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

Abstract

Key uncertainties in the global carbon cycle are reviewed and a simple model for the oceanic carbon sink is developed and described. This model for the solubility sink of excess atmospheric CO2 has many enhancements over the more simple 0-D and 1-D box-diffusion models upon which it is based, including latitudinal extension of mixed-layer inorganic carbon chemistry, climate-dependent air-sea exchange rates, and mixing of dissolved inorganic carbon into the deep ocean that is parameterized by 2-D eddy diffusion. By calibrating the key parameters of this ocean carbon sink model to various "best guess" reference values, it produces an average oceanic carbon sink during the 1980s of 1.7 Pg yr-1, consistent with the range estimated by the IPCC of 2.0 Pg yr~1 ± 0.8 Pg (1992; 1994; 1995). The range cited in the IPCC study and widely reported elsewhere is principally the product of the structural uncertainty implied by an amalgamation of the results of several ocean carbon sink models of varying degrees of complexity. This range does not take into account the parametric uncertainty in these models and does not address how this uncertainty will impact on future atmospheric CO 2 concentrations. A sensitivity analysis of the parameter values used as inputs to the 2-D ocean carbon sink model developed for this study, however, shows that the oceanic carbon sink range of 1.2-2.8 Pg/yr for the 1980s is consistent with a broad range of parameter values. By applying the Probabilistic Collocation Method (Tatang, et al. 1997) to this simple ocean carbon sink model, the uncertainty of the magnitude of the oceanic sink for carbon and hence atmospheric CO2 concentrations is quantitatively examined. This uncertainty is found to be larger than that implied by the structural differences examined in the IPCC study alone with an average 1980s oceanic carbon sink estimated at 1.8 ± 1.3 Pg/yr (with 95% Confidence). It is observed that the range of parameter values needed to balance the contemporary carbon cycle yield correspondingly large differences in future atmospheric CO2 concentrations when driven by a prescribed anthropogenic CO2 emissions scenario over the next century. For anthropogenic CO 2 emissions equivalent to the IS92a scenario of the IPCC (1992), the uncertainty is found to be 705 ppm ± 47 ppm (one standard deviation) in 2100. This range is solely due to uncertainty in the "solubility pump" sink mechanism in the ocean and is only one of the many large uncertainties left to explore 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 for policy-making purposes. Since atmospheric CO 2 levels are one of the primary drivers of changes in radiative forcing this result impacts on the uncertainty in the degree of climate change that might be expected in the next century.