1) CO2 levels are going up due to man's emmisions.
2) CO2 is a greenhouse gas.
3) It is getting warmer.
The unproven claims are:
1) The current warming is unusual (this only began to be claimed in the mid-1990s, when Mann and others used flawed data to claim the MWP didn't happen)
2) CO2 has caused the current unusual warming (very dubious; CO2 levels generally trail warming historically)
3) CO2 levels will cause unprecedented warming in the future (even more dubious; CO2 levels have been much higher in the past, even during Ice Ages, and the climate has increasingly strong negative feedbacks at higher temps, as evidenced by the small range of temperatures over the last billion or so years)
4) This unpredecented warming will have large net negative consequences (possible, but historically warmth has been a net benefit)
5) Emissions controls will have an effect on #4 greater than their economic cost (very, very unlikely)
The problem is that even if SOME of the above are true, it is very unlikely ALL of those things are true, and they all have to be true for the solutions they are proposing to make sense.
Now, you'll notice 3,4,5 are all predictions about the future. As it happens, there is a scientific field devoted to future predictions, the science of forecasting. Forecasting scientists looked at the IPCC predictions and said they had, quote, "no scientific basis" and that they violated 72 essential principles of scientific forecasting in a situation where zero violations are acceptable.
http://wattsupwiththat.com/2009/01/28/f ... g-climate/