The November issue of Scientific American lays out three of these plans. "A Sunshade for Planet Earth" by Robert Kunzig illustrates the ups and downs of these hypothetical projects.
The first involves the same scenario that quite possibly killed the dinosaurs. Sulfur, Brimstone, the Devil's smoke is the key but may become a culprit. The theory that volcanic activity killed the big lizards gained credence when Mt. Pinatubo in the Philippines erupted in 1991. The earth's overall temperature went down by one degree that year. Rainfall also dropped because evaporation was retarded by the lack of sunlight. This may help topsoil maintain its humidic integrity, but it also robs the rest of the planet of rainfall and fresh water. It also requires less carbon emissions or the upkeep would be more than it's worth.
The second involves making our clouds' metaphorical silver lining a thing of reality. Huge, satellite-guided ships would troll the oceans, shooting sea water into turbines (fueled by the churning of water and wind - NICE!), shooting vaporized sea water into the air. The sodium would bond to the clouds, making them thicker to block out solar radiation. However, rainfall would drop, brightening of the atmosphere may be unpredictable and the political repercussions are unknown. What happens in the ocean would affect airspace not necessarily belonging to the country involved and could cause problems. And, again, if carbon emissions are not curbed, it may be a moot point.
The third involves launching a cloud of satellite controlled plastic disks out of the atmosphere to deflect harmful sunlight while letting enough through to ensure survival of the flora and fauna that rely on it (i.e., every living thing on the planet). The prototype is a silicon nitride ceramic paid for by the Discovery Channel. Fractions of the width of saran wrap, it is far stronger and channels the good energy in, bad energy out. Set at L1, a Lagrangian point where the sun's gravity is equal to the earth's, the discs would act like a cosmic parasol, letting some energy in and scattering the rest.
All amazing, awesome ideas to cool the planet and keep the polar bears and penguins living in the style to which they are accustomed, but the underlying problem still exists.
Carbon emissions must be restricted, outlawed or replaced altogether. The above are referred to as a "quick fix."
As cool as they are, does that ever really work?