Artificial night lighting is one of the most pervasive — and yet under-recognised — causes of environmental pollution.
Although technological advances have vastly improved the cost effectiveness and energy efficiency of lighting over recent decades, these potentially positive changes are offset by an ever-increasing use of cheaper lights (the so-called rebound effect). Globally, external lighting is increasing by around 6% per year; in the UK we have more than 9 million street lights, and more and more buildings are illuminated at night on the grounds of security or aesthetics, and there is a trend towards constructing buildings with large, unshielded windows. The consequent light pollution is so intense that it is literally visible from outer space (see image), and the sky glow from cities can be seen hundreds of kilometers away. In addition, the spectral composition of night-time lighting is shifting, with a move from familiar orange street lights to white lights that are more similar to daylight. These changes have profound effects on the natural environment.
There is now overwhelming evidence that bat communities are profoundly affected by light pollution. Many species — including all those currently on the British Red List — avoid lit areas for foraging or commuting, while the illumination of historical buildings at night can prevent bats accessing their roosts. Moths are also disrupted by light pollution, an effect so profound that, historically, lamp lighters were paid to sweep up the piles of dead moths and cockchafers that would accumulate nightly below their lamps. This effect continues with our modern lights (albeit with smaller starting populations, so we no longer see large piles of dead insects); and the attraction can extend up to several kilometres from the source. In addition to causing direct mortality, street lights also alter the behaviour of a host of species, ranging from birds migrating at night, to the nocturnal foraging of predatory insects —with cascading effects on entire ecological communities.
Some Local Planning Authorities are taking steps to decrease light pollution, particularly in areas with sensitive bat species, for example by requiring forms of lighting that direct the illumination to only those areas where it is needed. It is also possible for us all to lessen light pollution by switching off our exterior lights at night, focusing security lights more precisely, choosing bulbs with lower light intensity, and avoiding placing outside lights on white walls, where their effect is magnified. Simply closing curtains at night also makes an important difference!
Professor Fiona Mathews
Mammal Society Chair
Professor of Environmental Biology, University of Sussex