Uncertainty in atmospheric CO₂ predictions from a parametric uncertainty analysis of a global carbon cycle model

Abstract

Key uncertainties in the global carbon cycle are explored with a 2-D model for the oceanic carbon sink. By calibrating the key parameters of this ocean carbon sink model to widely referenced values, it produces an average oceanic carbon sink during the 1980s of 1.94 Pg/yr, consistent with the range estimated by the IPCC of 2.0 Pg/yr +/- 0.8 Pg/yr. A sensitivity analysis of the parameter values used as inputs to the 2-D ocean carbon sink model developed for this study suggests that the IPCC's range for the oceanic carbon sink of 1.2 to 2.8 Pg/yr during the 1980s may be too conservative. By applying the Probabilistic Collocation Method to this simple ocean carbon sink model, the uncertainty in the size of the oceanic sink for carbon and hence future atmospheric CO₂ concentrations is quantitatively examined. An average 1980s oceanic carbon sink of 2.06 ± 0.9 Pg/y (with 67% confidence) is estimated. This uncertainty is found to be dominated the uncertainty in by the rate of vertical mixing of dissolved carbon from the surface into the deep ocean which is parameterized in this study by vertical diffusion. It is observed that a wide range of parameter values can be used to balance the contemporary carbon cycle due to the large uncertainties in the total oceanic and terrestrial sinks. For a reference set of emissions similar to the IS92a scenario of the IPCC, the uncertainty in the atmospheric CO₂ concentration in 2100 is found to be 659 ppm +/- 35 ppm (with 67% confidence). This uncertainty is solely due to uncertainties identified in the "solubility pump" mechanism of the oceanic sink, which is only one of the many large uncertainties lacking a quantitative examination in the global carbon cycle. Such uncertainties have implications for the predictability of atmospheric CO₂ levels, a necessity for gauging the impact of different rates of anthropogenic CO₂ emissions on climate and for policy-making purposes. Because of the negative feedback between the natural carbon uptake by the terrestrial ecosystem and atmospheric CO₂ concentration, taking changes in the former into account leads to a smaller uncertainty in the latter compared to that in the case with the fixed terrestrial uptake.