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The concentration of atmospheric carbon dioxide at Mauna Loa Observatory exhibits a seasonal pattern that repeats with striking regularity from year to year. The amplitude of this seasonal signal, expressed either by peak-to-peak changes in concentration or as a series of harmonic terms, increased at an average rate of about 0.7% per year from 1958 to 1982. The estimated standard error in the linear coefficient of increase is 0.09%. Thus the increase appears to be highly significant statistically. A detailed examination of methods of calibration and of data analysis during this long record do not reveal any inconsistencies large enough to be responsible for the increase. Because the seasonal cycle of CO2 in the northern hemisphere is thought to be due principally to the metabolic activity of terrestrial vegetation, it is likely that at least part of the increase is a result of increasing plant activity.