See:

• Cooling the Earth: CO2, SO2, and The Sunscreen Fix by technopolitical on October 16th, 2006
  
• Geoengineering: basic principles, some thoughts, some questions by a siegel on March 6th, 2008
  

And then the following illustration (h/t Fran in the March 31 Salon) of how a complex system responds to forcing

According to Indian and German researchers, an experiment that involved dumping tons of dissolved iron into the Southern Ocean does not appear to be a viable way to prevent global warming.

...

The iron stimulated growth of planktonic algae called phytoplankton, which researched had hoped would absorb and reduce the amount of carbon dioxide in the atmosphere.

But the scientists from India's National Institute of Oceanography (NIO) and Germany's Alfred Wegener Institute (AWI) did not expect the phytoplankton to be eaten by crustacean zooplankton.

Most economists teach a theoretical framework that has been shown to be fundamentally useless. -- James K. Galbraith