The strength of feedbacks between a changing climate and future carbon dioxide concentrations are uncertain and difficult to predict using Earth System Models (ESMs). We analyzed emission-driven simulations in which atmospheric carbon dioxide levels were computed prognostically for historical (1850-2005) and future periods (RCP 8.5 for 2006-2100) produced by 15 ESMs for the Fifth Phase of the Coupled Model Intercomparison Project (CMIP5). Comparison of ESM prognostic atmospheric carbon dioxide over the historical period with observations indicated that ESMs, on average, had a small positive bias in predictions of contemporary atmospheric carbon dioxide. A key driver of this persistent bias was weak ocean carbon uptake exhibited by the majority of ESMs, based on comparisons with observations of ocean and atmosphere anthropogenic carbon inventories. We found a significant linear relationship between contemporary atmospheric carbon dioxide biases and future carbon dioxide levels for the multi-model ensemble. We used this relationship to create a contemporary carbon dioxide tuned model (CCTM) estimate of the atmospheric carbon dioxide trajectory for the 21st century. The CCTM yielded carbon dioxide estimates of 600 å± 14 ppm at 2060 and 947 å± 35 ppm at 2100, which were 21 ppm and 32 ppm below the multi-model mean during these two time periods. Uncertainty estimates derived from this approach were almost 6 times smaller at 2060 and almost 5 times smaller at 2100 than those from the ESM ensemble. The CCTM also significantly narrowed the range of carbon dioxide-induced radiative forcing and temperature increases during the remainder of the 21st century. Because many processes contributing to contemporary carbon cycle biases persist over decadal timescales, our analysis suggests uncertainties in future climate scenarios may be considerably reduced by tuning models to the long-term time series of carbon dioxide from Mauna Loa and other atmospheric monitoring stations.