The projections are based completely on the temperature record, and changing the temperature record would produce different projections. The projections are essentially fudged to get the result that is desired for political reasons.
I'm guessing you haven't been exposed to many arguments against AGW, or maybe not many good ones. But since you have an open mind, I'd suggest you look into the absorbtion of IR By CO2, the actual proportion of CO2 in the atmosphere, the lack of tropophere warming (where it should show up) and the historical correlation of CO2 and temperasture-- CO2 has been a lagging indicator and has, in the past, been much higher than it is now from natural events without causing a runaway greenhouse effect. These are basic facts that don't take much analysis and greatly undermine the AGW theory. Also, the planet is getting colder in recent years (past decade) while CO2 has been rising.
Radiation transfer is what I do my research on (though not in the atmosphere), and it's something we know how to do. I really doubt the scientific field has screwed that up. Certainly all arguments I've seen regarding that have had crackpot quality. (I have some hope of getting around to actually running that problem myself, but "real" work always seems to get in the way.)
The projections were not based entirely on the Mann temperature record. There are satellite data, other analyses, etc., and I haven't seen to what extent changing one of those data sets impacts the projections.
I'm guessing you haven't been exposed to many arguments against AGW, or maybe not many good ones. But since you have an open mind, I'd suggest you look into the absorbtion of IR By CO2, the actual proportion of CO2 in the atmosphere, the lack of tropophere warming (where it should show up) and the historical correlation of CO2 and temperasture-- CO2 has been a lagging indicator and has, in the past, been much higher than it is now from natural events without causing a runaway greenhouse effect. These are basic facts that don't take much analysis and greatly undermine the AGW theory. Also, the planet is getting colder in recent years (past decade) while CO2 has been rising.