Lighting up the city or protecting biodiversity: a false dilemma?

Léa Tardieu, Chloé Beaudet, Léa Mariton, Maia David, June 2025

The Conversation

Light pollution has many harmful effects. It threatens biodiversity, human health and even astronomical observations. However, to be effective, the measures put in place must go beyond the binary opposition between lighting and switching off lights. The challenge is to adapt to each local situation.

Light pollution has increased significantly in recent years (by at least 49% between 1992 and 2017) and continues to grow at an alarming rate (7 to 10% per year). This rapid increase is due to the proliferation of artificial light sources, resulting from urban expansion and changes in the light spectrum of lighting (e.g. cooler colours that have a greater impact on insects). The rebound effect of the switch to LED technology, which provides more light for the same cost, is exacerbating the situation by increasing the number of light points.

However, artificial light at night has many harmful effects, which have now been well demonstrated by the scientific community. It has an impact on biodiversity, human health, astronomical research and even, indirectly, on greenhouse gas (GHG) emissions due to the increased energy consumption of lighting.

The debate on lighting is currently reduced to an ‘all or nothing’ approach, depending on whether the priority is residents or biodiversity. Overcoming this binary thinking is the challenge addressed in the study we published in Nature Cities.

It shows that compromises between biodiversity and society are possible, but that no uniform policy will be effective. Only a night lighting policy designed at the local level, for each lighting point, and adapted to the environmental and social context, will make it possible to reconcile the needs of all stakeholders. These needs are sometimes concurrent and sometimes conflicting.

The multiple ravages of light pollution

In terms of biodiversity, first of all, night-time lighting disturbs a wide range of taxa (groups of species), both nocturnal and diurnal.

It masks the natural day-night cycle. Nighttime is a habitat for nocturnal species and a time of rest for diurnal species. Nighttime is, in a way, a hidden side of environmental policy that is too often overlooked.

However, the effects of light pollution on biodiversity have been documented at various levels: at the individual level (alterations in physiology, reproductive behaviour, feeding behaviour and orientation), at the community level (impacts on competitive and predatory interactions), and even at the ecosystem level. For example, it affects pollination processes, plant diversity and the functioning of temperate and tropical coral reefs.

But light pollution is also a public health issue. It interferes with circadian rhythms and melatonin production in humans, affecting sleep cycles, wakefulness, eating habits and metabolism.

It also interferes with astronomical observations by reducing the visibility of stars to the naked eye. From an average of 250 today, this number could fall to just 100 within eighteen years. While only 200 stars can be seen in a city like Milan (Italy), an unpolluted location offers around 2,000. Not only does this disappearance impair scientific astronomical observations, it also damages the cultural link that unites us with the night.

Finally, lighting associated with light pollution is a source of excessive energy consumption that generates CO2 emissions. Energy consumption linked to artificial lighting represents around 2,900 terawatt hours (TWh), or 16.5% of global annual electricity production and around 5% of CO2 emissions. This makes this sector a key issue in meeting the Paris Agreement targets.

Restoring night-time landscapes: a political challenge

Restoring night-time landscapes is possible, even in large cities. However, this requires political will: it involves raising awareness of these issues, taking decisions that are not solely dictated by the energy cost of lighting, and steering urban planning towards more sustainable lighting systems.

Ambitious policies, at both global and local levels, are therefore essential to significantly reduce and mitigate light pollution. In some countries, such as France, national legislation prescribes measures and thresholds for adjusting public lighting to illuminate target areas more directly and reduce light spill (decree of 27 December 2018 or the new bill for the preservation of the night environment).

However, compliance with regulatory thresholds in terms of colour temperature, light intensity and switching off remains, at this stage, at the discretion of local elected officials. The proposed law envisages giving this power to the French Biodiversity Agency (OFB). This raises questions, given the agency’s existing responsibilities, the increasing obstacles its staff encounter in the course of their work, and recent calls for its abolition.

Moving beyond an ‘all or nothing’ approach

Of course, it cannot be ignored that the urban context presents a challenge for urban planners. They may have to arbitrate between the preferences of residents and the needs of biodiversity.

‘Potentially’ because most light pollution reduction measures are welcomed by the population. Resistance (or the perception of resistance) to change, linked to safety concerns and crystallised around the issue of light extinction, is often the main obstacle to their implementation for local elected officials. It is also the main argument for backtracking.

Our study published in Nature Cities was conducted in the Montpellier Méditerranée Metropolis (3M), which comprises 31 municipalities, 507,526 inhabitants, and has the highest population growth in France (1.8% per year).

Light pollution is particularly problematic there due to its proximity to the Cévennes National Park, one of six French parks designated as International Dark Sky Reserves (Rice).

Adapting locally to needs

In the study, we are developing a spatial analysis that focuses on two aspects:

The needs of species were assessed using multispectral satellite images (i.e. images taken at several wavelengths of the light spectrum) with very high spatial resolution.

This makes it possible to define two dimensions of light pollution:

We then use local expert knowledge and naturalist inventories to predict landscape connectivity (i.e. the ability of species to cross the landscape to move from one habitable environment to another), with and without light pollution, for six groups of species that are particularly sensitive to light pollution or of interest to the region. These included: insects dependent on wetlands and Lampyridae, two groups of chiroptera (bats), Rhinolophus and Myotis, a group of amphibian species Pelodytes, Pelobates and Epidalea calamita, and the European nightjar (which belongs to a family of nocturnal birds, editor’s note).

This was carried out in collaboration with three associations of naturalist experts: the Office for Insects and their Environment (Opie), the League for the Protection of Birds (LPO) and the Languedoc-Roussillon Chiroptera Group.

This enabled us, first of all, to classify lighting points according to the urgency of reducing light pollution in order to protect biodiversity. Citizens’ preferences for different light pollution reduction measures (reducing intensity, switching off at different times of the night, changing colours, etc.) were then mapped based on the results of a large-scale choice experiment, i.e. a survey in which individuals were asked to choose their preferred option from several composite scenarios. This was conducted among 1,148 residents of the metropolitan area.

Finally, we cross-referenced the needs of biodiversity and citizens to identify mutually beneficial public lighting measures and those requiring compromise. The results were incorporated into an interactive application, SustainLight, designed to help decision-makers and citizens explore the various possible scenarios.

Adapting strategies to each area

Three main situations emerge from our analysis:

Our work confirms that, to be effective in preserving biodiversity against light pollution, sustainable lighting policies must be socially accepted and take into account the needs of both biodiversity and society.

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