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https://doi.org/10.5194/egusphere-2024-2893
https://doi.org/10.5194/egusphere-2024-2893
23 Sep 2024
 | 23 Sep 2024
Status: this preprint is open for discussion.

Three-Compartment, Two Parameter, Concentration-Driven Model for Uptake of Excess Atmospheric CO2 by the Global Ocean

Stephen E. Schwartz

Abstract. This paper develops, applies, and examines a transparent three-compartment model for the amounts of CO2 (dissolved inorganic carbon, DIC) in the mixed-layer and deep ocean, over the Anthropocene driven by the observed amount of atmospheric CO2. The model has two independent parameters, the piston velocity vp characterizing the rate of water exchange between the mixed-layer ocean (ML) and the deep ocean (DO), and the atmosphere-ocean deposition velocity for low- to intermediate-solubility gases kam. The net uptake of CO2 into the ocean is only weakly dependent on kam, so the net uptake rate depends almost solely on vp. This piston velocity is determined from the measured the rate of uptake of heat by the global ocean from the 1960's to the present as 7.5 ± 2.2 m yr‑1, 1-σ. The resultant modeled net uptake flux of anthropogenic atmospheric CO2 by the global ocean at year 2022 is 2.84 ± 0.6 Pg yr-1; the corresponding net transfer coefficient, the net anthropogenic uptake flux divided by the stock of excess atmospheric CO2 is 0.010 ± 0.002 yr-1. This net transfer coefficient appears to decrease slightly (~ 17 %) over the Anthropocene, attributed to the decrease of the equilibrium solubility of CO2 (as dissolved inorganic carbon) in seawater due to the uptake of additional CO2 over this period and to increasing slight return flux from the DO to the ML. Modeled DIC in the global ocean and net atmosphere-ocean fluxes compare well with observations and with current carbon cycle models (both concentration-driven and emissions-driven). Uptake of anthropogenic carbon by the terrestrial biosphere is calculated as the difference between emissions and the sum of increases in atmospheric and ocean stocks. The model is examined for radiocarbon over the industrial era, over the period during which radiocarbon was influenced by emissions of 14C-free CO2 mainly from fossil fuel combustion, and the period dominated by 14C emissions from atmospheric weapons testing. A variant of the model with only two compartments and one parameter, vp, treating the atmosphere and the mixed-layer ocean as a single compartment in equilibrium, performs essentially as well as the three-compartment, two-parameter model. Although the concentration-driven model developed here cannot be used prognostically (to assess model skill in replicating atmospheric CO2 over the industrial period or to examine response to changes in emissions), it is useful diagnostically to examine the disposition of excess carbon into the pertinent global compartments as a function of time over the Anthropocene and for confidently representing ocean uptake of excess CO2 in emissions-driven models.

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Stephen E. Schwartz

Status: open (until 07 Nov 2024)

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Stephen E. Schwartz

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Short summary
The net uptake coefficient of anthropogenic CO2 by the global ocean (net uptake flux divided by excess atmospheric CO2 stock above preindustrial) calculated with a simple, transparent, three-compartment concentration-driven model with two independent parameters is determined to be 0.010 ± 0.002 yr-1 (net uptake flux at year 2022 2.84 ± 0.6 Pg yr-1). This result compares well with observations and with much more complex carbon cycle models.