Numerical Representation of Contemporary Atmospheric Δ ¹⁴ CO ₂ : 2. Three‐Dimensional Simulation and Comparison With Observations

The 14 C:C ratio in atmospheric CO 2 (expressed as Δ 14 C) is a powerful tracer of Earth system carbon cycle processes. In the 21st century, spatio‐temporal variations of atmospheric Δ 14 C are mainly the result of anthropogenic fossil CO 2 emissions, but the oceans and terrestrial biosphere also exert significant influence on its variations. Here we present a complete three‐dimensional representation of the impact of 14 CO 2 and CO 2 fluxes on atmospheric CO 2 and Δ 14 C for 2000 through 2012. We compare simulated atmospheric Δ 14 C with approximately 5,000 measurements from both the remote atmosphere and continental areas strongly influenced by fossil CO 2 emissions. These comparisons demonstrate that the spatio‐temporal characteristics of input surface fluxes developed in Part 1 of this study have high fidelity. Based on good model‐observation agreement, we used the model's ability to determine the relative contributions of fossil, oceanic, and terrestrial fluxes to simulated Δ 14 C to help explain the origin of the observed variations. During our study period, the pole‐to‐pole difference in atmospheric Δ 14 C increased, which our analysis indicates results from changes in both fossil and oceanic fluxes. Over the continents, we show that most short‐term variation of Δ 14 C in the PBL results from atmospheric mixing acting on fossil CO 2 fluxes. Overall, the validation of our simulations by comparison with observations demonstrates that we understand the processes affecting atmospheric Δ 14 C at a variety of spatial and temporal scales. This suggests that, especially with an expanded set of measurements, we can use Δ 14 C to better quantify and understand key carbon cycle processes, especially fossil CO 2 emissions.