Numerical Representation of Contemporary Atmospheric Δ ¹⁴ CO ₂ : 1. Time‐Varying Global Fluxes and Atmospheric Mass Balance

Precise measurements of 14 C:C in atmospheric CO 2 (expressed as Δ 14 C) are increasingly used in contemporary carbon cycle studies as a tracer of various processes, including anthropogenic CO 2 emissions from combustion of fossil fuels and cement production. Here we develop a complete representation of the global atmospheric Δ 14 CO 2 budget and use a combination of economic inventories, geophysical observations and process models to create monthly 1° × 1° gridded estimates of 14 C and associated CO 2 mass fluxes into the atmosphere for use in atmospheric transport models. The fluxes impacting Δ 14 C we include are (in order of impact on the global trend): fossil CO 2 emissions, cosmogenic production of 14 CO 2 , oceanic and terrestrial isotopic disequilibrium fluxes, 14 C production from nuclear power generation, and the impact of net terrestrial and ocean CO 2 fluxes. The total impact of these fluxes from 2000 through 2012 underestimated the observed decline in global average Δ 14 C indicating that the sum of derived fluxes was too positive. We thus optimize two global and time‐invariant scalars (for cosmogenic 14 C production and terrestrial disequilibrium) that allow us to match observed atmospheric trends and close the global 14 C budget. While our optimized scalars are not a unique combination, our optimized fluxes represent a geophysically plausible set that conforms with surface ocean 14 C observations and newly convergent constraints on the magnitude of global cosmogenic 14 C production. In Part 2 of this study, we implement this set of fluxes in a global atmospheric transport model and compare simulated Δ 14 CO 2 to a large array of global observations.