One genre of question I always try to work in to my conversations with environmentally-minded people goes something like the following: "So, how much of the ozone layer (or how much mercury in a certain lake, etc) would you sacrifice to reduce atmospheric C02 by 10 ppm?"
Denying this question is valid is a black mark, at least in my books. I don't particularly encourage you to try it out, though - I haven't found it much use at making friends. In fact, I have yet to see it spark a productive debate, but it remains interesting to. The notion of tradeoffs is always relevant.
POSTSCRIPT: Posting from St. John's.