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

Abstract

Abstract 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.