The carbon uptake period (CUP) is the time period when land absorbs more CO₂ than it emits. While atmospheric CO₂ mole fraction measurements can be used to assess CUP changes, atmospheric transport and asynchronous timing across regions reduce the accuracy of the estimates. Forward model experiments show that only ~ 50 % of prescribed shifts in CUP timing applied to surface fluxes (ΔCUP_NEE) are captured in simulated CO₂ mole fraction data at monitoring sites like the Barrow Atmospheric Baseline Observatory.