There is a sure-fire gambit I use on Climate Change Deniers (CCDers) that has yet to be confounded, and which doesn't involve all that tedious mucking about with graphs and arguing with religious nutters.
Does the CCDer accept the existence of chaos and complexity, encapsulated by Lorenz and Merilees as the Butterfly Effect? i.e. A flap of a butterfly’s wings in Brazil can set off a tornado in Texas (In the unlikely event that the CCDer denies the existence of chaos, refer them to the laws of thermodynamics).
OK, that was the hard part. Once the CCDer accepts the Butterfly Effect's existence, it's a fairly short step to point out that digging up carbon deposits built up over hundreds of millions of years and squirting it into the biosphere over a couple of centuries is bound to have an effect.