Radiocarbon in the Land and Ocean Components of the Community Earth System Model

Abstract

Large amounts of the carbon‐isotope 14C, entering Earth's carbon cycle, were produced in the atmosphere by atomic bomb tests in the 1950s and 1960s. Here, we forced the ocean and land components of the Community Earth System Model with atmospheric 14CO2 over the historical period to constrain overturning time scales and fluxes. The uptake of bomb 14C by the land model is lower than observation‐based estimates. This mismatch is likely linked to too‐low 14C uptake by vegetation as the model overestimates 14C/C ratios of modern soils. This suggests model biases in forest productivity or wood carbon allocation and turnover, and, in turn, a bias in the forest sink of anthropogenic carbon. The ocean model matches the observation‐based global bomb 14C inventories when applying the quadratic relationship between gas transfer piston velocity and wind speed of Wanninkhof (2014), https://doi.org/10.4319/lom.2014.12.351 and the wind products from Large and Yeager or the Japanese Reanalysis Project. Simulated natural radiocarbon ages in the deep ocean are many centuries older than data‐based estimates, indicating too slow deep ocean ventilation. The sluggish circulation causes large biases in biogeochemical tracers and implies a delayed deep ocean uptake of heat and carbon in global warming projections. Our study suggests that 14C observations are key to constrain carbon fluxes and transport timescales for improved representations of land and ocean biogeochemical cycles and Earth system model projections.

Bomb radiocarbon evidence for strong global carbon uptake and turnover in terrestrial vegetation

“…1C). CLM5.0-unc results are similar to another offline simulation of CLM5.0 that suggested the 14 C accumulated in the terrestrial biosphere in the 1960s could be too small (28).…”

Changes in Oceanic Radiocarbon and CFCs Since the 1990s

“…In the northwestern Pacific (P02), in CESM2 surface pCFC‐11 is positively biased while pCFC‐12 is negatively biased, and the biases are reversed in ECCOv4‐TM (Figure S17 in Supporting Information ). A prominent feature is the strong negative ∆ 14 C bias in CESM2 throughout the deep ocean but most notably in the Pacific (Figures S10–S12 in Supporting Information ), which stems from AABW formation in the CESM2 being too weak (Danabasoglu et al., 2020; Frischknecht et al., 2022). This also leads to potential densities in the deep Pacific that are too shallow in CESM2 (Figure S11 in Supporting Information ).…”

“…10.1029/2023JC020387 most notably in the Pacific (Figures S10-S12 in Supporting Information S1), which stems from AABW formation in the CESM2 being too weak (Danabasoglu et al, 2020;Frischknecht et al, 2022). This also leads to potential densities in the deep Pacific that are too shallow in CESM2 (Figure S11 in Supporting Information S1).…”

Making the case for an International Decade of Radiocarbon

“…Moreover, 14 C measurements have largely been contained within specific disciplines or within expert groups, resulting in a lack of freely accessible datasets and modelling tools. Although 14 C is recommended to be included in international observation programmes (https://gcos.wmo.int/en/essential-climate-variables/; [18]) as well as in earth system modelling [19,20], only one model incorporating 14 C in both land and ocean components has reported output in the latest version of the carbon-climate model intercomparison effort CMIP6 (CESM2, https://esgf-node.llnl.gov/projects/cmip6/; [14,21]). There has also been a perception in some research communities that the utility of 14 C has diminished as atmospheric 14 C/ 12 C has dropped over the decades since the bomb tests.…”