W

Figure 5,2< The upper panel approximates (he decline of the buffer capacity (and hence the capacity for excess CO2 uptake) of surface ocean water as a function of atmospheric COj content. The lower panel approximates the changc in the fraction of the sea*s buffer capacity utilized as a function of the mean age of fossil fuel CO? molecules. The square root relationship comes from the observation based on natural radiocarbon that the entire sea is ventilated on the lime scale of 10 ' years and the observation that on the time scale of a decade, bomb l4C and 'H were spread through about 10% of the ocean's volume.

40 50 60 70 80 90 MEAN AGE EXCESS COg (yrs)

Figure 5,2< The upper panel approximates (he decline of the buffer capacity (and hence the capacity for excess CO2 uptake) of surface ocean water as a function of atmospheric COj content. The lower panel approximates the changc in the fraction of the sea*s buffer capacity utilized as a function of the mean age of fossil fuel CO? molecules. The square root relationship comes from the observation based on natural radiocarbon that the entire sea is ventilated on the lime scale of 10 ' years and the observation that on the time scale of a decade, bomb l4C and 'H were spread through about 10% of the ocean's volume.

two-sixths of the CO2 we have produced has been taken up by the sea. Most of the other four-sixths remains in the atmosphere.

The partitioning betw een air and sea w ill evolve with time. Two opposing tendencies will influence this evolution. First, CO2 is taken up mainly because it reacts with COJ and HBO^ present in the sea to form HCO^ and H>B(>j* Because the amounts of CO; and 11BOJ in the ocean are finite, the uptake of CO2 is driving down their concentrations and hence also the sea's capacity for further uptake (see Figure 5.2), The uptake capacity of surface seawater (per ppm of atm CO2 rise) can be approximated as inversely proportional to the CO? partial pressure in the overlying air. Thus, were the atmosphere's COi content to reach tw ice today's (i.e., 720 ppm), then, at that time, the uptake potential for excess CO2 by surface waters would drop to about half of todays value. As can be seen by comparison with the exact calculations listed in Table 5.1, although not exact, this approximation is useful for first-order thinking.

The second aspect of the evolution of the air-sea partitioning has to do w ith the fraction of the sea's capacity that is utilized. This fraction depends on the time available

Maiepwan, 3ainwmeHHbiw asTopcKUM npaBOM

Tabic 5.1. CO2 Uptake Capacity of Ocean Water as a Function of Temperature and Carbon Dioxide Partial Pressure. As can be Seen, a Tripling of the Atmosphere s CO2 Content Leads to a 2.5-Fold Reduction in the COjf Ion Content and a Fourfold Reduction in the Uptake Capacity Per Unit Rise in Atmospheric CO? Content for Thermocline and Deep Ocean Water Source Waters. These Calculations were Carried out Using the Accepted Equilibrium Constants for Carbonate and Borate. The Starting Point was the Composition for Three Arbitrarily Selected Samples Measured as Part of the GEOSF.CS Program.

T Alk PCO2 SCO2 COJ A Y,CO2/ ApCOi

°C ¡.te q/kg /¿atm fimol/kg /imol/kg /imol/kg/jiatm

Warm Surface Ou can

26,7 2375

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