Has the Amount of Carbon Dioxide in the Atmosphere Changed Significantly Since the Beginning of the Twentieth Century?

The search for causes of the rising temperatures in some geographic areas during the twentieth century has directed interest toward the amount of atmospheric carbon dioxide (CO2). If the carbon dioxide added by the combustion of fossil fuels remains as a net increase, any temperature-changing effects of its presence as a minor constituent of the atmosphere should be cumulatively operative as the amount increases. In this paper, the physical knowledge of atmospheric CO2 is examined and the available nineteenth and twentieth century observations of the atmospheric CO2 concentration are summarized to ascertain the extent to which they corroborate claims that the amount of atmospheric CO2 has increased since the nineteenth century. In the light of the uncertainty of both physical knowledge and of statistical analysis, it is concluded that the question of a trend in atmospheric CO2 concentration remains an open subject.