Six of the SRES emissions scenarios have been used to project possible future changes in atmospheric concentrations. For the six illustrative SRES scenarios, IPCC (2001) projected the concentration of in the year 2100 as ranging between 540 and 970 parts-per-million (ppm) . Uncertainties such as the removal of carbon from the atmosphere by "sinks" (e.g., forests) increase the projected range to between 490 and 1,260 ppm. This compares to a pre-industrial (taken as the year 1750) concentration of 280 ppm, and a concentration of 390.5 ppm in 2011.