Curb light pollution in your city

Why light pollution?

Light pollution is increasing globally, with an estimated 80% of the world's population currently living under a “lit sky” - a figure closer to 99% in Europe and North America. The amount of artificial light on the Earth’s surface is increasing by at least 2% annually, while the use of artificial light at night is substantially increasing all over the world. To understand why light pollution is a problem, we first need to understand what it is.

Light pollution is a significant and growing threat to wildlife, including many species of migratory birds. Every year, light pollution contributes to the death of millions of birds by altering the natural patterns of light and dark in ecosystems. As a result, light pollution can change birds’ migration patterns, foraging behaviors, and vocal communication. Birds are attracted by artificial light at night, particularly when there is a low cloud, fog, rain or when they’re flying at lower altitudes. This causes migrating birds to become disorientated and potentially circling in illuminated areas. Consequently, their depleted energy reserves put these birds at risk of exhaustion, predation, and fatal collision with buildings.

Satellite monitoring revealed that from 2012 to 2016,

artificially lit outdoor areas increased by 2.2% per year. Recent studies show that this number could be much greater.

Light pollution can change birds’ migration patterns, foraging behaviors, and vocal communication.

What is light pollution?

According to CMS Resolution 13.5, “light pollution refers to artificial light that alters the natural patterns of light and dark in ecosystems”. Artificial light is used to illuminate streets, commercial, residential and industrial properties, particularly in cities. Although artificial light is often used for security or safety reasons, monuments, churches, bridges and other landmarks may be illuminated at night for aesthetic purposes.

Light pollution and wildlife

The increasing use of lighting has modified the natural environment dramatically impacting wild animals, plants and the functioning of entire ecosystems. There is a lack of information regarding how artificial light at night impacts most species. Research so far has largely focused on some species of birds, bats, marine turtles and insects. As they are nocturnal, bats are particularly susceptible to light pollution which can disrupt their foraging, commuting, drinking, roosting and migrating behaviors. Light pollution can affect adult female marine turtles when they come ashore to nest, and hatchling turtles may be unable to find their way to the sea or to disperse successfully to the open ocean under conditions where light pollution is present. Light pollution is implicated, alongside other drivers such as pesticide use, in the massive global decline in insects. The physiology, behavior and fitness of diurnal and nocturnal insects can all be affected, with knock-on effects on pollination and food webs.

Further research is needed to investigate how light pollution impacts other groups of animals including fish, reptiles (other than turtles) and mammals (other than bats).

How does light pollution impact migratory birds?

Each year, light pollution contributes to the deaths of millions of birds from collisions with buildings and other infrastructure. Long distance migrants, such as the blackpoll warbler (Setophaga striata), the Asian stubtail (Urosphena squameiceps) and the oriental plover (Charadrius veredus), may start and end their migrations in areas with relatively low levels of light pollution, but during migration they may fly over areas of intense urban development where they experience high levels of artificial light. Migrating birds can be attracted to lights, particularly when there is low cloud, fog or rain and they are flying at lower altitudes.

Cities and light pollution

In urban areas, artificial light comes from many sources and light levels may vary during the night. For example, street lighting is sometimes kept at a constant level throughout the night, whereas light spilling from the windows of residential buildings may only be a contributing factor during the first hours of the night. In some locations, lighting at open air sports facilities can be a large contributor to light pollution. Other sources of light pollution include industrial, institutional and commercial (e.g. shopping centers, hotels etc.) sources, vehicles, airfields and construction sites. Wildlife near coastal cities may also be impacted by artificial light emission from vessels and offshore infrastructure.

In cities, it is important to consider the cumulative effects of different light sources rather than separating each type of lighting. In a city center street, for example, where signs and lights from shop windows provide enough light for people to walk, streetlights could be kept switched off.

What can be done to curb light pollution?

International efforts are underway to reduce the impacts of artificial light on migratory species. At the last meeting of the Conference of the Parties to the Convention on the Conservation of Migratory Species of Wild Animals (CMS COP13, 2020), the COP adopted Resolution 13.5 “Light Pollution Guidelines for Wildlife”, calling for the development of new guidelines, including for migratory landbirds. A draft of these guidelines has already been prepared and is under further revision following an expert workshop that was convened in March 2022. The guidelines will ultimately be presented to CMS Parties for adoption at CMS COP14 in 2023.

Guidelines on light pollution covering marine turtles, seabirds, and migratory shorebirds were endorsed by the CMS Parties in 2020. Among their recommendations, and particularly useful to cities, the guidelines set forth six principles of best lighting practices (see our Checklist) and call for Environmental Impact Assessments for relevant projects that could result in light pollution. These should consider the main sources of light pollution at a certain site, the likely wild species that could be impacted, and facts about proximity to important habitats and migratory pathways for different species.

Numerous cities have undertaken different approaches to curbing light pollution to protect migratory birds. These have mostly relied on creating awareness to facilitate policy changes. View our case studies here.

What can you expect from this Guide?

This guide aims to inform cities about the threats of light pollution to different species, but migratory bird species in particular. The guide also presents case studies and provides lessons learned to share knowledge and expertise across urban stakeholders to curb light pollution and protect bird species globally.

This guide was produced through collaboration with the following partners: