Six-fold reduction in ocean heat content estimate uncertainty since 1960

Ocean heat content (OHC) is a crucial climate indicator and provides an estimate of Earth’s energy imbalance, but its accuracy and uncertainty have not been fully understood. Here, we constructed a Large-Ensemble to comprehensively quantify the OHC uncertainty from global to regional scales since 1955, and consider eight groups of error sources based on the community’s efforts. We find a six-fold reduction in the global upper 2000 m OHC estimated uncertainty (5–95% quartile error range) since 1955 from ~ 111 ZJ (1955–1965) to ~ 19 ZJ (2014–2023), attributed to the reduction of errors from all sources especially mapping/sampling. This indicates a robust upper 2000 m OHC increase since 1955 of 449 [385, 519] ZJ. Quality-Control (QC) has emerged as a primary error source in the recent decade, because of the difficulties in detecting outliers in large perturbations in eddy-rich regions. The trend uncertainty has different sources from the annual OHC: from 2005–2023, the upper 2000m OHC trend is 10.9 [9.2, 11.4] ZJ yr ^− 1 or equivalently 0.68 [0.57, 0.71] Wm ^− 2 , with uncertainty mainly stemming from spatial gaps and mapping errors, QC, and climatology choice. An increase in ocean warming rate from 2005–2023 is small but robust: 0.34 [0.19, 0.46] Wm ^− 2 dec ^− 1 . The comprehensive OHC uncertainty estimates provide a basis for model evaluation and a guide for future improvements in the global ocean observing system.