Figure 5.1, Potential carbon sources and sinks. The first three bars show estimates of the sizes of the as >ct untapped reservoirs of fossil fuel carbon. The next two bars show the amounts of carbon currently stored in forests and soil humus. The nevt bar shows the total buffer capacity of the sea, that is,

The next bar shows the amount of CiaCOj in the upper meter ot deep sea sediments (i.e., that available for dissolution on the time scale of several thousand years):

finjllv. the last bar shows the amount of extra COj required to raise its content in the atmosphere by 400 ppm.

coal, oil shale, and tar sand reserves will remain. Second, there is, of course, no limit to how much CO? the ocean-atmosphere-biosphere system can hold. Rather, the question is how the COi we release w ill distribute itself among these reservoirs.

5.2 Atmosphere-Ocean Partitioning

Were the rate of (X)? exchange between air and sea and the rate of mixing within the sea infinitely rapid, then five-sixths of the CO2 we have produced to date would have taken up residence in the sea. Only one-sixth would remain airborne. Hut based on the observed distribution of anthropogenic tracers (14C, Tl ,..) in the sea, it has been shown that only about 15% of the sea's capacity for CO? uptake is being utilized. The reason is that the resistance to uptake is dominated by the finite rate of transfer of water from the surface to the interior of the sea. Because of this limitation, onlv about

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