The seasonal component of atmospheric CO₂: Information from new approaches to the decomposition of seasonal time series

CO 2 concentration data in the atmosphere are widely known to possess a seasonal cycle, largely due to plant photosynthesis and respiration, superimposed 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 atmospheric CO 2 data by applying modern techniques of time series decomposition to monthly average CO 2 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 ∼5.6 ppm in 1958 to ∼6.2 ppm 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 CO 2 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 CO 2 seasonal behavior reflects an increase in global photosynthetic activity.