size of the photosynthesizing carbon pool remains fixed and that only the carbon pool of forest wood and detritus increases in size in response to higher atmospheric CO2 levels, they obtain a prediction that the biota carbon pool will increase by no more than about twice the amount of carbon in the preindustrial atmosphere (see Figure 10.3 in Chapter 10). This second model does not oscillate noticeably, nor does it predict an increase without limit in biota carbon for larger and larger inputs of industrial CO2.
Revelle and Munk also consider the effect on atmospheric CO2 of clearing of lands for agriculture and timber harvest. For the model with no limit on biota pool size, clearing has only a small influence on their predictions (see Figure 10.2 in Chapter 10). The model with a limit on the biota, which includes the effect of clearing, would probably be similarly insensitive to its exclusion, although this question was not investigated. Thus the most significant difference between the models almost certainly lies in the differing properties assigned to the photosynthesizing pool.
A possible limitation on the accuracy of these models is their simplified treatment of fluxes and storage times for short-, long-, and very long-cycled carbon species. Revelle and Munk have almost doubled the assumed quantity of organic carbon in the land biota by increasing the amount of soil humus, some of which may have very long cycling times. They have included this additional humus within a single box model reservoir that also includes short-cycled detrital carbon. The latter inclusion scarcely increases the assumed total amount of biotic carbon but approximately doubles the assumed flux to and from the detrital pool. This pool, with its single transfer time (of about 50 years), has a transfer function significantly different from that of a biota model with separate compartments for short-, long-, and very long-cycled carbon. This aggregating of fluxes and masses could result in an overestimate of biotic CO2 uptake.
On the other hand, the predicted CO2 uptake probably would be increased substantially by relaxing the requirement that the size of the photosynthesizing pool be constant. In the upper limit, with no restriction on pool size, the total land biota carbon pool is predicted eventually to take up all the industrial CO2 regardless of the amount. Little evidence exists from studies of plant communities to choose between such different models. Acceptance of a particular model seems therefore to depend mainly on the reasonableness of the prediction of biotic uptake of CO2. The work of determining the degree of uptake of CO2 that is reasonable probably must be done by ecologists, botanists, and soil scientists. When that work is done, the models can be adjusted to agree with their findings.
To prepare for this eventuality, we have investigated the result of assuming various amounts of biotic CO2 uptake using the 6R model with the biota growth factor linearly reduced to zero after A.D. 2000 over different time intervals. These predictions are based on quite arbitrary assumptions, but they reveal a feature of the carbon system that is probably only slightly model-dependent: for biotic uptake less than or of the order of that predicted by Revelle and Munk’s model with a limit on the biota, the sum of the uptake by the atmosphere and the biota is nearly the same regardless of the increase in biotic carbon. For example, with a cumulative increase in the biota in A.D. 2250 of 2.6 times the amount of preindustrial atmospheric CO2, the sum is only 3.7 percent greater than if the biota carbon pool grows negligibly.
Thus the predictions of atmospheric CO2 increase that we describe above in the main part of this chapter can be reduced by whatever the reader believes ought to be the uptake assigned to the land biota without seriously upsetting the model prediction in other respects. A similar option exists with respect to biota-limited models of the Revelle-Munk type, if assumed mechanisms controlling the photosynthesizing carbon pool are suitably varied.
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