DISTRIBUTION OF RADIOCARBON AS A TEST OF GLOBAL CARBON-CYCLE MODELS

Accurate global carbon cycle models are needed to estimate the future change of atmospheric CO2 for specified scenarios of CO2 emissions. Mo del accuracy cannot be tested directly because of the difficulty in es timating the carbon flux to the oceans and the terrestrial biosphere. However, one test of model consistency is the requirement that the mod el reproduce past changes and spatial distributions of C-14. A model f or carbon exchange within and among the atmosphere, oceans, and terres trial biosphere is found to satisfy this test. The ocean is modeled as an upwelling-diffusion column capped by a mixed layer with recirculat ion of the polar bottom water to complete the thermohaline circulation . This ocean advection scheme contains only two key dynamic parameters , the vertical eddy diffusivity kappa and the upwelling velocity w, wh ich are calibrated to match the vertical distribution of preanthropoge nic C-14. Th, thermocline depth scale kappa/w = 1343 m found by calibr ation is considerably deeper than that required to match the steady ve rtical temperature profile (500 m). This is consistent with the hypoth esis that isopycnal mixing, which is much more rapid than diapycnal mi xing, has a stronger effect on C-14 than on temperature since isopycna ls are nearly isothermal. This model is found to match measured values , within measurement error, of the prebomb decrease in C-14 in the atm osphere and the mixed layer due to the Suess effect, the bomb C-14 in the mixed layer, the bomb C-14 penetration depth, the bomb C-14 ocean inventory, and the vertical distribution of total carbon. Results are compared to those of other schematic carbon cycle models as well as th ose of ocean general circulation models.