The Southern Ocean carbon sink has been overestimated in the past three decades

• Published in: Communications Earth & Environment
• Main Authors: Zhong, Guorong, Li, Xuegang, Song, Jinming, Wang, Fan, Qu, Baoxiao, Wang, Yanjun, Zhang, Bin, Ma, Jun, Yuan, Huamao, Duan, Liqin, Wang, Qidong, Xing, Jianwei, Dai, Jiajia
• Format: Report
• Language: English
• Published: SPRINGERNATURE 2024
• Subjects: Environmental Sciences & Ecology; Geology; Meteorology & Atmospheric Sciences; Environmental Sciences; Geosciences; Multidisciplinary; GRIDDED DATA; CO2; SEA; PCO(2); VARIABILITY; CLIMATOLOGY; EXCHANGE; RECONSTRUCTION; SEAWATER; COASTAL; Sea ice; Southern Ocean
• Online Access: http://ir.qdio.ac.cn/handle/337002/185318; https://doi.org/10.1038/s43247-024-01566-6

Employing machine learning methods for mapping surface ocean pCO2 has reduced the uncertainty in estimating sea-air CO2 flux. However, a general discrepancy exists between the Southern Ocean carbon sinks derived from pCO2 products and those from biogeochemistry models. Here, by performing a boosting ensemble learning feed-forward neural networks method, we have identified an underestimation of the surface Southern Ocean pCO2 due to notably uneven density of pCO2 measurements between summer and winter, which resulted in about 16% overestimating of Southern Ocean carbon sink over the past three decades. In particular, the Southern Ocean carbon sink since 2010 was notably overestimated by approximately 29%. This overestimation can be mitigated by a winter correction in algorithms, with the average Southern Ocean carbon sink during 1992-2021 corrected to -0.87 PgC yr-1 from the original -1.01 PgC yr-1. Furthermore, the most notable underestimation of surface ocean pCO2 mainly occurred in regions south of 60 degrees S and was hiding under ice cover. As the surface ocean pCO2 under sea ice coverage in the winter is much higher than the atmosphere, if sea ice melts completely, there could be a further reduction of about 0.14 PgC yr-1 in the Southern Ocean carbon sink. Surface partial pressure of carbon dioxide in the Southern Ocean has been underestimated, in particular beneath sea ice, because of uneven observational density in winter and summer, suggesting that the carbon sink may weaken further in the future, according to analyses of in-situ observations with a neural-network-based method.