(Part of the How to Talk to a Global Warming Skeptic guide)
Objection: The Hockey Stick graph -- the foundation of global warming theory -- has been shown to be scientifically invalid, perhaps even a fraud.

Answer: The first order of business here is to correct the mischaracterization of this single paleoclimate study as the "foundation" of global warming theory.
What's going on today is understood via study of today's data and today's best scientific theories. Reconstructions of past temperatures are about, well, the past. Study of the past can be informative for scientists, but it is not explanatory of the present nor is it predictive of the future. The scientific foundation of global warming theory contains much more than a few tree-rings and the temperature during the Medieval Warm Period.
RealClimate has an interesting article about what it would mean for today's climate theories if the MWP had indeed been warmer than today.
Now, about that pesky bit of sporting equipment ...