Light Pollution
For most of human history, anyone who looked up on a clear night could see the Milky Way. Today around a third of the world's population can no longer it from where they live. Street lamps, illuminated advertisements and office buildings that stay lit all night a glow that spreads far beyond the cities themselves. The consequences are not only aesthetic. Many birds by the stars when they migrate, and bright lights can draw them off course. Turtle hatchlings, which instinctively head towards the brightest horizon, may crawl inland instead of towards the sea. In humans, exposure to artificial light late at night has been to poor sleep. The encouraging news is that, unlike many forms of pollution, this one can be almost immediately: shielding lamps so that they point downwards and switching off lights that nobody needs would make a difference within a single night.
