Causes

  Since the advent of electric lighting, artificial light has increasingly forced its way into previously unlit nighttime environments (Longcore and Rich). Due in part to the rapid global urbanization that has taken place over the past few decades, the world continues to grow brighter. Light pollution is, in fact, one of the fastest growing forms of environmental alteration today, with artificial light in certain geographic regions growing by as much as 20 percent per year (Holker et. al.). Artificial light is obviously something that modern society demands. Much of this light, however, is excessive and misdirected, harming the environment while providing little benefit for mankind. It is this light that is at the heart of the pollution problem.

Increasing Sky Brightness in the US Due to Artificial Lighting
 http://www.nature.nps.gov/night/assets/images/lightPollutionGrowthLarge.jpg

Artificial light comes in many forms, ranging from streetlights, to homes, to cities and everywhere inbetween (Galloway, Olsen, and Mitchell). Every potential source of artificial light, no matter how seemingly insignificant, can have a large impact on the surrounding environment. Nevertheless, some forms of artificial lighting are more significant contributors to light pollution than others. The main source of light pollution is, as may be suspected, outdoor lights, particularly those that emit light upwards or sideways (National Parks Service). One study demonstrated that the lights that contribute most significantly to light pollution are those that point just above the horizontal plane. The study shows that the sky glow produced by these rays is a remarkable 6 to 160 times greater than the sky glow produced by equally powerful rays pointing directly at the ground (Lubinbuhl, Walker, and Wainscoat). The figure below displays how a more "sideways" emmitance (angles closer to perpendicular) scatters light over a larger area, increasing the influence of light pollution.

The Influence of Lighting Angle on Sky Glow
(Courtesy of Luginbuhl et. al.)

Lights pointing upward, particularly those that are unshielded, increase light pollution 
(Courtesy of National Parks Service)

Although light pollution is more severe in certain areas of the world, the problem is a global one, and not limited to more industrialized areas. According to the first World Atlas of the artificial night sky brightness, nearly two-thirds of the world population lives in areas significantly impacted by light pollution. For the United States and European union, this number rises to 99 percent (Cinzano). Light pollution is clearly an issue that cannot be ignored, as its influence is seen virtually everywhere.

In many countries throughout all parts of the world, light pollution is not confined to urban areas. As displayed above, significantly more people live under polluted night skies than just the people in urban areas.
(Novel figure: Sky brightness data courtesy of Cinzano et. al., urban population data courtesy of UN Department of Economic and Social Affairs)