A deep-learning estimate of the decadal trends in the Southern Ocean carbon storage

Abstract

Uptake of atmospheric carbon by the ocean, especially at high latitudes, plays an important role in offsetting anthropogenic emissions. At the surface of the Southern Ocean south of 30∘S, the ocean carbon uptake, which had been weakening in 1990s, strengthened in the 2000s. However, sparseness of in-situ measurements in the ocean interior make it difficult to compute changes in carbon storage below the surface. Here we develop a machine-learning model, which can estimate concentrations of dissolved inorganic carbon (DIC) in the Southern Ocean up to 4 km depth only using data available at the ocean surface. Our model is fast and computationally inexpensive. We apply it to calculate trends in DIC concentrations over the past three decades and find that DIC decreased in the 1990s and 2000s, but has increased, in particular in the upper ocean since the 2010s. However, the particular circulation dynamics that drove these changes may have differed across zonal sectors of the Southern Ocean. While the near-surface decrease in DIC concentrations would enhance atmospheric CO2 uptake continuing the previously-found trends, weakened connectivity between surface and deep layers and build-up of DIC in deep waters could reduce the ocean’s carbon storage potential.

An Ice Sheet‐to‐Ocean Analysis of Carbon Stores and Fluxes in Earth's Polar Regions (RECCAP2, Polar Ice Sheets)

“…Around 70% of benthic PIC falls in the SPSS biome, which is consistent with its intersection with the “great calcite belt” (Balch et al., 2011) of the polar frontal zone compared with more siliceous sediments of Antarctic Zone “opal belt” (Chase et al., 2015), reflecting differing planktonic assemblages and export to the deep ocean (Krumhardt et al., 2020; Sachs et al., 2009). Higher stocks of DIC in the SPSS biome are consistent with its larger volume (65% of DIC in the SPSS, for 65% of Southern Ocean volume), but DIC concentrations in upper ocean layers rise polewards (Figure S16 in Supporting Information S1) as has been reported in other studies, reflecting the importance of upwelling and cold temperatures toward the Antarctic continent (Gregor & Gruber, 2021; Wu et al., 2019; Zemskova et al., 2022).…”

“…10.1029/2025GB008677 concentrations in upper ocean layers rise polewards (Figure S16 in Supporting Information S1) as has been reported in other studies, reflecting the importance of upwelling and cold temperatures toward the Antarctic continent (Gregor & Gruber, 2021;Wu et al, 2019;Zemskova et al, 2022).…”

An Ice Sheet‐to‐Ocean Analysis of Carbon Stores and Fluxes in Earth's Polar Regions (RECCAP2, Polar Ice Sheets)

“…Around 70% of benthic PIC falls in the SPSS biome, which is consistent with its intersection with the “great calcite belt” (Balch et al., 2011) of the polar frontal zone compared with more siliceous sediments of Antarctic Zone “opal belt” (Chase et al., 2015), reflecting differing planktonic assemblages and export to the deep ocean (Krumhardt et al., 2020; Sachs et al., 2009). Higher stocks of DIC in the SPSS biome are consistent with its larger volume (65% of DIC in the SPSS, for 65% of Southern Ocean volume), but DIC concentrations in upper ocean layers rise polewards (Figure S16 in Supporting Information S1) as has been reported in other studies, reflecting the importance of upwelling and cold temperatures toward the Antarctic continent (Gregor & Gruber, 2021; Wu et al., 2019; Zemskova et al., 2022).…”

“…10.1029/2025GB008677 concentrations in upper ocean layers rise polewards (Figure S16 in Supporting Information S1) as has been reported in other studies, reflecting the importance of upwelling and cold temperatures toward the Antarctic continent (Gregor & Gruber, 2021;Wu et al, 2019;Zemskova et al, 2022).…”

Revealing Long-Term Multi-Factor Climate Impacts on Antarctic Phytoplankton: A Trend-Based Approach Using STL and Piecewise SEM

“…Recent studies have highlighted ongoing ocean acidification in the Southern Ocean. Zemskova et al[52] showed that dissolved inorganic carbon decreased in the 1990s and 2000s but has increased in surface waters since the 2010s south of 60 Metzl et al [53] reported a long-term decline in annual mean surface pH from 1985 to 2020 south of the Antarctic Convergence, confirming sustained acidification. This study demonstrates that ocean acidification is not only ongoing but pronounced across the entire Southern Ocean south of the Polar Front over multi-decadal timescales.…”

Increased methane emissions from oil and gas following the Soviet Union’s collapse

“…The deep learning model we use is adapted from the U-net model, which was originally proposed for biomedical segmentation problems ( 46 ). The U-net model has been recently widely applied in the field of earth science ( 47 – 50 ). The schematic diagram of the model architecture is shown in SI Appendix , Fig.…”

An Ice Sheet‐to‐Ocean Analysis of Carbon Stores and Fluxes in Earth's Polar Regions (RECCAP2, Polar Ice Sheets)

“…Around 70% of benthic PIC falls in the SPSS biome, which is consistent with its intersection with the “great calcite belt” (Balch et al., 2011) of the polar frontal zone compared with more siliceous sediments of Antarctic Zone “opal belt” (Chase et al., 2015), reflecting differing planktonic assemblages and export to the deep ocean (Krumhardt et al., 2020; Sachs et al., 2009). Higher stocks of DIC in the SPSS biome are consistent with its larger volume (65% of DIC in the SPSS, for 65% of Southern Ocean volume), but DIC concentrations in upper ocean layers rise polewards (Figure S16 in Supporting Information S1) as has been reported in other studies, reflecting the importance of upwelling and cold temperatures toward the Antarctic continent (Gregor & Gruber, 2021; Wu et al., 2019; Zemskova et al., 2022).…”

“…10.1029/2025GB008677 concentrations in upper ocean layers rise polewards (Figure S16 in Supporting Information S1) as has been reported in other studies, reflecting the importance of upwelling and cold temperatures toward the Antarctic continent (Gregor & Gruber, 2021;Wu et al, 2019;Zemskova et al, 2022).…”

Revealing Long-Term Multi-Factor Climate Impacts on Antarctic Phytoplankton: A Trend-Based Approach Using STL and Piecewise SEM

“…Recent studies have highlighted ongoing ocean acidification in the Southern Ocean. Zemskova et al[52] showed that dissolved inorganic carbon decreased in the 1990s and 2000s but has increased in surface waters since the 2010s south of 60 Metzl et al [53] reported a long-term decline in annual mean surface pH from 1985 to 2020 south of the Antarctic Convergence, confirming sustained acidification. This study demonstrates that ocean acidification is not only ongoing but pronounced across the entire Southern Ocean south of the Polar Front over multi-decadal timescales.…”

Increased methane emissions from oil and gas following the Soviet Union’s collapse

“…The deep learning model we use is adapted from the U-net model, which was originally proposed for biomedical segmentation problems ( 46 ). The U-net model has been recently widely applied in the field of earth science ( 47 – 50 ). The schematic diagram of the model architecture is shown in SI Appendix , Fig.…”

An Ice Sheet‐to‐Ocean Analysis of Carbon Stores and Fluxes in Earth's Polar Regions (RECCAP2, Polar Ice Sheets)

“…Around 70% of benthic PIC falls in the SPSS biome, which is consistent with its intersection with the “great calcite belt” (Balch et al., 2011) of the polar frontal zone compared with more siliceous sediments of Antarctic Zone “opal belt” (Chase et al., 2015), reflecting differing planktonic assemblages and export to the deep ocean (Krumhardt et al., 2020; Sachs et al., 2009). Higher stocks of DIC in the SPSS biome are consistent with its larger volume (65% of DIC in the SPSS, for 65% of Southern Ocean volume), but DIC concentrations in upper ocean layers rise polewards (Figure S16 in Supporting Information S1) as has been reported in other studies, reflecting the importance of upwelling and cold temperatures toward the Antarctic continent (Gregor & Gruber, 2021; Wu et al., 2019; Zemskova et al., 2022).…”

“…10.1029/2025GB008677 concentrations in upper ocean layers rise polewards (Figure S16 in Supporting Information S1) as has been reported in other studies, reflecting the importance of upwelling and cold temperatures toward the Antarctic continent (Gregor & Gruber, 2021;Wu et al, 2019;Zemskova et al, 2022).…”

Revealing Long-Term Multi-Factor Climate Impacts on Antarctic Phytoplankton: A Trend-Based Approach Using STL and Piecewise SEM

“…Recent studies have highlighted ongoing ocean acidification in the Southern Ocean. Zemskova et al[52] showed that dissolved inorganic carbon decreased in the 1990s and 2000s but has increased in surface waters since the 2010s south of 60 Metzl et al [53] reported a long-term decline in annual mean surface pH from 1985 to 2020 south of the Antarctic Convergence, confirming sustained acidification. This study demonstrates that ocean acidification is not only ongoing but pronounced across the entire Southern Ocean south of the Polar Front over multi-decadal timescales.…”

Increased methane emissions from oil and gas following the Soviet Union’s collapse

“…The deep learning model we use is adapted from the U-net model, which was originally proposed for biomedical segmentation problems ( 46 ). The U-net model has been recently widely applied in the field of earth science ( 47 – 50 ). The schematic diagram of the model architecture is shown in SI Appendix , Fig.…”