Whether you think you might win over the crowd who're watching from the sidelines or change a denialist's mind, John Timmer's flowchart presents tried-and-true tactics for using science, reason, and facts to overcome ignorance and fear.
Crucially, Timmer also tells you when to walk away (when you're talking with a "hardcore, committed denialist" and there's no audience to hear your argument and realize how poor the other side's arguments are when viewed against them).
They will say something absurd—it will either be something that's wrong, or something with absurd consequences, or something that they don't really understand. Latch on to that and don't let it go. Force them to address it. Ken Miller, a biologist who's involved in a lot of creationism/evolution debates, said his favorite tactic is to take some of the things that creationists say to their logical end. If creationists think that there was a global flood followed by an ice age, make them explain where the water came from, calculate where all the energy involved in freezing all that water went in a few hundred years—and then have them figure out where the energy that melted the ice came from.
Another good thing to latch on to is a logical inconsistency. Climate change denialists tend to deride computer models, apparently not realizing they're used ubiquitously in science. So, there's a chance that some of the studies they like (because they suggest a lower climate sensitivity or something similar) also used a computer model. Call them on this, and don't let it drop. They'll also object to the policy implications of the science; they're separate, and it's often easy to make that clear.
Ars Science Q&A: How to deal with science denialists
[John Timmer/Ars Technica]
(Image: I Can't See You…, Peter, CC-BY-SA)