The ocean carbon sink under record-high sea surfacetemperatures in 2023/24

In 2023, sea-surface temperatures (SST) reached record highs. Historically, the years with highest global mean SST anomalies were associated with a slight increase in oceanic CO₂ uptake, primarily due to reduced CO₂ outgassing from the tropics during El Niño. In contrast, our observation-based estimates reveal that the global non-polar ocean absorbed about 10% less carbon in 2023 than expected (+0.16±0.28 PgC yr^-1).

This weakening of the ocean carbon sink occurred although the CO₂ outgassing in the tropics was indeed as low as expected. Instead, the decline in CO₂ uptake was concentrated entirely in the extratropics, driven largely by elevated SSTs in the Northern Hemisphere. While thermally induced reductions in CO₂ uptake are well-documented in the extratropics, our analysis using two ocean biogeochemical models highlights a mitigating process in the subtropical North Atlantic: the depletion of dissolved inorganic carbon in the surface mixed layer. Such negative feedbacks caused an overall muted response of the ocean carbon sink to the record high SSTs, but this resilience may not persist under long-term warming or more severe SST extremes.

By the time of this presentation, we anticipate confirming – or refining – our expectation that the ocean carbon sink in 2024 remained unusually weak, because the CO₂ outgassing from the tropics revived, whereas remaining high SSTs in the extratropics continued to suppress the CO₂ uptake.