Light pollution is increasing almost everywhere in the world. [1,2] We know that this has serious consequences for society and the environment. [3] Its rate of increase is faster than that of population growth in many countries. This implies that the rate of consumption of artificial light at night is growing. [1]

Researchers think that this results from the lower cost of operating light-emitting diode (LED) lighting compared to earlier lighting technologies. If this is true, it undercuts the claimed environmental benefits attributed to the high energy efficiency of LED. We don't yet know what this means for differences in energy consumption, if indeed energy savings are offset by the growth of new lighting.

​We also know that harms associated with light pollution are reversible. It does not persist in nature like other forms of pollution. When we deploy proven solutions, we see the reductions in light pollution that we expect. [4] Through concerted efforts, we can imagine a future in which it reduces in intensity according to predictions.

This is not only limited to slowing the further growth of light pollution. Activists are beginning to talk about 'restoring' natural nighttime darkness in some places. But what does this mean, and what would the world look like if it were successful? This month we look at the notion of nature restoration and what lessons the broader conservation movement has for dark-skies advocacy.

When we talk about 'restoring' nature, it's important to agree on what that term means. It is often discussed in the context of another word, 'rewilding'. It refers to reverting to environmental conditions resembling those that existed before human manipulation. One finds the term applied often in situations in cities where removal of development traces encourages natural processes to return. [5]

It is important to distinguish rewilding from another kind of restoration referred to as "Nature-based Solutions". [6] The International Union for the Conservation of Nature defined this term in 2006 as "actions to protect, sustainably manage, and restore natural and modified ecosystems that address societal challenges effectively and adaptively, simultaneously benefiting people and nature." [7]

But it's not always synonymous with ecological restoration. In contrast, one definition of 'ecological restoration' is "assisting in the recovery of ecosystems that have been degraded or destroyed, as well as conserving the ecosystems that are still intact". [8] The authors who suggested the definition argue that the rising use of the term 'Nature-based Solutions' "may partially be a rhetorical response to shifting priorities and terminology by funders of research and practice". Yet they say that Nature-based Solutions "reflect a genuine commitment to achieving societal benefit." While the terms are distinct, they "are similar and can be mutually supportive."

Why does this matter? Some scholars have criticized Nature-based Solutions as a relic of colonialism or even a form of greenwashing. [9] It centers humans, in the sense of achieving social benefits, over any intrinsic value nature may have on its own. It relies on truly sustainable environmental management, which is rarely achieved in practice.

It also implies that humans won't conserve something unless they value it, presumably for selfish reasons. In this sense, it follows the implications of a famous quotation from the Senegalese forest engineer Baba Dioum. In 1968, Dioum said "In the end we will conserve only what we love, we will love only what we understand, and we will understand only what we are taught." [10]

All these ideas proceed from the assumption that there is some value to nature restoration, whether for humans or nature itself. Appeals to human wants and needs may be a means to an end in this regard. Dark-sky advocacy often focuses on the purported human benefits of improved outdoor lighting, such as security, public safety, human health and more.

While advocates also point out the harm that light pollution poses to ecosystems, they tend to frame the argument in a way that caters to human motivations. Examples of this include focusing attention on so-called 'charismatic megafauna' like sea turtles or nocturnal insects that pollinate food crops. In this sense, advocacy efforts seem to align with Nature-based Solutions as a matter of pragmatism.

Efforts to bring dark night skies back to areas affected by light pollution usually take the form of "landscape scale nature restoration". The Scottish Nature Agency defines this term to mean "land managers working together to restore nature on a large scale, across multiple land holdings and resulting in multiple benefits for nature and people." [11] It further suggests that "this approach is more effective at achieving environmental, economic and social benefits than working in isolation, on smaller, single sites."

DarkSky International has pursued this for years through its International Dark Sky Places program. [12] The program can be described as Nature-based Solutions with two main goals. First, it aims to increase public awareness of light pollution and reward participating sites for taking steps to address the issue. The second goal — restoring natural nighttime darkness over typically large geographies — is more outcome-based.

The evidence for success in achieving the restoration goal is mixed. [13,14] In recent years it has put increasing emphasis on cities as a source of both problem and solution. Through new designation categories like its "Urban Night Sky Places", it advances the idea that restoration of darkness in more distant sites depends on changes in the lighting of cities. But this approach is still so new that we don't yet know if it works as advertised.

Would a stronger appeal to Nature-based Solutions still result in meaningful restoration of dark skies? One might look toward Attention Restoration Theory for clues. It holds that a connection exists between access to nature and total human wellbeing. [15] But for one thing, we don't know for certain whether the natural nighttime space is a "restorative environment".

It's also unclear whether experiences in nighttime darkness helps move the needle on nature restoration initiatives. That's particularly true given a broad (and maybe innately human) fear of the dark that stokes hesitation toward some nighttime restoration initiatives. It might be an obstacle to moving people to care about this aspect of nature to the point where they would give up something else they valued to protect it.

​Other social concerns can override the human valuation of natural spaces. A recent example is the U.S. migrant detention center popularly known as "Alligator Alcatraz". In mid-2025, this facility opened at a disused airstrip in Big Cypress National Preserve, a federally protected land in south Florida (and an International Dark Sky Park). Aside from concerns about light pollution due to detention activities at the site[16], activists alleged other environmental harms that were not subject to review or meaningful oversight. One might well ask whether the American public would have accepted this activity in a higher-profile U.S. national park like Yosemite or the Grand Canyon.

​Although public attention to light pollution has risen steadily for decades, it is still climbing the "hill of awareness". We have identified a collection of best conservation practices, and there is some evidence for their efficacy. Yet we're a long way from implementation in many parts of the world because it's just not a priority for most of the public.

This argues for a more top-down approach as compared to countries like the U.S., where outdoor lighting policy is distinctly local in flavor. At the same time conservation is often focused on larger landscapes where few people live. To not only slow the growth of light pollution around the world but also reverse the trend requires different ways of thinking.

What about tying restoration to legal requirements? The recently enacted EU Nature Restoration Law aims to do so, obligating EU member states to develop plans to restore at least 30% of habitats in "poor condition" by 2030, 60% by 2040, and 90% by 2050. It even contains some language about light pollution. [17] For now the law is forward-looking, and we don't know if it will yield good results.

Globally, the successful strategy may well instead be to "bend the curve" of light consumption per capita. That is the amount of light emitted in a territory per head of population. In earlier times, that figure seemed to be stable. The advent of LED, with its extreme energy efficiency, seems to have prompted its increase through what economists call "elastic demand".

But human behavior proves difficult to change through heavy-handed regulation. Instead, the more appropriate goal may be to change the human relationship to ALAN. We recently wrote about a combination of hard and soft law in the form of so-called "lightshed management" as a possible approach. It sets targets for light pollution reduction while engaging in extensive community outreach to nudge people toward different behaviors in terms of how they use outdoor lighting. Here, too, the idea remains a theoretical one only for the moment.

​Without targets — preferably binding ones — it is going to be difficult to achieve true restoration of nighttime darkness. Considering the motives for restoration carefully and asking critically whom they benefit is an important step. The implementation best practices remain mostly to be determined. The stakes remain high if we do nothing, but our confidence in successful results once we apply suitable efforts is equally high.

Light pollution is increasing over much of the world. It is a trend that goes back decades and shows no sign of slowing down. We wrote here about the situation as recently as early 2023. At that time, new research showed that skyglow was then increasing at a worldwide average rate of 10 percent each year. A much newer notion is the idea of 'light pollution' from space. We also wrote about that situation in 2023. The concern was then about the changing appearance of the night sky. 

Fast forward only two years, and a completely new concern is now emerging. What happens when satellites can turn night into day? The result is a novel threat we explore in this month's post.

We live in an era some that some call "New Space". This describes activities in outer space dominated by private commercial actors. It contrasts with previous decades when only national space programs could pay the high cost of launch.

That cost plummeted in the 2010s, enabling private companies to launch payloads into space. At first, the main service private satellites provided was global telecommunications. But as investor money poured into the industry, entrepreneurs began to come up with audacious new ideas.

One of those ideas is to harness solar power in space and send it to the ground. This takes different forms including converting sunlight to radio waves. A simpler concept would deploy large reflective surfaces in orbit around the Earth. These would redirect sunlight to the ground to illuminate solar power stations on the ground. The scheme only works near sunrise and sunset, when the reflectors are in sunlight but the sun has set on solar farms.

Proponents of this technology note that it can provide light for other uses. For instance, they say, it could function in place of street lighting over cities. It would provide an illumination level about four times the brightness of full moonlight. That could aid in disaster recovery when utility power is unavailable.

You would be forgiven for thinking this is the first time anyone has proposed such a thing. But it's not. In the Second World War, a group of German scientists at the German Army Artillery proposed building a Sonnengewehr ("sun gun"). They designed a focused beam of sunlight from an orbital reflector as an offensive weapon. And they expected the devastation to be akin to what the world later saw resulting from the U.S. atomic attack on Japan.
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In October 1992, the Russian Federal Space Agency launched the Znamya 2 satellite. Designed as a 'solar sail' testing a new kind of propulsion, the RFSA repurposed it as a 20-meter-wide solar mirror. It consisted of panels of stretched, highly reflective film that deployed after launch.

During a demonstration of the technology, it successfully directed a beam of sunlight to the ground. It yielded a spot 5 kilometers wide with the intensity of full moonlight. The beam swept across Europe from southern France to western Russia at a ground speed of 8 kilometers per second. Though weather along much of the ground track was cloudy, some observers reported seeing bright flash of light as the beam swept past them.

The RFSA tried another test of this technology in 1999 with a larger spot size and a brighter beam. The test failed when the reflective film panels tore after snagging a piece of the Progress spacecraft used to deploy the object. Although the RFSA planned future satellites with even large collecting surfaces, the 1999 failure was the death knell for the program.

More than 25 years later, the concept is again inching closer to reality. An American company called Reflect Orbital applied for clearance to launch and operate a test satellite in 2026. It consists of a thin-film reflector 18 meters on a side and a system to control its orientation in space. By changing that orientation relative to the direction of the Sun, it can direct a beam of sunlight toward Earth.

Like Znamya 2, its beam has a diameter of 5 kilometers on the ground, enough to illuminate a large solar power installation. The beam shines for about four minutes before the satellite disappears over the local horizon. The company plans to eventually launch a flotilla of thousands of these reflectors. That would enable continuous sunlight for about an hour after dusk and an hour before dawn.

Will it work? We don't yet know. The Russian test from the 1990s suggests that something like this is technically workable. But if it does, a host of concerns are expected to follow.

Some problems have to do with the idea of shining a beam of light with the intensity of sunlight onto specific locations at night. A simple calculation suggests that the reflectors will appear on the sky as sources several times brighter than a full Moon. Far from the beams, they'll still look like bright stars, and dozens of them may appear in the sky at any moment.

Scattering of light in the atmosphere will make the edges of the beams 'fuzzy'. The mirror-like surfaces of the reflectors will deteriorate with time due to exposure to the space environment. The beams of light they produce becoming more diffuse, spreading out and losing intensity. This may direct light well away from the intended targets of the beams. Some of that light may end up in ecologically sensitive areas.

As described above, the mirrors have to change their orientation in space as they pass over a given location. Although the company claims that the reorientation will happen quickly, it involves sweeping the light beam across the Earth at high speed. A 2000 study published in the Journal of the Royal Astronomical Society of Canada found that "space-mirror experiments reflecting sunlight to Earth can produce resolved images having surface-brightness sufficient to damage human eyes looking through telescopes or binoculars". Even if the flash is instantaneous, it could cause real harm to the viewer.

In a similar vein, these light beams could be a problem for aviation safety. Pilots have for years reported intense flashes of light from laser pointers on the ground shining up at their planes. At the altitudes of commercial planes, beams of sunlight from space will be larger in size and brighter than they appear on the ground. This could become a significant hazard for civilian and military pilots alike.

There is great potential for misuse of this technology. Sunlight could be weaponized in a way that conventional solar power cannot be. It may be directed toward people and communities that don’t want it. It could also be used to convey battlefield advantage in military conflicts. This adds another layer of complexity in deciding how the technology should be regulated.

The reflective material could be subject to fraying or other kinds of damage on orbit. They may shed material that becomes space debris. Over time, the satellites will inevitably fall back to Earth, whether from atmospheric drag or deliberate action to de-orbit them. Where, when and how they will come back down is not clear.
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Lastly, despite prognostications to the contrary, there is no “inevitability” about this or any other idea for furthering renewable energy sources through methods such as sunlight as a service. Improvements in battery storage are slowly improving renewable energy supplies at night. We may be better off deploying more solar panels on the ground, even knowing that there is a life-cycle cost for producing/recycling the equipment. 

"Greenwashing" is a label describing efforts made to persuade the public that an organization's products or actions are friendly to the environment. Usually they are not, and the greenwashing effort is deceptive.

That seems to be happening in the case of sunlight as a service. The sales literature is full of starry-eyed claims having to do with a purported social good. But it focuses on the notion of sustainability in a narrow sense: providing sunlight from space for "clean" electricity generation. It doesn't consider the full lifecycle of the project from launch to de-orbit.

It doesn't account for the negative externalities of directing light to spaces where it doesn't naturally exist at night. There is abundant evidence for the harms associated with artificial light at night. That in principle includes sunlight directed to the night side of Earth. It interferes with natural rhythms as much as sources of electric lighting.

At the same time, there is no national or international prohibition on this kind of activity in outer space. The international legal framework governing activities in space takes the view that 'what isn't forbidden is allowed'. Norms often change to discourage a harmful activity only when the harm has already been inflicted.

In that sense it resembles obtrusive space advertising, which we wrote about here recently. Dangerous activities in space can occur with only an ill-defined liability system on the other side.

It's unclear how serious any of this is, and projects like Reflect Orbital may not fly at all. Many such audacious ideas never become reality. Some are meant only to attract the intention of investors looking to make quick money as companies are bought and sold.
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It remains to be seen whether there is any successful business model for selling "sunlight as a service". Yet the lure of new wealth from space may prove irresistible. This is a challenge for defenders of the night everywhere, and one that is worth watching closely.

Our world is one of generally complicated problems that lack simple solutions. Reality is often messy and confusing. Yet politics gravitates toward easy fixes and even promises them. Ill-conceived attempts to solve such problems can make things worse by generating new ones.

Such is the case where the interaction of outdoor lighting and crime are concerned. We have written here in the past about that interaction. We pointed out how inconsistent the data are. And we explained why "feelings of safety" may have more to do with lighting and crime than any other underlying relationship.

New research by Paul Marchant and Paul Norman published in Applied Spatial Analysis and Policy challenges the narrative again. They challenge the underpinnings of much of the scholarly literature on the subject. They also show in a well-researched case study how 'common sense' assumptions about lighting and crime may be flat wrong. This post is as much about how scientists do research as it is about the result of the specific study used as an example.

Marchant and Norman examined data from the city of Leeds, UK, during a massive municipal street lighting retrofit. Between 2005 and 2013, Leeds replaced some 80,000 street lights over its administrative territory. The city exchanged its old sodium-vapor lighting for ceramic metal halide luminaires.

First, in 2022 the pair looked into whether changing to white light street lamps improved road safety. They found "no convincing evidence ... for an improvement (or detriment) in road safety by relighting with white lamps, despite the extensive, city-wide installation efforts and associated costs." In other words, there was no evident effect one way or another. The study did not find evidence that the new lighting had affected traffic safety despite there being over 19,000 road traffic collisions in the data set.
To secure UK Private Finance Initiative funding for the retrofit, the city touted expected benefits due to crime reduction. A key claim involved a 20% reduction in night-time crime. This, in turn, would lower social and economic opportunity costs associated with crime. And it would yield a Benefit to Cost Ratio (BCR) of 3.75 — an impressive return on investment. The retrofit cost was borne at least in part by this forecast.

The Leeds case is one of the largest-scale municipal lighting retrofits studied rigorously to date. But the results show that its influence on crime was close to zero. "The upshot of all the fitted models," the authors wrote, "is that the effect of the new replacement white lamps on crime is small."

We recently interviewed lead author Paul Marchant for more on his paper and its significance. His illuminating (!) answers below are lightly edited only for length and clarity only.


​DSC: Can you briefly summarize the method used in your paper for a lay audience?

​Marchant: The study analysed the weekly counts of crimes that were recorded by the police in each of the 107 geographical areas comprising the whole of the UK city of Leeds, as streetlighting was changed from orange to white light, over a period of nearly 9 years. The method uses the fact that the replacement new white lighting is introduced into each of the 107 areas in different amounts at different times.

Therefore, each area is at a different stage of completion at any given time and so it is possible to assess what these differences in lighting between areas lead to in terms of differences in the occurrence of crime.

Different areas suffer different amounts of crime, with some experiencing a lot whilst others only a little. The multilevel analysis treats the 107 series as a ‘family’, so that the overall levels of crime can be modelled as coming from a statistical distribution with a certain average weekly crime rate and a certain spread in rates. Similarly other distributions are formed for aspects of the underlying area-specific trends that are either increasing or decreasing the amount of crime, due to causes other than the nature of the streetlighting.

Crucially the modelling involved a term for the amount of new lighting each area had received at a given time point. This allows the effect of the new lighting to be separated from the underlying trends in crime in the different areas. Therefore, the effect on crime of the full complement of new lamps can be determined.


DSC: At the same time it was important to you to show why earlier reports in the literature are often unreliable. You point to several factors, from 'untrustworthy controls' to unavailability of the source data. Do you think something nefarious is going on with respect to lighting and crime studies? Does it have something to do with who funds the research?

Marchant: Frankly I don’t know, but it certainly needs to be guarded against. Transparency and openness are paramount for good science. There are concerns about such as conflict of interest bias even in the regulated clinical trials area, as outlined in the Cochrane Collaboration Handbook. Anyone interested in issues around the matter of lighting and public safety, and associated concerns of poor quality research, might like to read my article ‘Investigating whether a crime reduction measure works’. It explains how and why I started pursuing the issue.


DSC: Given the many effects that complicate light/crime studies, one might assume that getting at underlying relationships is nearly impossible. Do you think anything like a dose-response relationship between light and crime will ever be demonstrated? Or is human behavior just too complex for that?

Marchant: In principle it could be possible, as even very small effects can be found and estimated with enough of the right sort of data, sensible modelling and enough computing power to analyse it. Personally, I wonder whether some crime researchers talk of “dose-response” so as to (wrongly) give the impression that their work is just like that of a clinical trial. In a comprehensive study of lighting, one ought also take account of the characteristics of the places in which the different lighting is installed, to make its results more generalisable. One can conceive of a very elaborate study even involving randomisation but such would take a lot of resources!


DSC: The shortcomings of research study design can lead to poor-quality flooding the literature. How much research of a doubtful nature do you think is out there that never gets scrutinized (yet eventually becomes part of the folklore)?

Marchant: In this field, my answer is most of it! Indeed, researchers seem to have never heard of regression towards the mean. They wrongly assume the counts are Poisson [distributed] and this assumption can’t be checked as they only use ‘before and after studies’. I suspect that they may have only done basic statistics courses in which it is only statistically independent events that are analysed.

I believe ‘folklore' is an appropriate term as bad practice is just passed on with no adequate questioning of whether, for example, the method is appropriate or that the assumptions underpinning the method are met. The problem is that it is very easy to put some data into a computer, press the button for an unwittingly inappropriate analysis, get some other numbers out and call that the result. Reviewers of journal articles are also likely to have also been inculcated into the same folklore of the field.


DSC: The conventional wisdom about research is that one way to avoid at least some of these problems is to 'pre-register' study protocols. In this framework, scientists state their intended data collection, reduction and analysis procedures up front. Not only that, but they are further encouraged to publish these plans. This helps keep researchers honest, because any attempts to fit data to models after the fact will be more conspicuous. Yet comparatively few studies do this. Why do you think researchers avoid pre-registering their methodologies?

Marchant: Registration wasn’t always the case for clinical trials and it has taken a while for the practise to diffuse into other areas of inquiry. The ‘replication crisis’ has been a source of great concern about poor research. I think there has been progress in experimental areas in which a planned intervention is made, but registration is less common in observational studies, which just collect observations, when there is no planned / organised intervention. However, things are gradually changing and some organisations, like the Center for Open Science, are encouraging and facilitating the practice. Some journals like PLOS ONE also publish plans for research, which also allows comments like this to be made. The protocol for our study was not publicly registered but was sent to three competent [and] suitably qualified academics.

I am somewhat surprised in these cost-conscious times, that if a policy has been introduced on the basis of alleged evidence more checks are not made that the policy has achieved what it was intended to. I think that a big part of the problem is that the political class tends to be rather poor at statistical thinking.


DSC: What do you think would make for a good way forward in this field of research?

Marchant: To apply this multilevel method to datasets from other places, involving lighting changes, where there is appropriate data for ‘outcome measures’, such as crime or road traffic collisions. [And] get more statisticians involved. Many researchers use statistical analysis but this does not make them statisticians. Try to engage those who ideally have a higher degree in the discipline of statistics, are recognised by a professional body, such as the American Statistical Association and who undergo continuous professional development in the subject. Try to engage statistical epidemiologists as there are parallels between disease and crime in society.


DSC: Is the Leeds example a sufficiently cautionary tale that officials in other cities might think twice before acting more on instinct rather than evidence?

Marchant: Yes! Also, a good rule of thumb is not to believe that a salesperson is giving entirely unbiased information. It would be a step forward if policies were implemented in ways that would make an evaluation straight-forward and that post implementation performance is routinely checked by those who are statistically well-qualified and independent.


DSC: If your work offered one important takeaway message for readers, what would it be?

Marchant: Just because it’s a commonly held belief that lighting beats crime doesn’t mean that the claim shouldn’t be checked out. The recent Marchant and Norman study, which counters some of the shortcomings of other studies on the subject, suggests that any effect of changing to brighter white lighting, either detrimental or beneficial, was at most rather small, and not at all what was expected. There seems to be no reason to expect that the effect of such relighting would be any different elsewhere and so the anticipation of crime reduction is not a sound reason for increasing lighting.


​As we wait to find out where lighting and crime research is going next, this new work offers some appetizing food for thought. We thank Paul Marchant for sharing his insight with our readers.

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.