Magnitude, Trends, and Variability of the Global Ocean Carbon Sink From 1985 to 2018

Abstract

This contribution to the RECCAP2 (REgional Carbon Cycle Assessment and Processes) assessment analyzes the processes that determine the global ocean carbon sink, and its trends and variability over the period 1985‐2018, using a combination of models and observation‐based products. The mean sea‐air CO2 flux from 1985‐2018 is ‐1.6±0.2 PgC yr‐1 based on an ensemble of reconstructions of the history of sea surface pCO2 (pCO2 products). Models indicate that the dominant component of this flux is the net oceanic uptake of anthropogenic CO2, which is estimated at ‐2.1±0.3 PgC yr‐1 by an ensemble of ocean biogeochemical models, and ‐2.4±0.1 PgC yr‐1 by two ocean circulation inverse models. The ocean also degasses about 0.65±0.3 PgC yr‐1 of terrestrially‐derived CO2, but this process is not fully resolved by any of the models used here. From 2001‐2018, the pCO2 products reconstruct a trend in the ocean carbon sink of ‐0.61±0.12 PgC yr‐1 decade‐1, while biogeochemical models and inverse models diagnose an anthropogenic CO2‐driven trend of ‐0.34±0.06 and ‐0.41±0.03 PgC yr‐1 decade‐1, respectively. This implies a climate‐forced acceleration of the ocean carbon sink in recent decades, but there are still large uncertainties on the magnitude and cause of this trend. The interannual to decadal variability of the global carbon sink is mainly driven by climate variability, with the climate‐driven variability exceeding the CO2‐forced variability by 2‐3 times. These results suggest that anthropogenic CO2 dominates the ocean CO2 sink, while climate‐driven variability is potentially large but highly uncertain and not consistently captured across different methods.

The Southern Ocean Carbon Cycle 1985–2018: Mean, Seasonal Cycle, Trends, and Storage

“…The majority of models do not account for the river-induced outgassing of carbon (DeVries et al, 2023;Terhaar et al, 2023), hence the air-sea CO 2 flux in simulation A corresponds to the S OCEAN definition used in the Global Carbon Budget (Friedlingstein et al, 2022), which differs from pCO 2 -product estimates by the river-induced term. Note that the river-induced term will be discussed in greater detail in Section 4.1.…”

“…Other than 𝐴𝐴 𝐴𝐴CO 𝑠𝑠𝑠𝑠𝑠𝑠 2 , k w is the largest source of uncertainty in the calculation of bulk air-sea CO 2 fluxes (Wanninkhof, 2014;. However, most of the pCO 2 -products use a quadratic formulation of k w as described by Wanninkhof et al (1993) meaning that the product spread is reduced due to similar choices-details are shown in Global chapter's Table S2 in Supporting Information S1 (DeVries et al, 2023). An exception is the Watson2020 product (Watson et al, 2020) that calculates air sea CO 2 fluxes using the formulation described in Woolf et al (2016) where a cool and salty skin adjustment is applied.…”

“…Throughout this study, we report the air-sea CO 2 exchange as the net flux (FCO 2 ), which is the sum of natural, anthropogenic, and river-induced air-sea CO 2 flux (see e.g., Crisp et al, 2022;DeVries et al, 2023;Hauck et al, 2020). As the GOBMs vary widely in their choices on river carbon and nutrient input into the ocean and burial at the seafloor (see DeVries et al, 2023;Terhaar et al, 2023), an adjustment is applied to make all data classes comparable.…”

An Assessment of CO2 Uptake in the Arctic Ocean From 1985 to 2018

“…Thus, the Arctic Ocean sea-air anthropogenic CO 2 flux accounts for less than 1% of the global ocean sea-air anthropogenic CO 2 flux. Actually, for the years from 1985 to 2018, we find that the anthropogenic sea-air CO 2 flux into the ocean to be 19 ± 6 Tg C yr −1 , ∼1% of the global ocean anthropogenic sea-air CO 2 flux over the same period (DeVries et al, 2023). This relatively small contribution of the Arctic Ocean to the anthropogenic sea-air CO 2 flux may lead to the conclusion that the Arctic Ocean only plays a minor role in the global ocean carbon sink.…”

“…In the remaining regions, the climate effect is of similar magnitude or even larger than the CO 2 effect. In contrast to the Arctic Ocean, the CO 2 effect is much larger than the climate effect in the Southern Ocean (the other polar ocean; Hauck et al, 2023) and the global ocean where the CO 2 flux via the CO 2 effect is 2.1 PgC yr −1 of uptake and the flux from the climate effect is 0.2 PgC yr −1 of outgassing, respectively (DeVries et al, 2023). The Arctic Ocean is thus a unique ocean basin where climate change plays a role of similar magnitude as the increase in atmospheric CO 2 in controlling the sea-air fluxes of CO 2 .…”