Three-compartment, two-parameter concentration-driven model for uptake of excess atmospheric CO2 by the global ocean

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 oceans over the Anthropocene, driven by the observed amount of atmospheric CO2. The model has two independent parameters, a piston velocity vp characterizing the rate of water exchange between the mixed-layer ocean (ML) and the deep ocean (DO), and an 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 rate of uptake of heat by the global ocean from the 1960s to the present as 7.5 ± 2.2 m yr−1, 1σ. The resultant modeled net uptake flux of anthropogenic atmospheric CO2 by the global ocean in the year 2022 is 2.84 ± 0.6 Pg yr−1, and 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; this decrease is attributed to the decrease in the equilibrium solubility of CO2 (as dissolved inorganic carbon) in seawater due to the uptake of additional CO2 over this period and slightly increasing 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, used to calculate 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, compares well with available measurements of ocean radiocarbon and with other models. A variant of the model with only two compartments and a single 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), the model is useful diagnostically to examine the disposition of excess carbon into pertinent global compartments as a function of time over the Anthropocene. More importantly, the model and the parameters developed here can be used with confidence to represent ocean uptake of excess CO2 in emissions-driven models.