THE LIGHT DOCTOR 12: Light Pollution and Biodiversity
Chapter 12 discusses the harm caused to the other 8.7 million species on the planet by our unrestrained use of electric light, and what we can do to protect biodiversity
Early in the morning of February 4th, 1993, an orbiting Russian satellite named “Znamya 2” (meaning “Banner”), deployed a large mirror to reflect the sun’s rays onto the dark side of the earth1. The dream of the engineers was to banish the night and save billions of dollars in electricity. By increasing the length of the day, they sought to boost productivity in farms and cities in the then Soviet Union. A constellation of synchronized satellites was planned each carrying a 200-meter mirror illuminating a ten-square-mile area on earth with a brightness nearly 100 times greater than moonlight. Far from their thoughts was the devastating effects this would have on the health of humans and the other 8.7 million species2 which are evolution-optimized to live under the natural 24-hour light-dark cycle.
Life on earth dodged that bullet with the collapse of the Soviet Union, but now the profusion of cheap outdoor LED light presents an even bigger threat. As LED prices have plummeted and the cost of powering them has fallen because of escalating lumens per watt efficiency, the appetite to use more lights has increased, blunting the promised energy savings. Professor Lucas Davis at the UC Berkeley Haas School of Business believes we should embrace this, claiming “More and brighter lighting for our homes, businesses and public spaces can make us happier, more productive and safer. … LEDs are great even if they won’t necessarily result in large net energy savings.”3 But this viewpoint totally overlooks the negative impact of outdoor electric light at night on the earth’s flora and fauna.
By 2005, 1.5% of the world’s electricity production was used for outdoor lighting. We were pumping 11 quadrillions of lumen hours of light into the nighttime outdoor world (measured in Peta-Lumen-Hours/year, where a Peta is 1015 lumens)4. Total global electricity production has since increased at a rate of 3% per year, but the hope that energy efficient LEDs would reduce the use of electricity has not borne out. Any efficiency gains have been outstripped by the increased use of brighter outdoor LED light as prices fell.
To address this challenge to the millions of diverse lifeforms on this planet we need to think differently about indoor versus outdoor lighting.
Our rights indoors
We are entitled to control the lighting of our own indoor domain for the benefit of ourselves and our family members, without having to be concerned for other species, other than our pets and house plants. Most of us have no compunction about exterminating rodents or cockroaches and setting traps to catch flies within our own homes. In the same way the only consideration in selecting lighting for our homes, schools, hospitals and workplaces is what will most benefit the health of our family, our colleagues and ourselves. These lighting specifications are discussed in Chapter 10: How to be a Smart Consumer and Chapter 11: When You Don’t Control the Space.
8.7 million other species have rights outdoors
Outdoors it is a very different matter. We are part of a highly complex interdependent global eco-system that we only partly understand. All life forms on earth evolved to operate most successfully under the natural day-night and seasonal cycles conferred by our rotating planet. Increasing outdoor nighttime light levels above the 0.2 lux of bright moonlight disrupts the feeding and breeding behaviors of millions of species, and most of the time natural light levels are much less than that at night.
It is not just the immediate vicinity of outdoor lights that is affected. The skyglow from a major city can be seen more than 50 miles away5. In densely populated areas of the world, such as the eastern half of the United States and Western Europe, there are very few areas that are not impacted by light pollution at night. The brightness is increased on cloudy nights because electric light reflects off the clouds. By 2016, 88% of Europe and 47% of the United States, had night sky brightness at least 8% above the natural level6. And sky brightness continues to increase by 7-10% per year7.
Disrupted Food Webs
Nature is comprised of complex sets of food webs with food chains that are critical to the survival of each species. Not every species in a food chain has to be directly disrupted by light at night. All it takes for some microbes, plants, insects or animals in the food chain to be affected, and everything else above them in the food chain suffers.
The primary producers in terrestrial food chains are plants that use the energy of the sun to photosynthesize inorganic nutrients (e.g., phosphates, nitrates) into organic food (e.g., leaves and flowers) that other species can eat. The primary consumers are the herbivores which eat the plants (e.g., caterpillars or grasshoppers). The secondary consumers are the carnivores which eat the primary consumers (e.g., songbirds or mice). Above them in the food chain are the apex predators such as hawks which feast on the secondary consumers such as songbirds or mice. The players at each level, of course, depend on the ecosystem and climate.
Electric light at night (abbreviated as LAN) has significant impacts on these food chains. Primary producer plants in food chains have long been known to be disrupted by LAN, long before the introduction of LED lights. When sodium high pressure streetlights were first installed in the 1970’s maize crops alongside main roads grew rapidly but failed to flower8, and the normal development of soya beans near bright floodlights was disrupted9. The responses to LAN are highly varied. Some species of grass grow more rapidly while others are suppressed, changing the nature of the food supply to the detriment of some species and to the advantage of others.
Similarly, primary consumer populations are disrupted by LAN, both because of changes in food supply, and direct effects on their development. Caterpillar populations under white LED streetlights are reduced almost 50% in the hedgerows and by 33% in the grass verges of British country roads10. In turn this reduces the food supply to secondary consumers such as the British songbirds that nest in those hedgerows. And the impact of LAN ripples through the entire food chain.
In 1897, soon after the first electric lights were installed along the country streets of England, concerns were already being raised about their impact on songbirds:
“With the exception of the finches, all the English songbirds may be said to be insectivorous, and their diet consists chiefly of vast numbers of very small insects which they collect from the grass and herbs before the dew is dry. As the electric light is finding its way for street illumination into the country parts of England, these poor winged atoms are slain by thousands at each light every warm summer evening....The fear is expressed, that when England is lighted from one end to the other with electricity the song birds will die out from the failure of their food supply.”11
Songbirds may not have died out in England, but their populations have shrunken considerably. The population of the most famous of British songbirds immortalized in John Keat’s poem “Ode to a Nightingale” has declined by 92% since the first blue-rich lights were widely introduced in the 1970’s. Over the same time frame, turtle doves have declined 98%, willow tits by 94% and starlings by 82%, to name just a few12.
Aquatic food chains
In aquatic food chains the primary photosynthesizing producers include algae which live near the surface where they can harvest the most sunlight. Their primary consumers are zooplankton, thousands of species of small animals including shrimp and other crustaceans, which are highly sensitive to light and hide deep in the water during the day and come up to feed on the algae at night. The secondary consumers are the fish which eat the zooplankton.
Electric light at night falling on any body of water forces the zooplankton to stay deeper below the surface so they are less visible to the fish that prey on them13. This behavior starves the fish of their primary food supply, and at the same time allows the uneaten algae to bloom contaminating the surface of the water. Fish populations as a result plummet.
Sensitivity to Light
Worldwide, around 30% of vertebrates and more than 60% of invertebrates are nocturnal14. Nocturnal species in general are much more sensitive to light at night than day-active species. Their eyes are often adapted to detect lower levels of light at the expense of visual resolution.
The spectral composition of moonlight is quite different from sunlight15. The peak energy is deep in the violet at around 400 nm, as compared to sky blue 480 nm peak energy in sunlight – although the peak of the daylight spectrum can vary with weather conditions. It is therefore not surprising that nocturnal species gauge the brightness of light from a different range of spectral wavelengths than humans. Whereas humans experience maximum brightness at 555 nm in the green, nocturnal species are often more sensitive to shorter wavelengths sometimes down into the ultraviolet. Even small changes in lunar brightness from a new moon to a full moon can trigger critical reproductive and developmental effects.
Outdoor lights, besides raising brightness way above natural moonlight, introduce unnatural spectra into the nocturnal landscape, whether it is High Pressure Sodium lights emitting yellow-orange wavelengths, or conventional LEDs with a peak at 450 nm royal blue. We have only scratched the surface of how each of the millions of nocturnal species with diverse spectral sensitivities are affected. This has a major impact on biodiversity because electric lighting is fundamentally changing the photic environment.
Some species are attracted by the light, others are repelled by it, and yet others may have their life critical behaviors and physiological systems disrupted. Let’s look at some examples of electric light effects.
Circadian, Circalunar & Circannual Disruption: Both day-active and nocturnal species rely on the precise timing of natural light day cycles, whether they are the daily sunrise and sunset, the approximately monthly waxing and waning of the moon, or the changes in day length and night length with the seasons of the year. Disruption of these natural light-dark cycles impacts the metabolism, immunity and risk of cancer and other diseases, just like it does in humans. However, the sensitivity to low levels of light is much greater. For example, in nocturnal rodents it takes only 0.03 lux of light (moonlight levels) to significantly suppress melatonin16. Furthermore, LAN affects feeding behaviors, and reproductive success to a greater extent than in mankind.
Spatial Disorientation & Navigation: Many nocturnal species use the moon or stars to navigate. Electric light sources can obscure their view of the sky, or provide false signals. As a result, they may fail to find feeding grounds, or to mate and reproduce.
Attraction: Some species are attracted to light and others repelled by it. About a third of insects approaching outdoor lights die soon thereafter as a result of collision, overheating, dehydration, or predation”17. Millions of birds die each year after colliding with buildings, attracted from miles away by the lights18.
Loss of Dark Adaptation: As we discussed, nocturnal species are highly sensitive to light, and may take hours to regain full dark adaptation after coming close to electric lights. This temporarily blinds the creature and interferes with its ability to find nutrients or steer clear of predators.
Bioluminescence: Fireflies and certain other species use bioluminescence to emit flashes of light to attract mates. Outdoor electric light may mask these signals and interfere with reproduction.
Facing the Challenge
The LED revolution has given us plentiful cheap bright light, and the temptation to light up building facades, landscapes, sport facilities, highways and backyards. The arguments in favor - aesthetics, usability, safety and security - have to be tempered by the devastating impact of light at night on the flora and fauna of the natural world.
Lighting our own indoor day and night environments is a relatively simple challenge as compared to lighting the outdoor world that is shared with millions of other species each with its own unique spectral sensitivity to light. The human spectral sensitivity is now well known, and so indoor circadian lights can be designed with only one species in mind, as we have discussed in earlier chapters.
In 2023 Travis Longcore at the University of California, Los Angeles, Institute of the Environment and Sustainability, undertook an exhaustive exercise to document the spectral sensitivity of 320 species of terrestrial wildlife that could be affected by the nighttime lighting of California’s highways19. These included Spiders, Insects, Amphibians, Reptiles, Birds and Mammals. The lesson learned is that the diversity of responses to light is enormous, so there is no ideal one size fits all solution to lighting the natural landscape – other than returning landscapes wherever possible back to the lighting conditions of the pre-electric world.
Restore the Night
The environmental light pollution problem with outdoor lighting can be solved with the flick of a switch. Unlike most environmental hazard clean-ups, the solution is instantaneous, has no residual effects, and actually saves money. The contribution from indoor lighting emanating from windows can be addressed by using light proof curtains, shades and blinds.
But sometimes we do need to use light outside. It may be for safety and security, avoiding trips and falls by illuminating a pathway or steps, or lighting a roadway where there are pedestrians crossing the street. But when we do this, we should follow the Five Rules of Outdoor Lighting.
Five Rules of Outdoor Lighting
Rule #1: All Light Should Have a Clear Purpose
Outdoor electric light at night is harmful to many species. Therefore, when we use light outside it must have a clear purpose. Not all roadway lighting increases safety. It can be just as effective to have reflectors (cat’s eyes) marking the edges of each travel lane that reflect light from oncoming headlights, but emit no light themselves.
Rule #2: Light Should Be Directed Only Where Needed
Outdoor lights should be focused and shielded so light is only directed to where it is needed. For example, streetlights should only illuminate the roadway and sidewalks, and cast no light on hedgerows, grass verges and trees, or into the sky. Pathway lights should not shine into the rest of the garden, or the neighbor’s yard, with lighting as low to the ground as possible.
Rule #3: Light Should Be Used Only When Needed
If you cannot turn off the lights completely, consider using motion sensors and automatic timers so the lights are switched on, only when it is absolutely necessary. When lights are switched off, even temporally, it allows birds and insects to escape the fatal attraction of the lights. There is considerable experience in Europe with “dynamic roadway lighting” where lights are dimmed after peak traffic hours with no significant increase in collisions.
Rule #4 Light Sources Should Emit Less Than 2% Blue Content.
While there is considerable inter-species variation, on balance there is less disruption to flora and fauna when wavelengths are removed below 500 nm. As we discussed in Chapter 11: When You Don’t Control the Space , the Hawaiian island of Maui has passed an ordinance that outdoor lighting should have less than 2% 400-500 nm blue, and the major LED manufacturer Lumileds now produces LEDs that can meet this standard.
Rule #5: Light Should Be No Brighter Than Necessary
Even if you obey the other rules, you should always seek to use the dimmest light levels that will meet your objectives. Many of the species which co-habit this planet with us are highly sensitive to light levels above starlight, and our well-being depends on a flourishing biodiverse ecosystem.
In this chapter you have learned that:
1. Our unrestrained use of cheap outdoor LED light is increasing the brightness of the night sky by 7-10% per year.
2. We are entitled to control the lighting of our own indoor domain for the benefit of ourselves and our family members, but outdoors is a very different matter because light at night disrupts our planet’s fragile ecosystems which support 8.7 million other species.
3. We need to adopt policies which restrict when and where outdoor light is used and restrict its brightness and spectral content of short wavelengths.
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