Read here, here, and here. Some scientists involved in the Climategate scandal (emanating out of the UK's CRU facility) were also participants in determining the historical CO2 levels to be used in climate models. They established that pre-industrial CO2 levels (see solid green curve on chart below) were at a 270/280 ppm level in the atmosphere. This level was first determined by a single earlier scientist cherry-picking only selected CO2 data readings that matched his desired low CO2 level goal; and later, scientists had difficulties matching the ice core CO2 data with the previous cherry-picked CO2 records, so they just "adjusted" the ice core CO2 data to fit their needs. Obviously, as with the "adjusted" temperatures scandal, Climategate-type science has infected the CO2 data.xx
In summary, there is strong empirical evidence that pre-industrial CO2 atmospheric levels were some 15 to 30% higher than IPCC assumes. This means that IPCC climate model' formulas are based on incorrect, "adjusted" CO2 data.
This chart shows human CO2 emissions and CO2 atmospheric levels (IPCC estimated "adjusted" levels from ice cores and probable actual historical levels from non-ice core data). (click on image to enlarge)