The Earth’s radiative Energy Imbalance (EEI) is the difference between absorbed solar radiation and outgoing thermal radiation emitted by Earth at the top of the atmosphere (TOA). Integrated over the globe and over multiple years, the EEI provides an estimate of the net energy gain or loss to space by the climate system. From days to interannual time scales, EEI variations are dominated by the effects of internal climate modes of variability such as the El Niño Southern Oscillation (Loeb et al., 2018a). On decadal and longer time scales, changes in solar irradiance, large volcanic eruptions and increasing greenhouse gas (GHG) concentrations are, in part, responsible for EEI variations (Hansen et al.,2011; von Schuckmann et al., 2016, Meyssignac et al. 2019, Loeb et al., 2021). Over the past decades, anthropogenic emissions of GHG have been the dominant cause for a positive EEI (0.4–1 Wm−2) (Hansen et al., 2011; Trenberth et al., 2014, Charles et al. 2020, Kramer et al., 2021), which means that the current mean EEI, measured since the 2000s, represents a measure of the excess of energy that is stored in the climate system as a response to anthropogenic forcing (Trenberth et al., 2014; von Schuckmann et al., 2016, Charles et al. 2020, Tokarska et al. 2020). The EEI is a fundamental climate variable that characterizes the energy state of the climate system; measuring and understanding the causes of the current mean EEI, its time variability and long-term trend is absolutely essential to understanding the current state of climate change and predicting its future evolution.

The WCRP GEWEX core project initiated in 2021 under its Data Analysis Panel (GDAP), an assessment of Earth energy imbalance estimates and their associated uncertainties. The GDAP assessment builds on the recent community white paper published in 2019 on the Earth energy imbalance measurement (Meyssignac et al. 2019). Meyssignac et al. (2019) showed that on interannual and longer time scales, ocean heat uptake (OHU) estimates and TOA radiation measurements provide the most accurate and precise measurements of, respectively, the EEI and its time variations. For this reason, the GDAP EEI assessment focuses mainly on two sources of data: observations of the TOA radiative fluxes from space radiometry and observations of the ocean heat content from in situ data, satellite altimetry, space gravimetry and ocean reanalysis. Note that the energy stored in the climate system is not entirely stored in the ocean through the OHU. About 9% of the energy is actually stored in other components of the climate system such as the cryosphere, the continents and the atmosphere. Although these reservoirs of energy are small and thus play a minor role in estimating  EEI and its uncertainty, they are sizeable and so their values are needed to be estimated precisely typically with continental in situ measurements or atmospheric reanalysis data.

The overall purpose of the GDAP assessment is to design an intercomparison of EEI estimates and EEI uncertainty. This intercomparison should enable to progress on:   

1)      The understanding of the spread of global and regional ocean heat content and ocean heating rates among products;

2)       The detection of systematic errors that depend on assumptions, models, and combined observations;

3)      The understanding of the spread of uncertainties, which depend on the methods and formulas used;

4)      The understanding of error covariance, which depends on regions and ocean layer depth;

5)      The understanding of the differences between the OHU time variability estimated by ocean heat content products and the TOA radiation budget time variability.