Unexpected decline in the ocean carbon sink under record-high sea surface temperatures in 2023

In 2023, sea surface temperatures (SSTs) reached record highs, partly due to a strong El Niño. Based on historical responses to elevated global mean SSTs, oceanic CO2 uptake in 2023 should have increased (−0.11 ± 0.04 PgC yr−1), driven by reduced outgassing in the tropical Pacific Ocean. However, using observation-based estimates of ocean CO2 fugacity, we show here that the global non-polar ocean absorbed about 10% less CO2 than expected (+0.17 ± 0.12 PgC yr−1). This weakening was caused by the anomalous outgassing of CO2 in the subtropical and subpolar regions, especially in the Northern Hemisphere, driven primarily by elevated SSTs reducing the solubility of CO2. In most regions, this SST-induced outgassing was mitigated by 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. The ocean carbon sink strengthened in previous warm El Niño years due to reduced CO2 outgassing in the tropics. Here the authors show that the ocean carbon sink declined in 2023 despite record-high sea surface temperatures (SSTs), primarily due to SST-driven outgassing of CO2 in the subtropics.