The oceans currently absorb about a quarter of annual anthropogenic CO2 emissions from the atmosphere. However, by 2023, when Sea surface temperatures (SST) reached record highs, partly due to a strong El Niño phenomenon, there was unexpected decline in ocean carbon sinkThis year was a unique opportunity for scientists to study the impact of extreme temperatures on the functioning of this important absorber.

Based on historical responses to increased global mean SST, oceanic CO2 uptake was expected to increase in 2023. An anomalously strong CO2 uptake of −0.11 ± 0.04 PgC per year was predicted. This expectation was driven primarily by reduced CO2 degassing in the tropical Pacific Ocean, a typical response during El Niño events.

However, observations have shown that the global non-thermal ocean absorbed approximately 10 % less CO2 than expected, representing a decrease of +0.17 ± 0.12 PgC per year. The actual CO2 uptake was 0.27 ± 0.13 PgC per year weaker than expected. This decrease was due to anomalous CO2 degassing in subtropical and subpolar regions, especially in the Northern Hemisphere. The main factor was the influence of increased SSTs, which reduce CO2 solubility in seawater.

The impact of record-high SSTs on ocean CO2 uptake was assessed using four observation-based fCO2 products. These products are statistical machine learning models trained on in situ fCO2 observations.

Regional differences and the tug-of-war The overall response of the ocean carbon sink to unusual warming depends on the regional distribution of SST anomalies and the outcome of the “tug-of-war” between thermal and non-thermal factors.

• In the tropical Pacific Ocean (PEQU-E) The anomalous CO2 uptake was reflected in reduced CO2 degassing. This was primarily due to the reduced outflow of cold, CO2-rich waters, which is non-thermal factorHere, the non-thermal factor clearly won the "tug of war".
• On the contrary, in the North Atlantic Subtropical Permanently Stratified (NA-STPS) biome The highest and most persistent SST anomalies occurred in 2023, leading to a substantial decrease in CO2 uptake. Here, it was dominated by thermal factorInterestingly, Dissolved inorganic carbon depletion (DIC) in NA-STPS at the end of 2023 it acted as a compensatory measure against the thermal impact.
• All biomes North Pacific Ocean also recorded a weakening of CO2 uptake, with these anomalies being considerably stronger than expected based on SST anomalies alone.

Despite these anomalies, the overall response of the ocean carbon sink to record-high SSTs was dampened by negative feedbacks. Previous studies have shown that thermally induced CO2 degassing and reduced DIC outflows caused a negative feedback that compensated for the initial perturbation. However, the question remains whether this resilience will persist under long-term warming or more severe SST extremes. Continued and expanded monitoring is essential to monitor changes in the ocean carbon sink. fCO2 observations. JRi

Study  published in journal Nature Climate Change