The question

The ocean is one of Earth's two great carbon sinks, quietly removing a large share of our fossil-fuel emissions from the air each year. Its behaviour is not simple, though: warmer water dissolves less gas, but warming can also change currents, biology and how carbon moves between the surface and the deep. In 2023, sea surface temperatures spiked to record highs, driven partly by a strong El Niño.^[1] Based on how the ocean had responded to past warm years, scientists expected uptake to actually rise, because El Niño tends to suppress the CO2-belching upwelling of the tropical Pacific. The question this study set out to answer was straightforward: did the sink strengthen as expected during that record heat, or not?

What they did

Rather than run a single climate model, the team leaned on observations. They combined direct CO2 measurements collected from research vessels, cargo ships and moored buoys with satellite data, using machine learning to fill the gaps and build monthly maps of CO2 pressure across the sea surface.^[1] From those maps they calculated the air-to-sea flux of CO2 — where the ocean was inhaling and where it was exhaling — and compared 2023 against what the historical relationship between global temperature and ocean uptake would have predicted. That comparison is the heart of the paper: not just "how much did the ocean absorb," but "how much did it absorb relative to what a warm year like this should have delivered."

What they found

The global non-polar ocean absorbed about 10% less CO2 than expected in 2023 — a gap of roughly a billion tonnes of CO2, comparable to about half the European Union's annual emissions.^[1] The expected El Niño boost from the tropical Pacific did happen, but it was overwhelmed elsewhere. The biggest culprit was the subtropical and subpolar ocean, especially an exceptionally warm North Atlantic, where high temperatures cut the solubility of CO2 and pushed surface water to outgas.^[1] Crucially, the damage was not as bad as raw warming alone would suggest. Several compensating mechanisms — the outgassing itself relieving pressure, stratification that kept carbon-rich deep water from surfacing, and the biological pump moving organic carbon downward — muted the response. The net effect was a real but restrained dip rather than a collapse.

Why it matters

Climate accounting assumes the ocean will keep absorbing a fairly predictable slice of our emissions. This study is a concrete, observation-based example of that assumption bending under heat. The same record temperatures that make headlines on land also reach into the water column and change the chemistry of carbon uptake, in this case turning an expected gain into a shortfall. It is a reminder that the ocean sink is not a fixed discount on our emissions but a responsive system that can weaken exactly when the climate is most stressed. The finding also lands alongside a parallel result on land: 2023's heat and the Amazon drought drove the land carbon sink to its weakest level in two decades, helping push the atmospheric CO2 growth rate to a record.^[2]

What this does not prove

This is a single anomalous year, not a trend. The authors are careful to frame 2023 as an extreme case study, and one year cannot tell us whether the ocean sink is entering long-term decline. The analysis is observation-based reconstruction, which means it depends on interpolating sparse measurements with statistical models — the maps are estimates, not a complete census of the sea surface. The compensating buffers that limited the dip are a genuinely reassuring sign of resilience, but the study cannot show whether those same buffers will keep working under stronger or more sustained warming; if anything the authors flag the opposite as an open worry.^[1] And a weaker sink in 2023 does not by itself prove that any particular future emissions pathway will unfold — it constrains how the ocean responded to one hot year, nothing more.

What happens next

The obvious next step is time: watching whether the sink rebounds as the 2023–2024 El Niño fades, or whether repeated warm years chip away at it. Longer observational records and continued ship- and buoy-based sampling will test how durable the compensating mechanisms really are, especially in the North Atlantic. The deeper open question the paper leaves is whether the ocean's resilience is a stable feature or a temporary reprieve — and that can only be answered by measuring more warm years as they come.^[1]

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References

1. Müller JD, Gruber N, Schneuwly A, Bakker DCE, Gehlen M, Gregor L, Hauck J, Landschützer P, McKinley GA. Unexpected decline in the ocean carbon sink under record-high sea surface temperatures in 2023. Nature Climate Change. 2025;15(9):978–985. doi:10.1038/s41558-025-02380-4
2. Ke P, Ciais P, Sitch S, et al. Low latency carbon budget analysis reveals a large decline of the land carbon sink in 2023. National Science Review. 2024;11(12):nwae367. doi:10.1093/nsr/nwae367