Over the last decades, greenhouse gases and aerosols concentrations have increased in the atmosphere, generating an energy imbalance between incoming and outgoing radiation fluxes at the top of the atmosphere (TOA). Part of the outgoing longwave radiation being blocked, the system has reemitted less energy towards space than it has received from the Sun (e.g. Hansen et al., 2011; Trenberth et al., 2014). This imbalance at the top of the atmosphere, known as the Earth energy imbalance (EEI), is about 0.5-1 W m-2 (e.g. Meyssignac et al., 2019, Loeb et al. 2021). It is challenging to estimate the EEI since it is two orders of magnitude smaller than the mean incoming solar radiation  and the mean outgoing longwave radiation at TOA (~340 W m-2, e.g. L’Ecuyer et al., 2015).

Positive values of the EEI indicate that an excess of energy is stored in the climate system. With its high thermal inertia and its large volume, the ocean acts as a buffer, accumulating in the form of heat most of the excess of energy that is entering the climate system  (>90%, e.g. von Schuckmann et al., 2020). The other climate reservoirs, the atmosphere, land and cryosphere, play a minor role (<10%) in the energy storage at interannual and longer time scales (e.g. Palmer and Mc Neall 2014). As a result, the global ocean heat uptake (OHU) prevails in the global energy budget on timescales longer than interannual. The global OHU is therefore a good proxy of the EEI variations at these time scales .

Measuring and understanding Earth Energy Imbalance (EEI), its time variability, and long-term trend is essential to understanding the current state of our planet and predicting its future evolution. Under its Data and Analysis Panel (GDAP), the World Climate Research Programme’s (WCRP) core project Global Energy and Water Exchanges (GEWEX) initiated the “Earth Energy Imbalance assessment”.

The GEWEX-EEI assessment’s objective is to evaluate the quality and the uncertainty of current EEI estimates. It focuses mainly on two sources of data:

1) observations of the TOA radiative fluxes from space radiometry, and

2) observations of the ocean heat content from in situ data, satellite altimetry, space gravimetry and ocean reanalysis.

The approach adopted in the GEWEX-EEI assessment is to design an intercomparison of EEI estimates and associated uncertainty. This intercomparison enables progress on:

•     Assessing the quality of different EEI records

•     Understanding the sources of uncertainty in different EEI estimates

•     Understanding the causes for the spread in EEI estimates

•     Quantifying and understanding EEI time variability including trends

•     Improving regional earth energy and heat uptake estimates