Earth Energy Imbalance Assessment Workshop

Background

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.

Objectives

As a core part of the EEI assessment, the submission of individual data sets started at the beginning of 2021. All the ocean heat content data and the TOA radiation data have been collected with similar data formats to ease the intercomparison. The data is now freely available here: https://www.gewex-eei.org/. First analysis from different groups are on-going. This workshop seeks to engage the wider community in using the available data for intercomparison and to participate in the EEI assessment.

The objective of the workshop is to foster the wider EEI community effort along the main lines of the GDAP assessment. This first workshop will enable to share among the community the first results on the EEI assessment, to identify the key elements that need to be worked out to better understand the sources of the spread among EEI estimates and to exchange on how to improve the estimates of EEI, their uncertainty and variability. The workshop will also serve as a place for discussion to identify the main challenges that the community will have to face in the coming decade to estimate the EEI and its variability with a better accuracy and a better precision.

Topics addressed during the workshop include:

1. Status of the different EEI records and their associated uncertainty
2. Sources of uncertainty in different EEI estimates
3. Causes for the spread in EEI records
4. Time variability in EEI records including trends
5. Regional earth energy budget, regional ocean heat uptake, regional ocean heat content

Considering the complementarity of the different science communities (among the satellite radiometry community, the ocean in situ community, the ocean reanalysis community and the satellite altimetry and space gravimetry community) providing insights on the EEI, this workshop presents an opportunity to explore joint solutions with a broader view, and to address key issues including:

• What are the new key climate science questions that rely on EEI estimates and what is the precision and accuracy in EEI that is needed to address them?
• What are the new challenges for the observing system to meet required precision and accuracy in EEI to answer science questions? How should we address these challenges?
• For each technique that allows to estimate the EEI (i.e., in situ technique, satellite geodesy technique, satellite radiometry technique) can we identify all the sources of uncertainty and can we provide a comprehensive estimate of the uncertainty to build a comprehensive error budget? If not, what are the key challenges to be addressed?
• Can we explain the differences between different estimates of the EEI derived from the same measurements (e.g., from in situ data or from reanalyses)?
• Can we explain the differences across different estimates of the EEI derived from different techniques?
• Do new algorithms and new advancements allowing to reduce the uncertainty in EEI estimates for each technique exist?
• Can we provide an estimate of the mean 2002-2020 EEI and the 2005-2020 EEI trend with associated uncertainty that is consistent across the different techniques?
• In the future do we need new types of instruments to measure the EEI with higher accuracy and precision?
• Do current observing systems provide sufficient measurements to explain the EEI change? What analytical methods exits and are needed to interpret change in EEI?
• In the future, can we take advantage of all available data from the different techniques and propose an optimized estimate of the EEI and its uncertainty based on observations only? What would be the best strategy?
• At regional scale, what is the effective spatial and temporal resolution we can achieve in Ocean heat content estimates from the in situ technique and from the space geodetic technique? Could we improve the spatial and temporal coverage by combining the in situ and the space geodetic technique?
• How do we strengthen the collaboration between the five critical science communities: (ocean in situ, space radiometry, space geodesy, reanalysis communities, and surface energy flux) to reduce the uncertainty in EEI estimates at all time scales?

The expected outcome of the workshop is to define an action plan for the future and converge on recommendations from the Scientific Community. A round table discussion is planned to cover the aforementioned seed-questions.