Research has revealed that light pollution, the presence of excessive artificial lighting at night, is causing the night sky to brighten at a rate of around 10% a year. This dramatic increase threatens to obliterate our view of all but the most brilliant stars in the next 20 years. Already, the Milky Way is no longer visible to more than a third of the world's population. Light pollution also disrupts the natural world and may be affecting the health of animals, including humans. External lighting, street illumination, advertising and brightly-lit sporting venues are to blame, as is the widespread use of light-emitting diodes (LEDs).

1st June 2023

Around 83% of the world's population—including 99% of Europeans and Americans—live under light-polluted skies (those that are more than 10% brighter than natural darkness). Some 23% of the world's land area is affected by what is called sky glow, the scattering of light in the atmosphere at night. Globally, this proportion has increased by about 50% from 1992 to 2017. It is a major side-effect of urbanization, the migration of population from rural areas to cities.

Blue light

Artificial light sources during normally dark periods can disrupt levels of melatonin production. Melatonin is a hormone that controls the circadian rhythm—the internal, 24-hour clock that guides day and night activities and affects body processes in animals—including humans. Increased amounts of light at night lower the production of melatonin, which results in sleep deprivation and may cause other health problems, possibly including obesity and diabetes. This is especially true of blue light. This is light from the blue end of the visible spectrum, emitted by mobile phones and other computer devices as well as LED (light-emitting diode) lights—now commonly used in streetlighting and advertising displays.

Light pollution also has serious consequences for the natural world. Migrating birds—often flying by night to avoid the dehydrating heat of the day—are guided by moonlight or, to some extent, by the position of the stars. Sky glow causes them to get confused and lose their way. Insects, a key source of food for birds and other animals, are drawn to artificial lights where they are instantly killed upon contact with them.

Marine turtle hatchlings find their way to ocean from the beach where they hatched out by moving away from the dark silhouette of sand dunes and their vegetation. Sky glow can interfere with this behaviour.

Studies indicate that light pollution around lakes prevents zooplankton (microscopic animals) from eating surface algae. This may cause algal blooms, which can kill off plant life in the lake and lower water quality.

Astronomy

People living in cities experience high levels of sky glow at night. This reduces the contrast between stars and the sky itself, making it much harder to see faint objects. As a result, city-dwellers can make out no more than a handful of the brightest stars at night. The view bears no resemblance to what can be seen in truly dark skies.
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For astronomers this is a serious problem as it reduces their ability to view celestial objects. To avoid light pollution (and the worst effects of atmospheric turbulence), ground-based observatories are located in remote, mountainous regions. But this cannot overcome another recent source of light pollution: artificial satellites. With the steep increase in numbers of satellite constellations such as OneWeb and Starlink, it is feared that ground-based observation will soon no longer be possible.

What can be done?

The introduction of a small number of inexpensive measures could easily tackle the problem. Ensuring streetlights are properly shielded and direct light downwards instead of up into the sky would make a significant difference. We can all help by using outdoor lighting only when and where it is needed and by closing blinds, shades and curtains at night to keep light inside.