Assessment of Global Ocean Biogeochemistry Models for Ocean Carbon Sink Estimates in RECCAP2 and Recommendations for Future Studies

Abstract

The ocean is a major carbon sink and takes up 25%–30% of the anthropogenically emitted CO2. A state‐of‐the‐art method to quantify this sink are global ocean biogeochemistry models (GOBMs), but their simulated CO2 uptake differs between models and is systematically lower than estimates based on statistical methods using surface ocean pCO2 and interior ocean measurements. Here, we provide an in‐depth evaluation of ocean carbon sink estimates from 1980 to 2018 from a GOBM ensemble. As sources of inter‐model differences and ensemble‐mean biases our study identifies (a) the model setup, such as the length of the spin‐up, the starting date of the simulation, and carbon fluxes from rivers and into sediments, (b) the simulated ocean circulation, such as Atlantic Meridional Overturning Circulation and Southern Ocean mode and intermediate water formation, and (c) the simulated oceanic buffer capacity. Our analysis suggests that a late starting date and biases in the ocean circulation cause a too low anthropogenic CO2 uptake across the GOBM ensemble. Surface ocean biogeochemistry biases might also cause simulated anthropogenic fluxes to be too low, but the current setup prevents a robust assessment. For simulations of the ocean carbon sink, we recommend in the short‐term to (a) start simulations at a common date before the industrialization and the associated atmospheric CO2 increase, (b) conduct a sufficiently long spin‐up such that the GOBMs reach steady‐state, and (c) provide key metrics for circulation, biogeochemistry, and the land‐ocean interface. In the long‐term, we recommend improving the representation of these metrics in the GOBMs.

An Assessment of CO2 Storage and Sea‐Air Fluxes for the Atlantic Ocean and Mediterranean Sea Between 1985 and 2018

“…The inferred northward C ant transport at the southern boundary between Atlantic and Southern Oceans obtained for each of the nine GOBMs with Simulations A and D shows a high correlation (r 2 = 0.61; p-level < 0.01) with the maximum AMOC values at 26°N of each of these GOBMs (Figure S12 in Supporting Information S1), indicating that the northward physical transport is the main driver of the northward C ant transport. We note additionally that, in comparison with observations, the GOBMs tend to underestimate the maximum AMOC values at 26°N (Figure S12 in Supporting Information S1) and hence the inferred northward C ant transport (Terhaar et al, 2024).…”

“…Further, using Earth System Models, Goris et al (2023) showed that the AMOC-strength drives the simulated seasonal variability in the North Atlantic. Altogether, this suggests that the Global Biogeochemical Cycles 10.1029/2023GB007862 underestimation of the AMOC in the GOBMs (Terhaar et al, 2024) could be an underlying cause for the underestimation of the role of biogeochemical variability for both IAV and seasonality by the GOBMs in the NA SPSS.…”

“…Though several of the GOBMs analyzed in this study include river inputs of carbon, not all processes relevant for the land-sea flux are adequately represented. In consequence, the average of the global imbalance between river input and flux to the sediment is small (<0.14 PgC yr 1 ) in the GOBM ensemble (Terhaar et al, 2024) compared to the observation-based global integral of RCO recommended in the RECCAP2 protocol, that amounts to 0.65 ± 0.3 PgC yr 1 (Regnier et al, 2022). Combining the spatial distribution of RCO by Lacroix et al (2020) and the globally integrated estimate by Regnier et al (2022) allows us, in principle, to estimate its contribution to FCO 2 at biome scale, albeit with a relative uncertainty that is most likely even larger than that of the global integral (>50% of the absolute value) and without considering the already-present land-sea fluxes of the GOBMs.…”

“…Its approximation would require several thousands of years of integration with a GOBM including a sediment module. None of the GOBMs used here includes such a long pre-industrial spin-up (Terhaar et al, 2024). Though several of the GOBMs analyzed in this study include river inputs of carbon, not all processes relevant for the land-sea flux are adequately represented.…”

“…The riverine carbon outgassing (RCO, see Section 2.4) hampers the comparison of the FCO 2 estimates from the GOBMs with those of pCO 2 products, since the input of riverine carbon and the burial of carbon is treated in various ways across the ensemble of GOBMs. Furthermore, relevant output from the GOBMs is missing to properly assess the contribution of carbon, alkalinity and nutrient input from land and their burial in sediments, resulting in a situation where only a rough approximation of the RCO in the GOBMs is possible (Terhaar et al, 2024). In the RECCAP2 protocol, it was recommended to apply the spatial distribution of the RCO of Lacroix et al (2020), scaled to a globally integrated RCO value of 0.65 ± 0.3 PgC yr 1 (Regnier et al, 2022).…”