This is a very good article on how the science behind the theory is supported more by ideology than facts.
Myron Ebell, who works for the Competitive Enterprise Institute (CEI) in Washington, D.C., one of the few groups that examines global-warming claims skeptically, says that environmentalism is now a $1.5 billion industry. In Washington, skeptics (like himself) are outnumbered by global warming advocates perhaps by a margin of 300 to one. Yet CEI, greatly underfunded by comparison with groups like the Sierra Club, tends to be characterized in the media as "industry supported." The enviros' problem is that they have "everything going for them except the facts," Ebell says.