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Fertilizing the Earth with CO2
Alexandrov, G. and Oikawa, T.  2002.  TsuBiMo: a biosphere model of the CO2-fertilization effect.  Climate Research 19: 265-270.

What was done
The authors construct what they call a "demonstrative" model of biospheric productivity, which they define as one in which "the logic of transition from level to level should be clear, the number of coefficients should be relatively small, and the coefficients should be based on empirical information" - or, as they alternatively describe the model's defining characteristics, "the structure of the model should allow the process of deduction to be traced and should reveal the cause-effect links between assumptions and conclusions."

What was learned
Applied over the period 1980-90, the model estimated the total terrestrial carbon sink induced by the aerial fertilization effect of the contemporaneous increase in the air's CO2 content to be 1.3 Pg C yr-1.  This number compares well with estimates of up to 1.1 Pg C yr-1 derived from independent empirical observations of same-period anthropogenic CO2 emissions, changes in land use, CO2 uptake by the world's oceans, and increases in the atmosphere's CO2 concentration.

What it means
As with all models, no matter how simple or complex they may be, there are always uncertainties associated with their estimates; and the Tsukuba Biosphere Model is no exception.  One of its important parameters, for example, is so difficult to assess that its creators say "one can hardly define a feasible confidence interval for it."  Viewed from another perspective, the good agreement between the model's ultimate output and what is observed in nature might possibly be considered a way of actually determining that parameter's true value.  In any event, more work will clearly be needed to see if this new approach to describing the biospheric consequences of the ongoing rise in the air's CO2 content has indeed revealed this phenomenon to be the primary stimulatory factor in creating the great carbon sink for which modern day scientist-explorers still search.

Reviewed 17 July 2002