An observed increase of the amplitude of the seasonal cycle of atmospheric CO2 at Mauna Loa Observatory has been examined. Previous global carbon cycle studies seemed to suggest, at least qualitatively, that the observed increase may be entirely due to a CO2 fertilization effect on the terrestrial biota. The present detailed theoretical investigation shows that a pure CO2 stimulation of the net primary production may indeed lead to an amplitude increase of 0.15 plus 0.17% yr−1 and minus 0.10% yr−1 as compared to the measured value of 0.67 ± 0.25% yr−1 for the period between AD 1958 and AD 1987. This result is based on the assumption of a long term fertilization factor β with a range between 0.15 and 0.60, taken to be equal to the mesaured short term fertilization factor β, obtained in CO2 enrichment studies with exposure times generally smaller than 1 year. The experimental evaluation of the difference between the relative increase of the summer (peak to trough) and winter (trough to peak) amplitude gives information on the carbon sequestered annually by the terrestrial ecosystems. With the estimated respiration response factor, which characterizes the fraction of the additional production that is consumed by ecosystem respiration, equal to 0.75, and an assumed long-term CO2 fertilization factor, β, equal to 0.375, acceptable agreement between prediction and measurements is obtained, amounting to a mean annual increase in living biomass of 0.7 Gt C, while a corresponding portion may be stored in soils. The fact that the fertilization alone predicts the observed difference but not the absolute value of both the summer and winter amplitude, leads to the conclusion that other external effects are operative, which are not directly related to the fertilization phenomenon of the vegetation and which influence to an equal extent the summer and winter amplitude. Two such external contributions have been identified; they are both related to the increased fossil fuel carbon input into the Northern Hemisphere: (1) the seasonality of the fossil fuel consumption in the Northern Hemisphere is approximately in phase with the relative uptake or release of CO2 by the land vegetation and leads to a small contribution to the increase in amplitude, in the range between 0.01 and 0.08% yr−1; (2) the seasonally different transequatorial transport of fossil fuel carbon creates a seasonal behavior which again is approximately in phase with the biota and leads to an additional contribution in the range of 0 to 0.31% per year depending on the wind fields used.