The models and concepts used to predict future climate are based on physical laws and information obtained from observations of the past. New paleoclimate records are crucial for a test of our current understanding. The Vostok ice core record <Petit et al., 1999> showed that over the past 420 kyr (1 kyr = 1000 years), Antarctic climate and concentrations of the greenhouse gases carbon dioxide (CO2) and methane (CH4) were tightly coupled. In particular, CO2 seemed to be confined between bounds of about 180 ppmv (parts per million by volume) in glacial periods and 280 ppmv in interglacials; both gases rose and fell with climate as the Earth passed through four glacial/interglacial cycles.