It seems longbets.org would be a more appropriate arena. "The average temperature of the earth will (not) rise by more than X degrees in the next twenty years."
That would be an excellent bet for the person betting that the temperature will increase. However, it doesn't prove that human actions are causing it. I think most deniers do believe that the climate is changing - even that the temps are going up. The problem is that causation is hard to measure. I'm not a scientist, but I think theories explain causes, and you can't directly measure a theory, only the observations that support it.
So, maybe a bet that includes more measurements (green house gas emissions, etc.) would be needed to prove man-made global warming. The outcome of the total of those bets would either support or not support the theory. But that might be a lot of bets and a mixed result could complicate things.
