There is an ongoing debate about the degree to which carbon to nitrogen (C:N) ratios of plant tissues and plant litter change with enhanced atmospheric carbon dioxide concentrations. Previous field and laboratory studies have obtained widely varying estimates of the effect of elevated carbon dioxide on vegetation nitrogen concentrations and the litter resulting from mortality of these plant tissues. Changes in plant C:N ratios have the potential to influence the amount of carbon that can be sequestered from the atmosphere into terrestrial ecosystems. To explore the potential consequences of this uncertainty on estimates of terrestrial net primary production (NPP) and carbon storage, we first constructed a probability distribution for the change in the C:N ratio of vegetation with doubled carbon dioxide (dC2N) based on a range of experimental findings. We then used five simulations of the Terrestrial Ecosystem Model (TEM) with the same projected climate, but different values of dC2N to develop a response surface of terrestrial NPP and net ecosystem production (NEP) to atmospheric carbon dioxide concentrations. Probability distributions for projected NPP and NEP were then developed by running 10,000 Monte Carlo simulations of the derived response function with values selected from the probability distribution of dC2N. The results of this analysis projected that terrestrial NPP in 2100 will be 51.0 Pg C per year with a 90 percent probability of being between 50.2 and 51.7 Pg C per year based on the uncertainty in plant C:N ratios; and terrestrial carbon sequestration in 2100, as represented by NEP, will be 2.1 Pg C per year with a 90 percent probability of being between 0.9 and 3.3 Pg C per year. The larger uncertainty associated with NEP suggests that the response of decomposition associated with plant C:N ratios may be an important factor to consider in projecting the response of terrestrial carbon storage to changes in atmospheric carbon dioxide.