The SRES scenarios have been used to project future atmospheric GHG concentrations. Under the six illustrative SRES scenarios, the IPCC Third Assessment Report (2001) projects the atmospheric concentration of carbon dioxide () in the year 2100 as between 540 and 970 parts per million (ppm). In this estimate, there are uncertainties over the future removal of carbon from the atmosphere by carbon sinks. There are also uncertainties regarding future changes in the Earth's biosphere and feedbacks in the climate system. The estimated effect of these uncertainties mean that the total projected concentration ranges from 490 to 1,260 ppm. This compares to a pre-industrial (taken as the year 1750) concentration of about 280 ppm, and a concentration of about 368 ppm in the year 2000.