Observation Based Budget and Lifetime of Excess Atmospheric Carbon Dioxide

Abstract. The global budgets of CO2 and of excess CO2 (i.e., above preindustrial) in the biogeosphere are examined by a top-down, observationally constrained approach. Global stocks in the atmosphere, mixed-layer and deep ocean, and labile and obdurate terrestrial biosphere, and fluxes between them are quantified; total uptake of carbon by the terrestrial biosphere is constrained by observations, but apportionment to the two terrestrial compartments is only weakly constrained, requiring examination of sensitivity to this apportionment. Because of near equilibrium between the atmosphere and the mixed-layer ocean and near steady state between the atmosphere and the labile biosphere, these three compartments are tightly coupled. For best-estimate present-day anthropogenic emissions the turnover time of excess carbon in these compartments to the deep ocean and obdurate biosphere is 67 to 158 years. Atmospheric CO2 over the Anthropocene is accurately represented by a five-compartment model with four independent parameters: two universal geophysical quantities and two, specific to CO2, treated as variable. The model also accurately represents atmospheric radiocarbon, particularly the large increase due to atmospheric testing of nuclear weapons and the subsequent decrease. The adjustment time of excess atmospheric CO2, evaluated from the rate of decrease following abrupt cessation of emissions, is 78 to 140 years, consistent with the turnover time, approaching a long-time floor of 15–20 % of the value at the time of cessation. The lifetime of excess CO2 found here, several-fold shorter than estimates from current carbon-cycle models, indicates that cessation of anthropogenic emissions atmospheric would result in substantial recovery of CO2 toward its preindustrial value in less than a century.