Mean, Trend, variability and uncertainty in Earth's Energy Imbalance over the last two decades

The Earth energy imbalance (EEI) is a fundamental climate variable that characterizes the energy state of the climate system. When integrated over multiple years, EEI estimates provide the net energy gain (or loss) by the climate system. In addition, measuring accurately the EEI along with surface temperature and atmospheric composition is essential to separate the role of different radiative forcing from the role of feedbacks on the global energy budget enabling further to constraint effective and equilibrium climate sensitivities. In this presentation I review the current EEI observing system performance and uncertainty. I intercompare the different EEI datasets, originating from in-situ and space-based observing systems to evaluate their differences and to assess their uncertainty. Since 2000 the Clouds and the Earths Radiant Energy System (CERES) project provides satellite-based observations of the Earth radiation budget and the EEI with the highest precision (0.3W.m-2 -1s- on a monthly basis). Nevertheless, because of limitation in the absolute calibration of CERES radiometers the CERES final product needs a bias correction (of about 2.5W.m-2 -1s-) on the EEI mean. The current best approach to estimating the mean EEI is to estimate the ocean heat uptake (OHU) which represent 89% of the energy storage due to the EEI. Today, the OHU can be derived with the highest accuracy (0.18W.m-2 -1s- on the mean OHU), from in situ ocean temperature measured by Argo or from the thermal expansion estimated by the difference between satellite altimetry sea level and ocean mass from GRACE. On 2-yr and longer time scales, OHU and CERES EEI estimates show good agreement in EEI variability. But OHU approaches cannot resolve the EEI variability below 1 yr because the energy gain (or loss) induced by EEI over such small time-scales is of the same order of magnitude as the global exchanges of energy between the atmosphere and the ocean. The different EEI measurements have enabled since 2005 a robust estimate of the mean EEI of +0.750.18W.m-2 that is essentially due to anthropogenic emissions of greenhouse gases (GHG). They have also allowed to detect a significantly positive trend in EEI of 0.40.3W.m-2 per decade, leading to a doubling of the EEI during the past 20 years in response to continued increases in GHG emissions combined with decreases in aerosol emissions. In addition, on interannual time scales, they showed that the variability in EEI is mostly sensitive to low cloud variability, with ENSO controlling the 0.5W.m-2 variability on the 4-7yr time scale. Today, new scientific challenges related to EEI are emerging like the closure of the energy budget from top of the atmosphere to the bottom of the ocean at monthly to decadal time scales, the estimate of the current effective climate sensitivity, the monitoring of the physical climate system response to GHG mitigation policies and others. These new challenges lead to new requirements on the EEI observing system ranging from sustained continuity to higher precision and accuracy. I discuss briefly the need to refine these requirements and some opportunities to meet them in the future.