"Lightshed" management: the next frontier in dark-sky conservation?

Image credit: Y. Beletsky (LCO)/ESO
1705 words / 7-minute read

Summary: Reducing light pollution involves well-known technical solutions, but public policies that implement them often fail. The definition and active management of landscape-scale "lightsheds" may offer a way to overcome this obstacle. 

"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.

​What we do now and why it often doesn't work well

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.

Other approaches (that probably also won't work well)

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. 

What is a "lightshed" and how does it work?

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. 

Watershed map of the Lost Creek Reservoir in Morgan County, Utah, U.S. Major stream channels are shown in blue and the watershed boundary in red. The boundary encloses only the territory whose natural drainage is into the reservoir at lower left. (Credit: Matthew Heberger, licensed under CC BY-SA 4.0.)

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. 

Photons without borders". This map of northwest Spain shows simulation results for a site called Xares, marked with a yellow star. The colors of the municipalities outlined in black indicate the relative contributions from their light at night arriving at Xares. Maps like these can help guide the definitions of lightsheds. (Adapted from Figure 3 in Bará and Lima (2018), courtesy of the authors.)

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. 

Regional light at night map for Pennsylvania, U.S., and surroundings from orbital satellite data. The location of Cherry Springs International Dark Sky Park is indicated with a label. (Credit: NASA/NOAA)

Shortcomings of this approach and where we can go from here

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.

We thank Dr. Richard Green (Steward Observatory, University of Arizona) for helpful discussions in framing the argument presented in this post.