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The Increasing Amplitude of the Atmosphere's Seasonal CO2 Cycle: A Minimal Role for Global Warming
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.

Each spring, when the Northern Hemisphere's vegetation awakens from the dormancy of winter and begins to grow again, it removes enough carbon dioxide from the atmosphere to reduce the air's CO2 content by several parts per million.  Then, in the fall, when much of this vegetation dies and decays, it releases huge quantities of carbon dioxide back to the atmosphere, raising the air's CO2 content by a small amount.  Together, these two phenomena produce a seasonal oscillation that is superimposed upon the yearly incremental rise in the air's mean CO2 concentration; and the greater the yearly growth of the planet's vegetation, the greater are the yearly down- and up-swings in the amount of carbon dioxide in the air.  Consequently, the amplitude of the atmosphere's seasonal CO2 oscillation serves as a good relative measure of the planet's total vegetative productivity in any given year.

Recent measurements indicate that the annual amplitude of the seasonal CO2 cycle has increased 20% in the mid-latitudes of the Northern Hemisphere, to as much as 40% in its upper-latitudes since the 1960s.  It has long been suspected that this phenomenon is primarily a direct result of atmospheric CO2 fertilization, but it may also be influenced by nitrogen-induced increases in the growth rates of earth's ecosystems and CO2-induced expansions in some of their ranges.  A slight temperature increase reported in some Northern Hemisphere land areas over this time period may also be a contributing factor.  In this paper, the authors set out to quantify the role of temperature in the increasing amplitude of the seasonal CO2 cycle that has been reported in the Northern Hemisphere over the last few decades.

What was done
Annual CO2 amplitude data from ten Northern Hemispheric locations were linearly regressed against mean near-surface air temperatures over land areas of the Northern Hemisphere over the last few decades to determine if a statistical relationship exists between the two parameters.

What was learned
Northern Hemispheric temperatures did not affect the CO2 amplitudes at the ten locations in the year in which they occurred, but exerted an influence two years later.  Temperatures in the lower latitudes were much more strongly correlated with the CO2 amplitude data than were temperatures in the middle or higher latitudes.  Using an equation based upon the statistical relationship between the two parameters, the authors were able to conclude that Northern Hemispheric warming could explain no more than about one-fifth of the annual CO2 amplitude increase observed in the 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.

What it means
This study indicates that the Northern Hemispheric warming of the last few decades "cannot have been the major cause of the long-term increase in the annual CO2 amplitude" observed there, thus adding to the growing list of evidence that suggests that the aerial fertilization effect of atmospheric CO2 is the primary agent responsible for the amplitude increases.

Reviewed 15 April 1999