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.

Not really, because you'd have to agree on how to measure average temperature of the earth.