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

Ocean heat content (OHC) dominates Earth's energy imbalance, but its uncertainty has not been fully understood. Here, we construct 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. The global upper 2000 m OHC estimated uncertainty (defined as 5-95% quartile error range) has reduced six-fold since 1955 from ~111 ZJ (1955-1965) to ~19 ZJ (2014-2023), mainly due to reduced mapping/sampling uncertainty. This indicates a robust upper 2000 m OHC increase since 1955 of 449 [385, 519] ZJ. Quality-Control (QC) has become a primary error source recently, due to the difficulties detecting outliers in eddy-rich regions. Trend uncertainty sources differ from annual OHC: for 2005-2023, the upper 2000m trend is 10.9 [9.2, 11.4] ZJ yr^-1 (0.68 [0.57, 0.71] Wm^-2), with uncertainties from spatial gaps, mapping, QC, and climatology choice. The ocean warming rate increase from 2005-2023 is small but robust: 0.34 [0.19, 0.46] Wm^-2 dec^-1. These comprehensive uncertainty estimates guide future improvements in the global ocean observing system.