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Numerical Representation of Contemporary Atmospheric Δ14CO2: 2. Three‐Dimensional Simulation and Comparison With Observations

John B. Miller,Scott J. Lehman,Arlyn Andrews,Colm Sweeney,K. McKain,P. Tans,J. Southon,Samuel Hammer,J. Turnbull,Xiaomei Xu

Published 2025 in Global Biogeochemical Cycles

ABSTRACT

The 14C:C ratio in atmospheric CO2 (expressed as Δ14C) is a powerful tracer of Earth system carbon cycle processes. In the 21st century, spatio‐temporal variations of atmospheric Δ14C are mainly the result of anthropogenic fossil CO2 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 14CO2 and CO2 fluxes on atmospheric CO2 and Δ14C for 2000 through 2012. We compare simulated atmospheric Δ14C with approximately 5,000 measurements from both the remote atmosphere and continental areas strongly influenced by fossil CO2 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 Δ14C to help explain the origin of the observed variations. During our study period, the pole‐to‐pole difference in atmospheric Δ14C 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 Δ14C in the PBL results from atmospheric mixing acting on fossil CO2 fluxes. Overall, the validation of our simulations by comparison with observations demonstrates that we understand the processes affecting atmospheric Δ14C at a variety of spatial and temporal scales. This suggests that, especially with an expanded set of measurements, we can use Δ14C to better quantify and understand key carbon cycle processes, especially fossil CO2 emissions.

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