How the Atmospheric Sun Shade Could Work

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?

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.