The residence times of carbon in plants, litter, and soils are required for partitioning land and ocean sinks using measurements of atmospheric δ13C and also for estimating terrestrial carbon storage in response to net primary production (NPP) stimulation by elevated levels of atmospheric CO2. While 13C-based calculations of the land sink decline with increasing estimates of terrestrial carbon residence times (through the fossil fuel-induced isotopic disequilibrium term in equations describing the global atmospheric budgets of 13CO2 and CO2), estimates of land sinks based on CO2 fertilization of plant growth are directly proportional to carbon residence times. Here we used a single model of terrestrial carbon turnover, the Carnegie–Ames–Stanford Approach (CASA) biogeochemical model, to simultaneously estimate 1984–1990 terrestrial carbon storage using both approaches. Our goal was to identify the fraction of the 13CO2-based land sink attributable to CO2 fertilization. Uptake from CO2 fertilization was calculated using a β factor of 0.46 to describe the response of NPP to increasing concentrations of atmospheric CO2 from 1765 to 1990. Given commonly used parameters in the 13C-based sink calculation and assuming a deforestation flux of 0.8 Pg C/y, CO2 fertilization accounts for 54% of the missing terrestrial carbon sink from 1984 to 1990. CO2 fertilization can account for all of the missing terrestrial sink only when the terrestrial mean residence time (MRT) and the land isodisequilibrium forcing are greater than many recent estimates.