"Climate-engineering" efforts could increase urban light pollution by 25 percent or so, a new study suggests.

Tinkering with the sky to fight climate change would make it more difficult for astronomers and skywatchers to observe the heavens, a new study suggests.

Spraying particles into the atmosphere to reflect sunlight and help cool the Earth down — a strategy known as stratospheric aerosol injection (SAI) — would brighten city night skies considerably and decrease light clarity in rural areas, the study said.

SAI would be "very, very bad news for astronomy," said study author Charles Zender, a professor in the departments of Earth system science and computer science at the University of California, Irvine. [Changing Earth: 7 Ideas to Geoengineer Our Planet]

A warming world

Earth's average surface temperature has risen by about 2 degrees Fahrenheit (1.1 degrees Celsius) since record-keeping began in the late 19th century, according to new analyses by scientists at NASA and the National Oceanic and Atmospheric Administration. Furthermore, 16 of the 17 hottest years in recorded history occurred from 2001-2016, and the last three years have been the hottest of all, the researchers found.

This warming trend — which climate scientists attribute primarily to the huge amounts of heat-trapping carbon dioxide that humanity pumps into the atmosphere every year— has already affected the planet in significant ways.

Glaciers are retreating around the globe, for example, and more and more Arctic sea ice is melting every summer. A variety of plant and animal species are shifting their ranges toward the poles or up mountain slopes as their habitats warm, and some birds are migrating significantly earlier in the spring than they used to.

Such impacts will intensify the more that Earth warms, scientists have said. Indeed, the latest report from the Intergovernmental Panel on Climate Change recommended that humanity work to keep average global temperatures from rising more than 3.6 degrees F (2 degrees C), in order to stave off climate change's worst effects.

Climate engineering

SAI provides one possible way to help accomplish this goal, some researchers have said. The basic idea involves using planes, balloons or artillery to seed the atmosphere with sulfates or other particles that would reflect sunlight back into space.

SAI and other "climate engineering" concepts have not yet been widely applied; scientists are still trying to determine how effective such efforts would be, and identify likely side effects, before putting the ideas into practice.

Zender's new study, which he presented last month at the annual fall meeting of the American Geophysical Union (AGU) in San Francisco, is part of this sussing-out effort. He used computer models to estimate how a large-scale SAI project would affect humanity's view of the night sky.

The results should interest professional and amateur astronomers alike, he said. For example, Zender found that SAI would increase the brightness of urban night skies by about 25 percent, bouncing artificial light back down to Earth and making it even harder for city dwellers to appreciate celestial sights.

"Basically, take a magnitude or two off the faintest visible galaxies and stars," he told Space.com at AGU.

Zender's modeling work also indicated that SAI would make rural night skies slightly darker, by reflecting starlight back into space. This would cause a large drop in light clarity, Zender said.

"It would be considerably more challenging to do optical ground-based astronomy," Zender said, though he noted that infrared instruments, which collect longer wavelengths of light, would not be affected.

Some astronomers may be willing to accept this trade-off — as long as SAI works as advertised.

"Fixing global warming is more important than astronomy," David Grinspoon, an astrobiologist at the Planetary Science Institute in Tucson, Arizona, said during a panel discussion at the 229th meeting of the American Astronomical Society in Grapevine, Texas, earlier this month.

"There's no question to my mind that saving our civilization and many other species is more important than our ability to do ground-based astronomy for a few decades," Grinspoon added.

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