Scripps Institution of Oceanography, La Jolla, CA

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The Early Keeling Curve
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The Early Keeling Curve

In the first part of the 20th century it was suspected that the concentration of atmospheric CO2 might be increasing in the atmosphere due to fossil fuel combustion. However there were relatively few measurements of this gas and the measurements varied widely.

In 1953 Charles (Dave) David Keeling began a post doctoral position at Caltech, Pasadena, California under Professor Harrison Brown. His initial project was aimed at extracting uranium from granite rock with applications in the nuclear power industry. He never really started this project but with encouragement from Professor Brown became involved in another project investigating the equilibria between carbonate in surface waters, limestone and atmospheric CO2. This involved the construction of a precision gas manometer to measure CO2 extracted from the air as well as acidified samples of water.

Dave Keeling found significant variations in CO2 concentration in Pasadena, probably due to industry, and later took his sampling equipment to Big Sur near Monterey. There he began to take air samples throughout the day and night and soon detected an intriguing diurnal pattern. The air contained more CO2 at night than during the day and after correcting for the effects of water vapor, had about the same amount of CO2 every afternoon, 310 ppm. He used stable isotope ratio mass spectrometry measurements of the CO2 he extracted to show that the 13C/12C ratio in CO2 at night was smaller than during the day and a function of plant respiration.

He repeated these measurements in the rain forests of Olympic peninsula and high mountain forests in Arizona. Everywhere the data were the same ...strong diurnal behaviour with steady values of about 310 ppm in the afternoon. The explanation for the results came from a book on meteorology describing diurnal patterns in turbulence in the atmosphere. In the afternoon Dave Keeling was measuring CO2 concentrations representative of the "free atmosphere", concentrations that prevailed over a large part of the Northern Hemisphere. At night time with a lower boundary layer the CO2 concentration was heavily influenced by respiration from local plants and soils.