The big thing we don't know is if there is some kind of "tipping point" where it will be very hard to correct the extra CO2 problem because it starts to get very bad, very fast. One such "tipping point" would be that the methane clathrates(methane trapped with frozen water) all over the earth start to release lots of methane causing more methane clathrates to melt and quickly we get a 3C or something increase in a decade.