Here is an idea for cooling the planet that I have talked about before.

http://www.technologyreview.com/Energy/18175/page3/

This idea will work. When a volcano erupts much dust is ejected and goes to high altitude. The earth then cools for a year or so. This has been observed many times. This caused the famous "year without a summer" in the early 1800's. I don't think anyone would do this unless the earth actually started getting much hotter because it would be relatively easy to put the earth into another ice age. (Just based on history we may be nearing the start of another ice age anyway. In the past the ice ages have developed over a very short time such as 20 to 50 years.) One aspect of this is that it is relatively inexpensive, especially compared to the cost and economic losses associated with draconian reductions in carbon dioxide politically imposed. But, the socialists wouldn't like this approach because it would eliminate the need for them to regulate life.