Observation-based estimates of the global carbon budget serve as important constraints on carbon cycle models. We test the effect of new budget data on projection uncertainty. Using a simple global model, we find that data for an additional decadal budget have only a marginal effect on projection uncertainty, in the absence of any constraints on decadal variability in carbon fluxes. Even if uncertainty in the global budget were eliminated entirely, uncertainty in the mechanisms governing carbon sinks have a much larger effect on future projections. Results suggest that learning about the carbon cycle will best be facilitated by improved understanding of sink mechanisms and their variability as opposed to better estimates of the magnitudes of fluxes that make up the global carbon budget.