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Seasonal CO2 Cycle -- Summary
Each year when the terrestrial vegetation of the Northern Hemisphere waxes and wanes in vigor along with the seasons, it removes considerable CO2 from the atmosphere in its productive growing phase, while it returns CO2 to the air when it dies and decomposes.  This phenomenon creates a seasonal ripple in the atmosphere's CO2 concentration, as it drops a few ppm when land plants are growing vigorously and rises a similar amount when the majority of these plants are senescing.  Over the past forty years that this seasonal oscillation of the air's CO2 content has been accurately measured, it has been noticed that the amplitude of the oscillation has been growing in size.  In fact, as we note in our mini-review of the Amplitude of the Atmosphere's Seasonal CO2 Cycle, this once-a-year "breath of the biosphere" has risen in strength by approximately 20% over the last four decades.

Most of the people who have studied this phenomenon have concluded that the increasing amplitude of the atmosphere's seasonal CO2 cycle is a reflection of increasing biospheric productivity; and a large proportion of them attribute this increase in vegetative vigor to the aerial fertilization effect of the ongoing rise in the air's CO2 content.  A number of other factors that could possibly be influencing the size of this seasonal signal have also been identified.  One of them is global warming.  However, based on a study of annual CO2 amplitude data from ten different locations, Idso et al. (1999) have demonstrated that Northern Hemispheric warming could explain no more than about a fifth of the annual CO2 amplitude increase observed in lower latitudes from 1960 to 1995 and no more than about a tenth of the amplitude increase recorded at higher latitudes over the same time period.  Likewise, in a study of surface carbon exchange over five natural and five disturbed sites in northeast Siberia, Zimov et al. (1999) also conclude that global warming can only have played a minor role in the historical increase in the amplitude of the air's seasonal CO2 cycle; but they identify disturbance itself as a potential major player in this regard.

More work will obviously be required to better define the roles of a number of different factors that may impact this phenomenon; but the major message of the data seems clear: whatever has happened to the planet over the past four decades has favorably impacted the biosphere as a whole.

References
Idso, C.D., Idso, S.B. and Balling, R.C.  1999.  The relationship between near-surface air temperature over land and the annual amplitude of the atmosphere's seasonal CO2 cycle.  Environmental and Experimental Botany 41: 31-37.

Zimov, S.A., Davidov, S.P., Zimova, G.M., Davidova, A.I., Chapin III, F.S., Chapin, M.C. and Reynolds, J.F.  1999.  Contribution of disturbance to increasing seasonal amplitude of atmospheric CO2Science 284: 1973-1976.