How the Atmospheric Sun Shade Could Work

By: John Fuller

A woman reads a book under the shade of a tree as she relaxes in Hyde Park in London, England. If a quick solution to global warming ever becomes necessary, some experts have suggested shading the Earth. See more green science pictures.
Oli Scarff/Getty Images

Ever since reports on global warming have cited humans as the likely cause of rising temperatures around the world, the debate about how much we're involved has hardly ceased. The production of carbon emissions, emitted after the combustion of fossil fuels, is most likely why more greenhouse gases (like carbon dioxide and methane) have become trapped in the Earth's atmosphere. This effect is like a double-edged sword, because while carbon dioxide and methane allow sunlight to enter the atmosphere, they also keep it from bouncing out, raising temperatures slightly across the globe.

Some scientists also warn about the problems associated with global warming and the likely consequences if nothing is done about it. The potential dangers of global warming range from the uncomfortable to the disastrous -- according to reports, an increase in the number of kidney stones is already underway thanks to higher temperatures, whereas rising sea levels, flooding and famine represent the apocalyptic version.


Many agree that the most significant solution to combat global warming is the development of renewable energy. While that may be a long time coming, for the moment, scientists, theorists and experimentalists are thinking up temporary fixes that might help the Earth in the event of an emergency. If temperatures start rising drastically, along with instances of disease, famine and conflict, something may have to be done -- and quickly.

Some of the ideas being tossed around have been inspired by a place people often seek when we're outside and it's too hot -- the shade. A tree, an umbrella, an overhang on a building or anything else that can block the sun's rays from shining directly on us (and damaging our skin) has proven a simple but effective way to cool down.

So this has led some scientists to wonder: What would it be like to block the sun's rays on a much larger scale? Could we actually shade the Earth in some way to lower temperatures? A few experts have offered up a potential solution -- the atmospheric sun shade. But is it just science fiction, or could it actually work?


Cool It Down: Aerosol Injection

Injecting light-scattering aerosols into the atmosphere could cool the planet, and it's actually happened before when Mt. Pinatubo erupted in 1991.
Injecting light-scattering aerosols into the atmosphere could cool the planet, and it's actually happened before when Mt. Pinatubo erupted in 1991.
StockTrek/Getty Images

The idea of an atmospheric sun shade isn't exactly new. Scientists have suggested a number of different ways to shade the Earth to cool it down over the years. They've ranged from the potentially plausible to the technically monstrous and incredibly expensive. In 1989, for instance, James Early at the Lawrence Livermore National Laboratory in California proposed building a massive 1,242.7-mile (2,000-kilometer) diameter glass shield that would hover along L-1, the orbit that aligns with the sun.

Other designs have been more lightweight. One popular suggestion has been the scattering of aerosols into the atmosphere. To some this might sound like a dangerous thing to do, especially when you think about aerosol cans and their link to air pollution. But does this mean we'd all have to point cans of hairspray into the sky and spray away? Not exactly.


Along with main gases nitrogen and oxygen, the atmosphere has a collection of both solid and liquid particles called aerosols, or particulate matter. These small pieces of matter are tiny in size, ranging between 0.01 and 10 micrometers in diameter, and they continuously float around the atmosphere. Larger aerosols are simply sea salt, dust and other bits of matter that get blown upward by the wind -- they're washed back down to the Earth's surface when it rains or snows. Smaller aerosols, on the other hand, include carbon particles, which get up there when we burn fossil fuels and, because they trap and absorb light, they play a part in global warming.

However, certain aerosols have an ability to scatter light instead of absorb it. Regular air molecules can't change the direction of radiation beams from the sun because they're too small, but larger aerosol particles, when they absorb water, increase in size and block light from the sun from entering the atmosphere.

This creates a cooling effect, and the Earth has actually witnessed a cooling effect like this before. In 1991, when Mt. Pinatubo erupted in the Philippines, enough aerosols were released into the air to cause a noticeable drop in average global surface temperatures for quite a while.

But are there any other alternatives to injecting the atmosphere with aerosols? And is an atmospheric sun shade a good idea in the end?


Lightweight Flyers and Side Effects

Flying discs might be able to form a large cloud between the Earth and the sun to block sunlight, but they'd also reduce solar power generation at the same time.
Flying discs might be able to form a large cloud between the Earth and the sun to block sunlight, but they'd also reduce solar power generation at the same time.
Pete Starman/Getty Images

Aerosols aren't the only possible theory for an atmospheric sun shade. The most recent method for giving the Earth a little rest from the sun's rays, proposed by Roger Angel at the University of Arizona, involves sending trillions of lightweight, transparent discs out into space where they could block some incoming radiation.

The discs would act like small spaceships hovering about one million miles (1,609,344 kilometers) above the Earth's atmosphere in one big cloud. The long line of discs would have a diameter of about half the Earth's (which is about 7,900 miles, or 12,700 kilometers) and be 10 times longer. Angel designed the lightweight flyers to be made of a transparent film, measure about 23.6 inches (60 centimeters) in diameter and covered with tiny holes.


The end result would be that the discs could direct as much as 10 percent of the sun's light passing through the cloud away from the Earth. This could reduce sunlight by 2 percent over the surface of the entire planet, cooling things down significantly. The lighter weight of the discs provides an advantage over the previously-mentioned glass shield theory because getting them outside the Earth's atmosphere wouldn't require quite as much effort. In fact, hydroelectric power could be used to power electromagnetic launchers, which Angel suggests firing every five minutes for a duration of 10 years in order to send off an acceptable number of flyers. The price tag for the project is anything but tiny -- the sunshade flyers might cost several trillion dollars, or about $100 billion every year, but Angel believes the benefits of his sun shade system would last for about 50 years.

Although scientists have been speculating for decades about how an atmospheric sun shade could work, many skeptics have pointed out that the scheme won't work at all or at least that it won't help other aspects of power generation. The unfortunate side effect of shading the Earth is the unintentional reduction of peak solar power productivity. An atmospheric sun shade, whether it involves scattering particles into the air or sending out millions of thin flying discs, could deflect as much as 20 percent of solar power from power producing plants.

If you'd like to find out more about the atmospheric sun shade or other related topics, follow the links on the next page.


Lots More Information

Related HowStuffWorks Articles

  • COSMOS Magazine. "Sunshade in space to cool planet." Nov. 6, 2006. (July 6, 2009)
  • "Space sunshade might be feasible in global warming emergency." Nov. 3, 2006. (July 6, 2009)
  • National Oceanic and Atmospheric Administration. "Atmospheric 'Sunshade' Could Reduce Solar Power Generation." March 11, 2009. (July 6, 2009)
  • Rosenthal, Elisabeth and Revkin, Andrew C. "Science Panel Calls Global Warming 'Unequivocal.'" The New York Times. Feb. 3, 2007. (July 6, 2009)
  • The Habitable Planet. "Unit 11 - Atmospheric Pollution." (July 6, 2009)