This study investigates the response of the global mean and spatial variations of the δ18O value of atmospheric CO2 (δCa) to changes in soil CO2 hydration rates, relative humidity, the δ18O value of precipitation and water vapor, visible radiation, temperature, and ecosystem flux partitioning. A three-dimensional global transport model was coupled to a mechanistic land surface model and was used to calculate isotopic fluxes of CO2 and H2O and the resulting δCa. The model reproduced the observed global mean and north-south gradient in δCa. The simulated seasonal amplitude and phases of CO2 and δCa agreed well at some but not all locations. Sensitivity tests with relative humidity increased by 3.2% from its original value decreased δCa by 0.21‰. Similarly, a global 3.3‰ decrease in the isotopic composition of both precipitation and water vapor (δWP and δWAV, respectively) caused a 2.6‰ decrease in δCa. A 1 K increase in atmospheric temperatures also affected δCa, but there was a very small δCa response to realistic changes in light levels. Experiments where leaf and soil CO2 fluxes were repartitioned revealed a nontrivial change to δCa. The predicted north-south δCa gradient increased in response to an increase in soil CO2 hydration rates. However, the δCa gradient also had a large response to global changes in δWP and δWAV. This result is particularly important since most models fail to deplete δWP enough at middle and high latitudes, where the influence of δWP and δWAV on the δCa gradient is strongest.