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JWG C.3: Assessing Earth’s Energy Balance with geodeticobservations

Chair: Maria Hakuba (USA)
Vice chair: Felix Landerer (USA)
Affiliations: Commission 2, GGOS

Introduction

The Earth’s Energy Imbalance (EEI), defined as the long-term global mean net radia-tive flux at the top-of-the-atmosphere (TOA), represents the rate at which our planet accumulates heat in response to radiative forcings and feedbacks, making it a key metric for quantifying ongoing global warming. EEI is often used as a target value in global climate model tuning and to constrain the equilibrium climate sensitivity, although its accurate quantification is challenging. In principle, the absolute EEI magnitude is the small difference between large incoming and outgoing radiative fluxes at the TOA. But due to calibration and retrieval uncertainties that are one order of magnitude larger than EEI itself, EEI cannot be derived from radiometric observations, unless adjustments are made to match the global net radiative flux to independent estimates of long-term planetary heat uptake. These heat uptake estimates have ranged between 0.4 and 1.2 W m-2 over the past few decades.Since the ocean absorbs about 90% of the incoming energy surplus, EEI assess-ments largely hinge on ocean heat uptake (OHU). To date, standard approaches to estimate OHU are:

  1. to derive ocean heat content (OHC) changes from direct sub-surface ocean temperatures through hydrographic profiles;
  2. to derive the oceans’ thermosteric expansion through sea level budget assessments using geodetic observations from space;
  3. to estimate the ocean state using global ocean models and reanalyses that assimilate various ocean and atmosphere observations.

The geodetic approach estimates OHU throughout the entire ocean volume as a residual of global-mean sea-level (GMSL) and ocean-mass changes (2002 to present).Changes in GMSL are caused by thermal expansion due to OHU (thermosteric changes) and by ocean mass changes (barystatic), the latter being driven by ice-mass and terrestrial water storage (TWS) changes. Subtracting the ocean-mass change from the change in GMSL yields an indirect estimate of full-column steric sea-level change, provided that corrections for glacial isostatic adjustments (GIA) and sea-floor defor-mations due to contemporary mass changes (GRD) are accounted for. The relation between steric sea-level change and OHU requires knowledge of the ocean’s expansion efficiency of heat. The difference between the upper-ocean OHU from hydrographic observations and the geodetic full-column estimate could provide information on the sparsely observed deep-ocean steric expansion; however, to unambiguously detect thesmall deep OHU, requires to reduce the combined uncertainties from the hydrographicand geodetic methods.

Objectives

The objectives are:

  • to exploit geodetic observations to close Earth’s energy budget and derive change in Earth’s energy imbalance;
  • to provide global and regional satellite-based Ocean Heat Uptake for model valida-tion/calibration and climate analysis (e.g., variability and trends, constraints on climate sensitivity, international EEI assessments).

Program of Activities

  • Take stock of and bring together groups who provide geodetic global and regional Ocean Heat Content (OHC) products.
  • Intercompare methods, uncertainty derivation, and investigate regional/global discrepancies.
  • Work toward best practices for deriving global Ocean Heat Uptake (OHU) and change therein.
  • Work towards regional OHC estimates from geodetic data: Treatment and computation of the halosteric component and expansion efficiency.
  • Assessment of global/regional OHC and OHU against in-situ observations and ocean models/reanalysis.
  • Application of geodetic OHC and OHU estimates in global climate model evaluation, and study of EEI variability.
  • Outstanding GRACE/GRACE-FO-centered investigations to improve EEI estimates.

Members

  • Anne Barnoud (France)
  • Gael Forget (USA)
  • Sebastien Fourest (France)
  • William Llovel (France)
  • Audrey Miniere (France)
  • Gavin Schmidt (USA)
  • Bernd Uebbing (Germany)
  • Karina von Schuckmann (France)