Significant uncertainty exists about the magnitude of carbon uptake in the next century. This uncertainty comes from a variety of factors, including economics of future emissions, ocean processes, temperature change, and ecosystem processes. The MIT IGSM contains detailed submodels to address each of these critical factors in the carbon cycle. A previous study showed that oceanic vertical mixing, parameterized by Kv, the coefficent of vertical diffusion, is the dominant uncertain parameter in the ocean component of the IGSM. In this new work we examine the relative contributions of the uncertainty in Kv, economic uncertainty, and climate sensitivity to the final values of carbon uptake by the ocean and the ecosystem. In a case where only climate parameters were allowed to vary and an economic scenario designed to achieve 550 ppm stabilization was used, regression analysis showed that about 96% of the ocean uptake resulted from variation in Kv. When uncertainty in the economic model was addressed, with the assumption of no climate policy, uncertainty in carbon dioxide emissions yielded about 55% of the variation in ocean uptake in 2100. Only about 40% of the variance was due to uncertainty in the coefficient of vertical diffusion. Therefore, the impact of climate sensitivity must be less than 5%. Under a sample policy regime based on extending the Kyoto protocol, emissions became more certain, and the dependence of the uncertainty on Kv increased to about 90%.