At World Water Week in August, water pollution was on the agenda at the forum in Stockholm. The EU issues guidelines on soil pollution. Search Google for 'air pollution' and you'll get over sixteen million references. But there's another kind of pollution that you don't hear so much about - light pollution. Even the name is careless, since it really ought to be called 'dark pollution'. It's the dark that is being polluted after all.

Light pollution, in the environmental context, is excessive light. It may be from streetlights, billboards, stadiums, houses or parking lots. It's worse in cities. I know a thing or two about light pollution myself - I live about a mile from an airport, and there's a permanent unearthly glow from the horizon.

It's a growing problem. A quick comparison between satellite photos from the eighties and from today tells you all you need to know - we just love lighting stuff up. There are good reasons for doing so. People feel safer in well-lit cities. It's easier to find where you're going, and safer for drivers. In winter in more northern regions, the sun has gone down by the time you get out of work or school, and artificial light is important in making the remaining hours of the day useful.

However, there are serious problems with artificial lighting too. The most obvious is waste. We've got ourselves into some typical cheap-energy bad habits, and now that energy efficiency is a priority again, we might have to smarten up. The worst contributors are street lights, of which the UK has 7.5 million, each one contributing 400kg of CO2 to the atmosphere annually. Up to 30% of street lighting is wasted upwards or ‘spilt' into areas it isn't needed. Across the country, that adds up to 830,000 tonnes of CO2 just from the wasted energy each year, according to the Campaign for Dark Skies. The same campaign points the finger at floodlights too. Just one 500W floodlight left on all night, either for security or to illuminate a monument, can account for more CO2 than some small cars.

The second big impact of excessive lighting is on wildlife. It is known to affect bats, whose navigation systems are disrupted by artificial light. Bats emerge at dusk, when insects are most active. If there is too much light, the bats cannot tell when dusk actually is, and they miss the prime feeding time. Birds have a similar problem at dawn. The Royal Society for the Protection of Birds has noticed that the ‘dawn chorus' begins at different times depending on how lit the surrounding area is. In areas of high light pollution, birdsong can continue through much of the night, and then start later in the morning. The light interferes with birds' sleeping patterns, so they are awake too long. When dawn comes, they don't notice it until later, again missing the prime dawn feeding times when worms and snails are on the surface. Light pollution has been suggested as the culprit in one of England's conservation mysteries - the almost complete disappearance of sparrows from the urban environment.

Illuminated buildings are also a collision hazard for migrating birds, as the Royal Ontario Museum highlighted recently with a display of 2000 dead birds killed in Toronto. Insects are also confused by artificial light, as we all know. The moths and midges attracted to light bulbs often die of exhaustion, reducing their numbers in well-lit areas, with impacts up the food chain. Other species disrupted by artificial light include glow worms and snakes, and even trees are affected. Trees that have been floodlit, in parks for example, lose their leaves later in the autumn.

So how do we turn down the lights? One of the most obvious solutions is to switch lights off during the night, at least during the hours when nobody needs them, between midnight and 5pm. The city of Rome did just this, and also dimmed its street lights in 2005, reducing the intensity of the brightness. Businesses are encouraged to turn off internal lights, and stores to turn off their signs after hours. (The Howies shop in London has an unusual solution. They have a light switch in the street. The shop lights are off at night, but if you want to see in for any reason, you can flick the switch and the lights will come on, on a timer, so you can see what you're missing.)

A number of towns and cities in the UK are currently experimenting with turning off street lights, and this is proving quite controversial. A borough near where I live is running a pilot scheme that will save 600 tonnes of CO2 a year, and £100,000. It's been very unpopular with residents, and with the press, who are worried about crime. Surprisingly though, the relationship between crime rates and lighting is complex. Sometimes crime is reduced, at other times it is simply displaced. Experiments in lighting alleys in Chicago (pdf) actually prompted a 21% increase in crime, because people don't hang around in the dark. An extensive study by the UK government (pdf) concluded that "lighting improvements are in general more likely to have a positive effect on the public's fear of crime than on the incidence of crime itself." In other words, we're still afraid of the dark. 

Despite this, the many campaigns to reduce light pollution are gaining ground. One solution is to make lights more effective. Since a lot of illumination escapes upwards into the sky, better designed street lights can make a big difference. Newly installed street lights are much more likely to be downward-directing, and many older ones are being replaced or upgraded. Dimming the lights would also help, or turning off one light in two.

A lot of monuments and statues are illuminated at night. The Church of England recently took a lead on this, publishing a book called ‘Don't stop at the lights' to educate churches about climate change - including switching off floodlit churches and saving the dramatic lighting for special occasions.

Perhaps the most important thing though, is to learn to appreciate darkness again. Light pollution obscures the stars. Perhaps we should teach astronomy more, and take a greater interest in the sky above us. Maybe we'd learn to value darkness that way, rather than seeing it as something to be feared. We've become accustomed to well-lit cities, but it didn't used to be that way. Small personal choices, like reducing the intensity of light bulbs, or switching off floodlights, will all help, but ultimately we need to learn to embrace the darkness.

Further Reading:

• Light and Dark in the Spotlight