Distribution of radiocarbon as a test of global carbon cycle models

Abstract

Accurate global carbon cycle models are needed to estimate the future change of atmospheric CO2 for specified scenarios of CO2 emissions. Model accuracy cannot be tested directly because of the difficulty in estimating the carbon flux to the oceans and the terrestrial biosphere. However, one test of model consistency is the requirement that the model reproduce past changes and spatial distributions of 14C. A model for carbon exchange within and among the atmosphere, oceans, and terrestrial biosphere is found to satisfy this test. The ocean is modeled as an upwelling‐diffusion column capped by a mixed layer with recirculation of the polar bottom water to complete the thermohaline circulation. This ocean advection scheme contains only two key dynamic parameters, the vertical eddy diffusivity κ and the upwelling velocity w, which are calibrated to match the vertical distribution of preanthropogenic 14C. The thermocline depth scale κ/w = 1343 m found by calibration is considerably deeper than that required to match the steady vertical temperature profile (500 m). This is consistent with the hypothesis that isopycnal mixing, which is much more rapid than diapycnal mixing, has a stronger effect on 14C than on temperature since isopycnals are nearly isothermal. This model is found to match measured values, within measurement error, of the prebomb decrease in 14C in the atmosphere and the mixed layer due to the Suess effect, the bomb 14C in the mixed layer, the bomb 14C penetration depth, the bomb 14C ocean inventory, and the vertical distribution of total carbon. Results are compared to those of other schematic carbon cycle models as well as those of ocean general circulation models.