Satellite data has shown that outdoor artificial light has risen steadily in recent years, according to new research. The findings raise alarm among experts over the still poorly understood effect that light pollution can have on humans and wildlife. The brightness and total area lit at night grew by roughly 2 percent each year in the years between 2012 and 2016. The growth in light pollution was more rapid in developing countries that were not already well lit.

The research was published Wednesday in the journal Science Advances, and writer Will Dunham detailed the findings for Reuters.

According to GFZ German Research Centre for Geosciences physicist Christopher Kyba, who led the new research:

“Earth’s night is getting brighter. And I actually didn’t expect it to be so uniformly true that so many countries would be getting brighter.”

Researchers cautioned that the data, from U.S. National Oceanic and Atmospheric Administration weather satellites, could be underestimating the extent of the problem. The satellites cannot detect the blue light of LED lighting, which is increasingly widely used.

Africa, Asia, and South America saw most of the expansion in nighttime light, which changed little in already well-lit countries such as the US, the Netherlands, and Spain. Again, however, the researchers warned that the satellites could be failing to pick up increases in LED light in those countries.

Some areas saw decreased lighting, such as Yemen and Syria, amidst war and political chaos. Wildfires decreased Australia’s outdoor lighting as well.

Some of the effects are alarming in principle. Four out of five Americans can no longer see the Milky Way in the night sky due to light pollution, and residents of brightly lit countries such as Qatar and South Korea can no longer see any stars in their night sky.

But on top of this, scientists have more practical concerns as well.

Both the World Health Organization and the American Medical Association have warned that exposure to light at nighttime can lead to increased risk of certain cancers, possibly due to interference with circadian rhythms and hormones. Nighttime lighting mat also have the potential for harmful effects on nocturnal wildlife, especially the relationship between predators and prey. The shift to LED lighting, while energy efficient, may be even worse for human health and ecosystems.

The growth of artificial lighting may be offsetting improvements from the use of LED lighting as well. According to Kyba:

“While we know that LEDs save energy in specific projects, for example when a city transitions all of its street lighting from sodium lamps to LED, when we look at our data and we look at the national and the global level, it indicates that these savings are being offset by either new or brighter lights in other places.”

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