Because Mars does not have a strong intrinsic magnetic field, the atmosphere is eroded by interactions with the solar wind. Early solar-system conditions enhanced the sputtering loss. It is calculated that similar to 3 bars of carbon dioxide (CO2) have been sputtered over the last 3.5 billion years. This significant increase over the previous estimate by Luhmann et al. of similar to 0.14 bar of CO2 is the result of the development of a more complete model. The model also predicts slightly greater loss of water-similar to 80 meters instead of the similar to 50 meters predicted by Luhmann et al. Because estimates of CO2 on early Mars range from 0.5 to 5 bars, the 0.14-bar estimate is insignificant but the similar to 3-bar estimate will have a large effect on our understanding of the planet’s evolution.