Trends and Variability in Earth’s Energy Imbalance and Ocean Heat Uptake since 2005

Earth’s Energy Imbalance (EEI) is a fundamental metric of global earth system change, quantifying the cumulative impact of natural and anthropogenic radiative forcings and feedbacks. To date, the most precise measurements of EEI change are obtained through radiometric observations at the top of the atmosphere (TOA). The quantification of EEI absolute magnitude is facilitated by heat inventory analysis, where ~90% of heat uptake manifests as an increase in ocean heat content (OHC). Various international groups provide OHC datasets derived from in-situ and satellite observations, as well as from reanalyses ingesting many available observations. The WCRP formed the GEWEX-EEI assessment working group to better understand discrepancies, uncertainties and reconcile current knowledge of EEI magnitude, variability, and trends. Here, 18 OHC datasets and ocean heat uptake (OHU) rates are intercompared, providing OHU estimates ranging between 0.40 to 0.96 Wm-2, a spread that is reduced when unequal ocean sampling is accounted for, and that is otherwise attributable to differing source data, mapping methods, and quality control procedures. Rate of increase in OHU varies substantially between 0.11 (an in-situ product) to 1.11 Wm-2 dec-1 (a satellite product). Products that either more regularly observe (satellites) or fill in-situ data-sparse regions based on additional physical knowledge (reanalysis and hybrid products), tend to track radiometric EEI variability better than purely in-situ-based OHC products. This paper also examines zonal trends in TOA radiative fluxes and the impact of data gaps on trend estimates. The GEWEX-EEI community aims to refine their assessment studies, to forge a path toward best-practices, e.g., in uncertainty quantification, and to formulate recommendations for future activities.