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?