself clear the air.
Some air pollution results from the direct action of burning fuels. For example, sometimes motor vehicles will emit visibly dark clouds from their tailpipes. Many people have in mind that this kind of scene is what air quality laws have in mind to prevent.

But some materials emitted by vehicles and through other activities do not immediately become "pollution". Rather, they dwell in the atmosphere for hours to years. Some of these chemicals react with sunlight or other compounds in the atmosphere to form visible hazes. Natural processes in the air can "clean" certain pollutants by reacting with them chemically.

One of those is the nitrate radical (NO3). Nitrate reacts with volatile organic materials that would otherwise become ozone molecules and smog. But it only works at night because ultraviolet light from the Sun in the daytime destroys it. In 2010, Harald Stark and his collaborators wondered whether ALAN could play a similar role. They set up a clever experiment to find out.

In Stark's experiment, the researchers outfitted a small airplane with sensors capable of detecting both nitrate radicals and ozone, as well as ALAN. They flew the plane over Los Angeles, California, at night. Then they flew the plane over the Pacific Ocean and compared the measurements with those from the city. The nitrate concentrations showed distinctly different patterns over the city versus the ocean.

From this the team concluded that city lights were interfering with the nitrate radical's nighttime work. [5] Although the effect was small — only 4% — the reliability of the measurements was high. Since then, other research has pointed toward ALAN's influence on harmful ozone [6] and nitrogen dioxide [7] levels.

​It's important to note the limitations of these studies. They have so far been carried out on relatively small scales, and they find small effects. But as city lights get brighter around the world, the significance of the issue is expected to grow. So it makes sense to consider which solutions are most effective.

In 2021 we wrote that "cleaner air not only has obvious public health benefits, but it could further reduce diffuse artificial light in the night sky and improve astronomical viewing after the utility of other methods such as lighting changes has been exhausted." [4] Connecting the two in this way might be a means for dealing with both issues at once.

Although cleaner air is the goal that resonates most with the public, "clean" night skies have some appeal. Dealing with the main sources of air pollution is difficult, but we may find it easier to score some wins around the edges of the challenge.

The best policy approaches for reducing ALAN's influence on air pollution have to do with timing. Much of the chemical mechanism involved in scrubbing pollutants from the air works during the overnight hours. Limiting the times in which outdoor lighting is used through curfews is an important tool. Another is insisting on full shielding of all lighting. This ensures that the direct emission of light into the night sky is limited or eliminated. The third leg of the stool is limiting total light emissions. Given that much of the light pollution we see in the world results from over-lighting relative to needs, we expect these methods to work well.

​Light pollution is perhaps the only environmental problem of our time that is fully reversible. What's more, we know that the solutions are cost-effective and produce the anticipated results when implemented correctly and consistently. Solving the problem might also have the added benefit of tackling air pollution in and near urban areas. This not only adds to the quality of life in cities, but it does so without any obvious downsides. It's an easy win that the world desperately needs.