This linked article is a good summary of why skeptics do not believe the theory that CO2 is causing global warming. The key assumption built into this theory are the huge feedbacks that have been asssumed. As can be seen in this paper, those feedbacks do not match reality. The author does a good job of presenting the science in layman terms.

http://joannenova.com.au/2012/01/dr-david-evans-the-skeptics-case/