The historical range, no the prehistoric range of CO2 in the atmosphere of earth has been between 180 ppm (parts per million) and 290 ppm for at least the last 650,000 years. This has been clearly established by the analysis of deep ice core samples from Antarctica.* As of early 2008, the level of CO2 in the atmosphere was already at 385 ppm and rising steadily. Discussion and debate have until recently been concentrated on the feasibility and cost of constraining the level of carbon dioxide at 450, 550 or 650 ppm and the respective catastrophic damage to the ecosphere of Earth that will come with each of these level. However one of the leading scientists in this debate, Dr. James Hansen of NASA has now gone on record as maintaining that we must immediately aim to reduce the level of CO2 in the atmosphere back to 350 ppm, a level which was last seen on earth in about 1983, or risk melting all the ice on earth, and raising the level of the ocean more than 200 feet for starters. This is a daunting task, but one that we absolutely MUST seek to achieve for the sake of life on Earth, including the future of our own species. * - The data which support this are part of the following papers: Science [Siegenthaler et al., 2005; Spahni et al., 2005]; EPICA Community Members, Eight glacial cycles from an Antarctic ice core, Nature, 429, 623-628, 2004; Indermühle, A., et al., Atmospheric CO2 concentration from 60 to 20 kyr BP from the Taylor Dome ice cores, Antarctica, Geophys. Res. Lett., 27, 735-738, 2000; Petit, J.R., et al., Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica, Nature, 399, 429-436, 1999. Siegenthaler, U., et al., Stable carbon cycle-climate relationship during the Late Pleistocene, Science, submitted, 2005; Spahni, R., et al., Variations of atmospheric methane and nitrous oxide during the last 650,000 years from Antarctic ice cores, Science, submitted, 2005.