That solution was raised a few years ago, proposal was using hundreds of thousands of small aluminium very thin foils to somehow decrease the amount of sunligth coming to earth, I believed they shelved it for two reasons, first was not being able stop blocking the sun after a given amount of time and the second reason was the blockage will effect different parts of the world very differently north climates can freeze to death and milder climates can see total environmental destruction due to very quick fluctuation in temperatures.

It was a good idea even then but massive water evaporating ships ( http://www.scientificamerican.com/blog/post.cfm?id=geoengine... ) was better IMHO at least you can stop doing it whenever you want :)