CO2 concentration data in the atmosphere are widely known to possess a seasonal cycle, largely due to plant photosynthesis and respiration, superimposted upon an upward trend that is largely due to increasing fossil fuel use. In this paper we assess the information contained in the seasonal component of atmosphere CO2 data by applying modern techniques of time series decomposition to monthly average CO2 observations at three locations. At Mauna Loa and South Pole, which have the longest time series, the amplitudes of the seasonal components are found to be increasing with time, from α5.6 ppm at 1958 to α6.2 pm in 1978 at Mauna Loa and from ~1.0 ppm in 1965 to ~1.3 ppm in 1978 at the South Pole. We consider four possible causes of the CO2 seasonal behavior-changes in the seasonal pattern of fossil fuel use, increasing vegetation, increasing global photosynthetic activity, and changes in ocean temperature-and conclude that it is most likely that the CO2 seasonal behavior reflects an increse in global photosynthetic activity.