There is a great article in the Atlantic Monthly about really cheap ways that the climate of the earth could be modified, even to the extent of creating another ice age. By really cheap, I mean hundreds of millions of dollars for some projects up to a few billion for others.
The article is interesting not only for describing a number of ways that climate modification can be done, but also for discussing a lot of potential adverse consequences. Be careful of a bargain is the message. Caveat Emptor.
Read the entire article here. I found it absolutely fascinating.