Ernie Schenck found this guerilla campaign from BUND, the German Friends of the Earth.The BUND people put these little globe balloons on the exhaust pipes of cars. The emissions inflate the balloons revealing the line, "The world can't take any more CO2." The balloon goes bang. The driver gets the message.
And Ernie has certainly a point here : "How, I'm not sure. Um, by the time I hear the bang and get out of my car, wouldn't it be kinda hard to read the line? Of course, that assumes the explosion doesn't scare the shit out of me and I drive smack into a bridge abutment. Now that would get my attention."
Besides that we've also seen the idea already in this campaign against climate change from Clemmow Hornby Inge.
UPDATE : and here is the idea again but just that bit smarter.