The world at night is changing rapidly. In 2023, scientists estimated that the night sky increased in brightness at a global average rate of 10% per year since 2011. Since night-sky brightness is determined by the amount of light on the ground, these results imply more artificial light at night in the environment. The resulting light pollution is now a worldwide problem with few parts of our planet untouched. And it affects much more than whether we can see the stars at night.

We have written here before about the complexities associated with measuring light pollution. We have global views of 'nighttime lights' from Earth-orbiting satellites. But their data are sometimes at odds with reports from ground-based observers. For example, despite the figure quoted above, a landmark 2017 study based on satellite measurements found a much slower rate of change. Can we reconcile these ways of knowing our planet at night?

​Satellite data from 2012-2016 indicated that during that time the radiance, or light intensity, of sources on the ground increased globally at an average rate of about 2% per year. That result relied on measurements made by the U.S. National Oceanic and Atmospheric Administration's Visible Infrared Imaging Radiometer Suite (VIIRS). This workhorse instrument flies aboard several NOAA satellites and sees our entire planet once a day.

The VIIRS "Day-Night Band" (DNB) detects sources of light on the night side of the Earth. But it wasn't designed for that application. As a result, it is insensitive to the blue wavelengths of light emitted in quantity by modern white light-emitting diode (LED) lighting technology. The outcome is that the VIIRS-DNB undercounts the light from newer installations of outdoor lighting compared to that from technologies that preceded LED.

Meanwhile, the Globe At Night program has for almost 20 years collected observations of night-sky brightness from citizen-scientists around the world. It asks participants to estimate the brightness by counting stars in particular constellations. Research has shown that the accuracy of these observations when considered in total is high, and thus they are scientifically valid.

Unlike the VIIRS-DNB, the human eye is sensitive to the blue light emitted by white LED lighting products. Scientists expected that a shift in the colors of light sources would result in a perceived change in the brightness of the night sky. Yet the 10% per year sky-brightness increase still came as a surprise. It suggested that the night sky was getting brighter at a rate few imagined to be so high.

How can this evident tension between satellite radiance and ground-based visual observations resolved? In a recent paper, Spanish professors Salvador Bará (University of Santiago de Compostela) José Castro-Torres (University of Granada) suggested a way forward. Their investigation found that "these different results are not incompatible". They offered an explanation having to do with the colors of the light sources on the ground. That includes light seen directly by satellites and indirectly by human observers as skyglow. And it appears that indeed, ground-based observers are sensing light that the VIIRS-DNB misses.

In particular, the transition from legacy lighting sources to white LED seems to be the cause. But it's not just because VIIRS-DNB misses some of the light that LEDs emit. There is reason to believe that there is more light in the nighttime environment from LEDs than in earlier times. That remains true even after adjusting for population changes.

Chris Kyba (GFZ Helmholtz Centre for Geosciences, Germany) and coworkers found that during the five-year period of their study, published in 2017, the rate at which countries' light emissions increased roughly matched the rate at which their economic output rose. The result implied that countries were emitting more light than before in part because that light became cheaper to consume. That made their observation "inconsistent with the hypothesis of large reductions in global energy consumption for outdoor lighting" because of the introduction of LED.

But Bará and Castro-Torres argue that the change in night-sky brightness could have gone either way. It could "potentially either increase as a result of increased atmospheric scattering of blue light or decrease as a result of improved lighting fixtures that reduce horizontal emission."

These are two competing effects. On one hand, blue light scatters more strongly during its flight through the atmosphere than other colors. That could make the sky apparently brighter at night even if the quantity of light emitted on the ground didn't change.

But we also know that light emitted at shallow upward angles contributes significantly to skyglow. During the transition to LED, many new light fittings were installed that feature fully shielded designs. These decrease light emissions at all upward angles, including those at small angles. But even light directed toward the ground can still end up in the night sky after reflecting from various surfaces. It appears that the net result of all these influences is a real increase in night-sky brightness.

One way to know for sure is to get a better handle on the connection between known light emissions on the ground and what the satellites see. We could then learn not only which kinds of sources contribute the most light that ends up in the night sky, but also the times of night when they are doing so. A group of scientists (and citizen-scientists) based in Germany recently did exactly that.

The Nachtlichter ("NightLights") team wrote a mobile device app to guide volunteers through the process of collecting basic information about outdoor lights they saw while walking defined transects through cities. Participants counted almost a quarter-million lights over a combined area of about 22 square kilometers. The recently published results offer great insight into the relationship between the number of lights on the ground and the resulting light emissions measured from space.

Nachtlichter also provided some support for the idea articulated by Bará and Castro-Torres that some of the light not detected by satellites is due to unfavorable geometry. While it is straightforward to model the effects of street lighting, other kinds of outdoor lighting are not as easy to understand. "Some of these lighting applications, such as decorative and advertising lighting, produce a larger fraction of horizontally propagating light than modern street lighting does," they wrote. These are sources of light emitted at near-horizontal angles that is so important to skyglow formation. Yet satellite detectors can undercount that light. "It is therefore likely that some of the differences between the rates of change for skyglow that we calculate and those estimated from satellite data arise from changes in lighting practices or deployment."

Bará and Castro-Torres asked whether the ongoing transition to LED, in changing the color of outdoor lighting, might account for the differences between the DNB and Globe At Night results. Their model took into account the physics of light scattering both in the atmosphere as well as inside the eyes of observers on the ground. In a previous paper, they found the latter is an important influence on the perceived brightness of individual light sources. And they speculated that nearby sources of light on the ground might bias the visual observations of night-sky brightness. The result could be inconsistent with satellite measurements.

They found that differences between the Globe At Night reports and DNB data seemed to disappear when observers were dark adapted. That is, their sensitivity to faint light increased by spending time in the dark before making their observations. It implied that there were no nearby, bright sources of outdoor light. To explain the observations "requires the existence of additional light sources that affect the Globe at Night observations but do not show up in the VIIRS-DNB data". This could be sources emitting in directions that the DNB doesn't see. Examples of this include lighted signs and indoor illumination escaping through building windows.

To make that work, the light emissions from sources that not detected by the VIIRS-DNB have to increase at a rate of about 6% per year. This adds to "the estimated 3% per year of the remaining lights deduced from the VIIRS-DNB measurements". It could be that Globe At Night observers were often located near light sources that raised the brightness of the night sky as they perceived it.

The work represents important progress in interpreting both the satellite and ground-based observations. But the case is not quite closed. "Settling this interesting issue requires gathering more detailed data on the lamp substitution processes in different regions of the world," Bará and Castro-Torres wrote. And we also need to know more about light-scattering conditions both in the atmosphere and in the eyes of observers.

With each passing year we learn a little more about how outdoor lighting is affecting nighttime conditions everywhere. Much of what we learn affirms basic principles about how to reduce light pollution. The most effective approaches use it only as needed in the proper places, times, amounts and colors. The recent work by Bará and Castro-Torres, and the Nachlichter team, draws us closer to the fuller picture we seek.

VIIRS-DNB data might be able to tell us. We modeled the effect of adding reflective materials to roadway surfaces, commonly referred to as “cool pavements”. These materials have a paint-like consistency and are applied directly atop existing street pavements. There is some evidence that they work, lowering air temperatures in treated areas by up to a few degrees Celsius. Based on field measurements of the cool pavement materials applied to some streets in Phoenix, Arizona, U.S., we calculated the expected change in nighttime brightness of the roadway surfaces at between about 2-6%. Although the materials appear bright to the eye, much of the radiation they reflect is in the infrared part of the spectrum, some of which we perceive as heat.

To test this model, we looked at VIIRS-DNB measurements of the nighttime brightness of Phoenix made over more than a decade, including the time when the City of Phoenix applied cool pavement materials in certain neighborhoods. We compared the measurements to those of nearby neighborhoods whose streets did not receive the pavement treatments. The results were recently published in the Journal of Quantitative Spectroscopy and Radiative Transfer. 

What we found was surprising and a little disappointing: we couldn’t tell whether application of the cool pavement treatments made any difference in the satellite data. We saw unexpectedly high variations in the brightnesses of all of the neighborhoods from one month to the next. Those variations were so large that the brightness changes due to the cool pavement treatments our model predicted were lost in the noise. In the end, we couldn’t rule either in or our real changes that might affect the brightness of the night sky over Phoenix.

All remote sensing measurements are obtained in imperfect conditions. We can control these circumstances to some extent; for example, the effect of moonlight on nighttime lights observations can be limited by acquiring images only when the Moon is below the local horizon. Some influences, like weather, can’t be completely avoided. Long-term observations can help even out the extremes in order to get at the true underlying trends.

But some of the difficulties we experience are due to satellite designs that are less than ideal for this kind of application. Facilities like the VIIRS-DNB are almost afterthoughts, added to satellites for reasons other than studying night lights. While they provide a trove of information about the distribution of artificial light at night on our planet, they’re not really built specifically for that reason. 
Consequently, some of their characteristics fall short of what we might want. A prime example of this has to do with the “blindness” of the VIIRS-DNB to blue light. The instrument is only sensitive to light with wavelengths of between about 500 and 900 nanometers (one-millionth of a meter). But a lot of modern LED lighting emits considerable light just below the 500-nanometer cutoff. 
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This presented a problem for light pollution researchers from the very beginning when the first copy of the VIIRS instrument was launched in 2012. It was around that time that white LED light was beginning to replace earlier lighting technologies. As lighting modernization projects proceeded, VIIRS-DNB observations saw changes that could easily be misinterpreted. Cities seemed to be getting darker in the satellite images, whereas measurements on the ground clearly indicated that they were no darker than before the lighting retrofits; see the example below. We can account for effects like this to some extent, yet we are left to guess the exact amount of light that the VIIRS-DNB simply doesn’t see.

In June 1969, the Cuyahoga River, bisecting Cleveland, Ohio, on its way to Lake Erie, caught fire. The level of flammable material in the river, resulting from dumping of industrial waste into the river and its tributaries, rose to the point where it could ignite. A Time magazine photographer was there to witness the event. The resulting photo spread in the magazine horrified Americans and galvanized the nascent environmental movement in the U.S. A short three years later the Clean Water Act became law and the health of American waterways began improving.

But, as the U.S. National Park Service tells the tale, the fire was not particularly shocking to locals because it was only one of many such instances historically. “The river had burned more than 10 times over the previous century. The first newspaper coverage focused on the damage, not the fact that the river had burned. At the time, people largely saw the river as a part of industrial infrastructure. In that light, a river fire seemed more normal. It is when we view a river as a natural system that a fire seems out of place." The notion that industrial pollution of rivers was an inevitable consequence of modernity and progress was gradually replaced by refusal to accept outcomes like rivers catching fire. Faced with a parallel situation involving light pollution, is a similar reorientation of thinking the best way forward?

In an earlier post here, we explored environmental law in the United States context particularly, noting how effective it was during the past half century in substantially reducing environmental pollution. We argued that this represented a kind of road map leading toward comprehensive management of a pollutant (artificial light at night) fouling a valuable resource (natural nighttime darkness).

We also previously wrote about Dark Skies and the "Rights Of Nature". The argument in part was that novel legal theories (at least in the West) that confer some legal rights on natural systems may be a way to pursue protection of nighttime darkness. But the idea has yet to gain significant traction. It relies on an assumption that the night sky is a kind of commons owned by no one, in tension with legal guarantees of private property rights.

With University of San Francisco professors Dana Zartner and Aparna Venkatesan, we recently reviewed what's known about this and related subjects. In our paper, we landed on three broad legal approaches to protecting dark night skies. In addition, we considered individual rights and community rights descending from concepts such as the right to a healthy environment and the rights of future generations to inherit from us a planet where humans can continue to live. 

In the end, we concluded, there are "needed cultural shifts in how we think about the impacts of increasing light pollution and what we are losing with decreasing darkness. ...Creative use of newer legal strategies may address this environmental justice issue and support the need to protect darkness." In short, we need to rethink the issue entirely and try new methods because the old ones aren’t working.

At some level the reason why there has been no meaningful breakthrough in advancing legal protections for dark night skies has to do with the way the sky is treated in our culture and in our law, which centers human needs and desires in our interactions with the natural world. The Western legal tradition places humans at the top of a created hierarchy, below which are the elements of the natural environment. In the Christian worldview humans were given dominion over the natural world and laws were created to govern the affairs of people; it's therefore no surprise that law historically favored human priorities over environmental protection.

But that began to change during the 20th century. Environmental law began to emerge after a seismic shift took place in thinking about how humans interact with the environment. Although humans have modified the environment for millennia, the scale of that modification tended to be local. Only recently has technology developed to the point that the effects were large-scale or even global. 

Industrialization began to yield environmental harms as early as the 18th century. This accelerated in the postwar years and by the late 1950s environmental destruction itself was industrialized. Science gradually showed how humans were modifying the natural environment in global and ultimately negative ways.

Environmental law was a reaction to the changes that became increasingly evident during the last century. But those laws mainly functioned only at the national level, even if most countries began adopting a canon of similar laws. Environmental pollution respects no political boundaries, so approaches that focus on single jurisdictions are usually inadequate to solve significant problems that result. 

Few international efforts have achieved success, although those that have suggested ways forward: witness the wild success of the Montreal Protocol (1987) that ended industrial fluorocarbon production in order to heal the ozone hole over Antarctica. While limited in scope, Montreal proved that major environmental goals could be achieved if the world committed itself to them. 

A turning point of swords was the recognition that human activity is inseparable from the environment. The U.S. National Environmental Policy Act of 1969 (NEPA) was one of the first laws to introduce the use of the term "human environment". NEPA defined this term "comprehensively" as "the natural and physical environment and the relationship of present and future generations with that environment." It flows from the the NEPA statement of legislative intent, which reads in part "to declare a national policy which will encourage productive and enjoyable harmony between man and his environment; to promote efforts which will prevent or eliminate damage to the environment and biosphere and stimulate the health and welfare of man; [and] to enrich the understanding of the ecological systems and natural resources important to the Nation."

Three years later the UN held its first global environmental conference, the "United Nations Conference on the Human Environment", resulting in the Stockholm Declaration (also here) that led to the establishment of the United Nations Environment Programme (UNEP). This began an ongoing dialogue among nations exploring links between economic growth, environmental pollution, and the well-being of humanity. It also started the process by which the world began reacting to the threat of global climate change.

The human environment is a coupled system: humans impact the environment, and the environment impacts humans. For most of the existence of our species, it was a largely closed system bounded by the atmosphere. If the atmosphere is used as a defining boundary, then even before space travel the human environment extended to the Moon. To the extent that such discoveries come as a surprise, it’s because they challenge our preconceived ideas about how the world is composed and structured.

Space travel changed that by establishing a human presence in outer space, even if only in the form of the artifacts produced by human hands. In the decades, since the launch of the first artificial satellite, orbital space has become an increasingly congested volume around our planet. In many respects, the condition of low-Earth orbit (LEO) is fully determined by human activities there.

In 2023 we wrote here about the effect of satellites on the appearance of the night sky, asking whether the rapid proliferation of satellites in LEO was worth worrying about. The answer at the time was that we didn't really know, given uncertainties about the future development of space. There are some encouraging signs involving voluntary actions taken by commercial space companies, but there are still considerable hazards presented by effects such as the so-called "Kessler Syndrome" of runaway space debris generation. The prospect of space warfare also looms on the horizon, most recently brought into focus by the November 2021 test of a destructive anti-satellite weapon by the Russian Federation. In short, space is an increasingly dangerous place.

Now we're also worried about a potential feedback cycle between the space and terrestrial environments from the spacecraft life cycle. That is to say, the entire process of building, launching, operating, and ultimately de-orbiting spacecraft has its own environmental footprint. A warming climate plus water vapor and black carbon soot emitted into low- to mid-altitudes could increase the prevalence of clouds. Rocket launches are punching "holes" in the ionosphere. Metals deposited in the upper atmosphere during spacecraft re-entry have unknown effects on the Earth's energy budget. All of this probably impacts the visibility of the night sky and comes on top of an alarming increase in terrestrial skyglow in recent years.

Yet existing legal mechanisms, continue to treat the earth and space like they were fundamentally different and decoupled environments unto themselves. That view is increasingly untenable in a world where the connection between those two spaces is demonstrably stronger than ever. Between them is the sky that represents humanity's portal to the stars, views of which inspired untold generations of people to reach for them. As threats to those views now come from both above and below, the need to change how we think of Earth, sky and space as elements of a single human environment is more important than ever.

If humans were to reach the conclusion that the night sky is “environment”, we might begin to treat it differently as a matter of law. That view finds synergy with the related contention that the night sky is “culture” unique to no particular society. In coming to these realizations, we might further decide that there is value in this resource that is diminished by its pollution. For now, we haven’t yet decided that a clean river is what we want — much less acknowledged that the polluted river we already have is on fire. 

​A visit to an astronomical observatory at night is a magical experience. On a far mountaintop, the starry vault overhead, they are like temples to the study of the cosmos. Telescopes silently scan the heavens all night, shutting their eyes before the first light of dawn. But with increasing frequency, these distant sites are under threat from artificial light at night. What were once thought to be the most defensible locations are less so now. Some suggest that a new management paradigm is the only way to save them.

Astronomical discovery depends on access to faint cosmic light. It is the one physical attribute of the universe directly accessible to science over very large distances. Information about physical conditions in faraway stars and galaxies is encoded in the light we collect with telescopes. Absent this fact, we would know very little about outer space. Because the intensity of light decreases with the distance to its source, these signals are very faint when they reach Earth. Telescopes with massive mirrors collect this light in enough quantity to make sense of the information it brings.

Yet it is now often the case that cosmic light must compete with light from artificial sources on the ground. This began as early as the 18th century, when astronomical observatories were still situated in cities. The installation of gas lighting systems, later replaced by electric ones, began brightening the night sky. Astronomers moved out of cities and took their telescopes with them to the countryside. Many such locations, especially in Europe were unfavorable for observations. Bad weather got in the way, and turbulence in the atmosphere made for poor-quality images. They then moved telescopes to the summits of mountains far from cities. In the 20th century, they started launching them into space. All this increased the difficulty and cost of collecting astronomical data.

Well into the last century, many important observatories in and near cities continued their work. Some sought to hold back the rising tide of light pollution with public policy interventions. Lowell Observatory, in Arizona, U.S., pioneered this approach in the late 1950s. It convinced the council of the city of Flagstaff, its home since the 1890s, to enact what became known as the "Searchlight Law". [1] It aimed to reduce the impact of the use of searchlights for advertising purposes on the observatory's work. But this could only go so far. Flagstaff's population now over four times that in 1960, and the observatory has moved its operations out of town.

Rising night sky brightness over observatories means that scientists need more time and money to achieve the same science outcomes. Consider a case in which the brightness of the night sky at an otherwise unpolluted observatory site doubles. The exposure time for a given telescope and camera needed to reach some science goal then doubles. [2] Given the dollar cost associated with operating a modern observatory, the cost to achieve that goal also doubles. Light pollution thus not only threatens to slow the pace of discovery, but it also makes that discovery more expensive.

There are some recent successes. In 1998, the Chilean National Congress enacted the "Norma de Emisión para la Regulación de la Contaminación Lumínica" ("Emission Standard for the Regulation of Light Pollution”). It required a few basic restrictions on the operation of outdoor lighting in three northern provinces of the country. The goal was to protect observatories in the mountains above the Atacama Desert. Successive updates to the 'Norma Lumínica' have proven rather successful. Yet even the most remote observatory sites there are now threatened. Light can travel hundreds of kilometers through the atmosphere, fouling the night sky far from where it is emitted. Now, no observatory is entirely safe from light pollution. [3] 

Astronomers began confronting this problem in a systematic way over a half-century ago. In 1973, the International Astronomical Union (IAU) established a 'commission' tasked with identification and protection of existing and potential observatory sites. At its 1976 General Assembly, the IAU adopted this statement of concern:

The IAU notes with alarm the increasing levels of interference with astronomical observations resulting from artificial illumination of the night sky, radio emission, atmospheric pollution, and the operation of aircraft above observatory sites. The IAU therefore urgently requests that the responsible civic authorities take action to preserve existing and planned observatories from such interference. To this end, the IAU undertakes to provide through Commission 50 information on acceptable levels of interference and possible means of control.

​It issued guidelines for what it considered 'acceptable' levels of light-pollution interference in 1977. By the following year, the issue attracted the attention of the International Commission on Illumination (CIE). As the international authority on light and illumination, it took note of the increasing problem faced by observatories. It adopted a statement acknowledging the problems caused by uncontrolled outdoor lighting near the best observatory sites.  Further, it urged authorities to take all possible action to protect these sites.

The IAU and CIE joined forces in 1980, releasing the joint publication "Guidelines for minimizing urban sky glow near astronomical observatories". [4] Among its main findings, the document recommended that:

The increase in sky brightness at 45° elevation due to artificial light scattered from clear sky should not exceed 10 per cent of the lowest natural level in any part of the spectrum between wavelengths 300 and 1000 nm.

A lot has changed in 45 years. For one thing, we know more about by how much the natural night sky varies in brightness. [5] That's true on timescales ranging from minutes to years. For example, the 1980 CIE-IAU report presumed a typical night sky brightness at unpolluted sites that is only found sometimes near the minimum of the 11-year solar activity cycle. At the cycle's maximum the brightness can be over 50% brighter, even in the absence of light pollution. Natural sources of light in the night sky varying from one night to the next can yield even bigger changes.

​The bottom line in all this is that the natural night sky itself is a dynamic system "alive with its own light," as the American astronomer Al Grauer says. The night sky, apart from stars and other sources, is not a pure black due to the absence of light. It varies in brightness and color like a noisy audio signal varies in intensity and tone. To continue the analogy, light pollution is like a loud sound atop this noise, with a pure timbre that stays mostly constant from night to night. In the midst of this racket, astronomers are trying to sense a mere whisper.

In 2019, astronomers reacted to a new threat from above: the light of thousands of new satellites launched into orbit around the Earth. They held an international conference in 2021 on the emerging idea of preserving "dark and quiet skies" (D&QS). The report of this conference make a series of recommendations for protecting observatory sites from encroaching light pollution. Some of these are technical in nature, while others leverage the power of public policy to regulate outdoor lighting installations.

The current IAU view, echoing the D&QS report, is:

Present-day professional observatories are located in remote, high-altitude locations; a key selection criterion is the actual sky darkness being as close to the natural background as possible. These sites have an artificial light contamination significantly below the 10% limit recommended by the IAU in 1979. Hence, this limit is not appropriate for the protection of modern professional astronomical sites.
Its new guidance is to "keep the total contribution to skyglow from ALAN substantially below the 10% dark site limit defined by the IAU" (emphasis in the original). At the same time, it recognizes that there is no "one size fits all" approach:

The core of the recommendations is that each major site has a unique limit that should not be exceeded by growing ALAN. It requires each observatory to know what its ALAN contribution is and the rate at which it is currently growing, quantities that can be easily measured.

We can monitor the light situation near observatories from the ground and space, but this involves making long-term observations. Changing the trajectory of that situation requires the commitment not only of governments, but also of the public they serve. There are various proposals for how to do this. We previously wrote about one such idea: the active management of so-called "lightsheds" near observatories.

Another approach is to incentivize reducing light pollution through means such as cap-and-trade schemes. But this still requires treating observatory sites as a special kind of reservation. "For example," the IAU writes,

if an observatory has a current ALAN growth rate of 0.04% per year, which is to be brought to zero within five to eight years, then the ALAN contribution will be less than 0.5% for the foreseeable future. The condition that the ALAN growth rate must be brought to zero and reversed at a site that now has an extremely low artificial contribution sets strong constraints; there will be no way to accommodate a new major artificial light source within these rules, as there is no offset for any sources that could be reduced.

There is now movement in the astronomical community to revise the existing 1980 standard with new information gleaned during the intervening decades. It is also informed by radical changes in the way the way the outdoor world is lit at night since the introduction of LED technology. The high directionality of LED and the ease by which active controls manipulate its light is a game-changer. And non-white light sources, such as amber LED, are more available than ever before. These sources emit light in a limited range of colors, leaving much of the rest of the spectrum dark.

​Astronomers are optimistic that these factors enable a tightening of the guidance that governs outdoor lighting. Over time, it could make a real difference for the wellbeing of the world's observatories. It may also extend their useful lifetimes. Controlling light pollution in this way could lead to a renaissance of science's "cathedrals of the night". 

The news reports are at first ominous. Purple glows appear over towns. Some headlines are even comical: "Tomato factory lights mistaken for 'lovely aurora'". But they share a common origin. To extend growing seasons and further the agriculture industry, more food crops are grown in greenhouses. Artificial lighting used at night aims to increase yields. And that light is having an unwanted effect on neighbors and the night sky. Yet some are betting the farm that new technologies and better regulations can solve this problem. 

Greenhouse farming is on the rise around the world. In the U.S. alone, its economic output was almost $6 billion in 2024. Rising at an annual rate of almost 9% per year, analysts expect it to top $11 billion by 2030. Industry experts make a case for investing further into this sector. Last year Agritechture wrote: "The agricultural landscape in the United States is undergoing a transformative shift, driven by supply chain weaknesses identified during the pandemic, the changes in our climate making it harder for field farmers to grow consistently, and the increasing demand from consumers for more sustainable and locally sourced produce. Greenhouses offer a viable solution to meet this shift in consciousness by providing a controlled environment for year-round cultivation."

With year-round cultivation comes an effort to grow plants faster by lengthening the growing day. Illuminating plants during overnight hours can make them bigger and more productive. "Horticultural lighting" can also reduce the time from seed to crop by accelerating growth. Rapid improvements in lighting technology better support year-round growing seasons.

In particular, the arrival of LED lighting was as much of a game-changer for agriculture as in other sectors. Earlier lamps consumed a lot of electricity and radiated much of their power in the form of heat. Besides being much more energy-efficient, LED has remarkable color characteristics. Its properties are selectable to give plants only the light they need for photosynthesis, explaining the pink or purple colors reported in news stories.

Given the challenge of providing adequate food for the global population, no one doubts the value of greenhouse farming. Food insecurity is a serious threat to economic development. According to the World Bank, "food security continues to be at alarming levels in most low-income countries." That is particularly true in Africa, where famines and rising food prices put millions at risk.
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At the same time — and ironically in part due to light pollution — populations of pollinating insects are in decline. Thus far, machines cannot replace the "ecosystem services" pollinators provide for free. These factors combine to create distinct threats to humans through potential disruptions in the global food supply. 

Their clear glass walls and roofs that make greenhouse ideal during the day is the source of a problem at night if owners use light to extend the growing day. Even with good lighting design and best-in-class lighting products, greenhouses are the source of "obtrusive" light. Light scatters and reflects from glass, interior surfaces, and plants themselves. Some of that light emerges from the greenhouses sideways and can cause light trespass. Light leaving from transparent roof panels travels unimpeded into the night sky. This causes the strange glows reported in news stories. And it can have ecological consequences of its own. For example, one recent study found that greenhouse lighting is harmful to songbirds.

What can be done about this? The fundamental solution involves keeping light contained within the building at night. That's an obvious challenge for structures whose very nature is to let outside (sun)light in. To address that particular aspect, some have experimented with so-called 'smart glass'. This method uses exotic materials that are alternately transparent or opaque to visible light. The state of transparency changes when, for example, an electric current is applied to a glass panel. But these materials are rather expensive, and their opacity is usually not enough to keep all interior light contained.

​A lower-tech solution involves a much lower-tech approach: close the blinds. Roof-mounted machinery deploys physical window coverings at dusk and retracts them at dawn. As shown in the example below, these can be an effective mitigation for much of the light that would otherwise escape the greenhouse. Yet even this trick isn't inexpensive, and it involves moving parts subject to wear and tear.

These mitigations can be combined with local regulations to enforce changes. The simplest laws impose a lighting "curfew" time, after which greenhouse owners must switch lights off each night. That may be most fair to people who live near greenhouses to reduce the impact of lighting. Of course, growers may take exception to such policies as unfairly limiting their operations. Zoning restrictions can help put some distance between greenhouses and their neighbors. And some jurisdictions may choose to prohibit greenhouses entirely within their territories.

​But regulation is challenging to put in place correctly and consistently. It often fails to completely address the problem. Like other examples of conflicting land uses, satisfying all those involved may be impossible. The benefits to society that greenhouse farming represents must be weighed against its social costs. In the ideal case, workable solutions respect the rights and wishes of all stakeholders.

Modern greenhouse lighting is here to stay, and by all accounts will only be more important to the global economy in the future. Yet there are clear challenges to enabling its future development while reducing its effect on the nighttime environment.

The controllability of LED could be the key to solving this problem, alongside other tech. But good old-fashioned "shutting the blinds" is the best approach. It presents an added expense to operators, which could as easily be counted alongside other cost of doing business. In that sense it's not unlike complying with building codes or workplace safety laws. And it's up to each jurisdiction to decide how much (and how best) to regulate.

Recent experience suggests that jurisdictions should get out ahead of this issue before it lands on their doorstep. Often the first sign of a new greenhouse locals notice is the nighttime glow. Reactionary efforts at regulation have a habit of proceeding rather poorly. Proactive attention to the problem can reduce that tendency.

There may be also some lessons learned here about things like land-use zoning. They may point the way to how to deal with other outdoor lighting challenges from 'speciality' lighting applications. Because of its associated effects, greenhouse farming might be subject to geographic restrictions. Regulators should, however, be mindful of the fact that light pollution can drift far from its sources. Other interventions may still be necessary to avoid conflicts.

​Can the future of greenhouse farming remain bright without compromising the dark? Any serious discussion of that question must consider both lighting design and regulation. It must further weigh its economic benefits against potential social and environmental harms. As in many such instances, the ideal solutions find the right balance between the needs of both people and planet.