Huge barriers exist unless you enjoy the sound of one hand clapping. PR firms wage campaigns in free media to change perceptions on issues, and nearly all media are on the AGW bandwagon. For instance the L.A. Times won't even publish letters to the editor that challenge Al Gore Warming.
If some wildly rich benefactor was willing to finance a paid commercial campaign it might make some inroads. But the giant main stream media, greedy though they are, would likely refuse to print/air such advertising. Fox would air commercials like that but it reaches viewers who generally don't need convincing -- one hand clapping.