Light pollution is a serious topic.
The term refers to wasted light that goes up into the sky instead of illuminating the ground. Almost everyone on the planet has seen it; anywhere near a city the glow from all that light goes into the sky, washing out the stars. Even far from urban areas, the effects are felt.
The obvious problem is for astronomers, who have to fight this added light when observing faint objects. To do this we have to build observatories far, far from cities, or even up in space. That's expensive, inconvenient, and honestly just a pain in the rump.
But there are other issues as well: people lose sight of the sky, lose their ties with it. That's bad enough, but there is also concern that, like with chemical or other forms of pollution, wildlife is affected. Mating cycles, hunters and hunted, sleeping cycles: all are affected by our wasted light.
In the journal PLoS ONE, a paper has just been published about the amount of light pollution in the sky. It's sobering. Clouds over cities are very efficient at reflecting the lights (the picture above was taken at night, to give you an idea of how bad this is), and the researchers found that the sky brightness can increase by a factor of ten times in Berlin on cloudy nights. Of course, astronomers don't observe when it's cloudy, but the other effects on wildlife are still there... and this effect ranges far outside the city.
Maps like that will help scientists better catalog the effects of light pollution.
Now, I know some folks may poopoo this whole thing, and not really care that much about how this affects astronomers and critters. Well, think of it this way then: every photon directed into the sky is a photon not helping you see the ground. It's wasted energy and wasted money. More efficient lighting systems -- ones that direct all the light down instead of up -- save a lot of money. In many cases all it takes is a cover or other sort of shade over the top of a street light. You'll need fewer lights, too, again saving money.
Better lighting helps everyone: astronomers, animals, and (city) administrators.
For more information on this, please go to the Dark Sky Association's website.