Atmospheric Carbon Dioxide

Through his worldwide industrial civilization, Man is unwittingly conducting a vast geophysical experiment. Within a few generations he is burning the fossil fuels that slowly accumulated in the earth over the past 500 million years. The CO₂ produced by this combustion is being injected into the atmosphere; about half of it remains there. The estimated recoverable reserves of fossil fuels are sufficient to produce nearly a 200% increase in the carbon dioxide content of the atmosphere. ...The climatic changes that may be produced by the increased CO₂ content could be deleterious from the point of view of human beings.

From Restoring the Quality of Our Environment: Report of the Environmental Pollution Panel, President's Advisory Committee, November 1965, Appendix Y4: Atmospheric Carbon Dioxide (PDF). The authors credit T.C. Chamberlain and Svante Arrhenius with the first speculation about the phenomenon, in 1899 and 1903, respectively.