Carbon dioxide is one of several greenhouse gases that can modify the earth's heat balance by absorbing outgoing radiation from the earth's surface, thereby increasing the amount of heat retained by the atmosphere (the so-called greenhouse effect). Changes in CO2 are therefore of considerable importance.  In this paper, the long-term trends are assessed at four baseline stations -- Mauna Loa (Hawaii), Barrow (Alaska), American Samoa and South Pole. The authors conclude that a parabolic model provides the best fit for the observed rates of CO2 concentration growth over the last 20-30 years.