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In 2019 the Combination Service for Time-variable Gravity fields (COST-G) started operation with the release of the complete time-series of combined GRACE monthly gravity fields COST-G GRACE RL01. Meanwhile additional time-series of monthly GRACE gravity fields have become available, mainly from Chinese analysis centers. The COST-G quality control confirms the high quality of these solutions that are derived with independent analysis tools.In the frame of the Horizon 2020 project Global Gravity-based Groundwater Product (G3P) the weighting scheme of the monthly GRACE-FO gravity fields was adapted to take into account the limitations on the solution space imposed by the specific observation geometry of the GRACE/GRACE-FO missions.We present the new COST-G GRACE RL02 combination, based on an extended set of time-series including COST-G's new Chinese partners, and combined applying the G3P weighting scheme., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Given the major role of the Atlantic Ocean in the climate system, it is essential to characterize the temporal and spatial variations of its heat content. The 4DATLANTIC-OHC Project (https://eo4society.esa.int/projects/4datlantic-ohc/) aims at developing and testing space geodetic methods to estimate the local ocean heat content (OHC) changes over the Atlantic Ocean from satellite altimetry and gravimetry. The strategy developed in the frame of the ESA MOHeaCAN Project (https://eo4society.esa.int/projects/moheacan/) is pursued and refined at local scales both for the data generation and the uncertainty estimate. At two test sites, OHC derived from in situ data (RAPID and OVIDE-AR7W) are used to evaluate the accuracy and reliability of the new space geodetic based OHC change. The Atlantic OHC product will be used to better understand the complexity of the Earth’s climate system. In particular, the project aims at better understanding the role played by the Atlantic Meridional Overturning Circulation (AMOC) in regional and global climate change, and the variability of the Meridional Heat transport in the North Atlantic. In addition, improving our knowledge on the Atlantic OHC change will help to better assess the global ocean heat uptake and thus estimate the Earth’s energy imbalance more accurately as the oceans absorb about 90% of the excess energy stored by the Earth system. The objectives of the 4DATLANTIC-OHC Project will be presented. The scientific requirements and data used to generate the OHC change products over the Atlantic Ocean and the first results in terms of development will be detailed. At a later stage, early adopters are expected to assess the OHC products strengths and limitations for the implementation of new solutions for Society. The project started in June 2021 for a 2-year duration. Visit https://www.4datlantic-ohc.org to follow the main steps of the project.

The energy radiated by the Earth toward space does not compensate the incoming radiation from the Sun leading to a small positive energy imbalance at the top of the atmosphere (0.4–1 Wm–2). This imbalance is coined Earth’s Energy Imbalance (EEI). At decadal time scales, it is mostly caused by anthropogenic greenhouse gas emissions and it is driving the current warming of the planet. Combined with surface temperature measurements the EEI measurement informs on the sensitivity of the climate system to GHG emissions (the so-called climate sensitivity). Thus monitoring precisely the EEI is critical to assess the current status of climate change, estimate the climate sensitivity and by this mean evaluate the future evolution of climate. But the monitoring of EEI is challenging as it is two orders of magnitude smaller than the radiation fluxes in and out of the Earth system. Over 90% of the excess of energy that is gained by the Earth in response to the positive EEI accumulates into the ocean in the form of heat such that the monitoring of Ocean Heat Content (OHC) and its long-term change provides a precise estimate of EEI. Today, global OHC changes can be tracked from space with a combination of the altimetric measurement of sea level change and the gravimetric measurement of ocean mass change. In this talk we review this current space method to estimate global OHC changes and evaluate its relevance to derive EEI estimates on different time scales. We compare its performance with an independent estimate from direct observations of in situ temperature. Then, we use both the space based and in situ based estimates of EEI along with the surface temperature record to derive estimates of the 20th century mean effective climate sensitivity. Accounting for the internal variability (with an explicit representation of the so called “pattern effect”) we derive from our observed 20th century effective climate sensitivity an observational constraint on the climate sensitivity of 3.4 [1.5;20.8] K (median, 5-95% CI) with the space geodetic data. With a longer in situ dataset, we obtain a tighter constraint of of 5.4 [2.4;35.6] K.

The altimetry data record (1993-2022) has enabled the global mean sea level (GMSL) rise to be quantified. Not only is the sea level rising, but it is accelerating (Dieng et al., 2017; Nerem et al., 2018; Ablain et al. 2019) and understanding this acceleration is necessary both to support society’s response to climate change and to inform climate research on the sea level budget, greenhouse gas forcing and the Earth energy imbalance. Here we describe work on quantifying the uncertainty on the sea level climate data record, and identifying the uncertainty needed to provide new answers to the climate research questions. Work by Ablain et al. (2019) and updated in Guérou et al. (2022) at a global scale, has quantified GMSL rise as +3.3 mm/yr (90% confidence uncertainties 0.3 mm/yr) over 1993-2021. Its acceleration has been quantified at 0.12 ± 0.05 mm/yr². At local scales, (Prandi et al. 2021), the sea level is rising almost everywhere over the globe, at rates ranging between 0 and 6 mm/yr, with uncertainties ranging from 0.8 to 1.2 mm/yr depending on the location. The local sea level accelerations are ranging between -1 mm/yr² and +1 mm/yr² with uncertainties between 0.057 and 0.12 mm/yr² (Prandi et al., 2021). Here we present more recent work refining these uncertainty estimates through a metrological approach that propagates uncertainties, and error covariance structures, through the full processing chain of the altimeter measurement from the raw waveform to GMSL, using methods described in Mittaz et al. (2019). We review the assumptions in the derivation of the retracking model from the radar equation, consider error correlation structures in the corrections, and provide fresh consideration of the error covariance matrix, building on the earlier research. The work was developed in the ASELSU project funded by ESA. The project has also reviewed the climate science research questions to understand the need for improved altimeter observations. There are three major science questions that require accuracies greater than those currently achieved. These questions are the closure of the sea level budget, the detection and attribution of the signal in sea level that is forced by greenhouse gas emissions (GHG) and the estimate of the Earth energy imbalance (EEI). Dieng et al. 2017: https://doi.org/10.1002/joc.4996. Nerem et al. 2018: https://doi.org/10.1073/pnas.1717312115 Ablain et al 2019: https://doi.org/10.5194/essd-2019-10 Guérou et al. 2022: https://doi.org/10.5194/egusphere-2022-330 Prandi et al. 2021: https://doi.org/10.1038/s41597-020-00786-7 Mittaz et al. 2019: https://doi.org/10.1088/1681-7575/ab1705

The International Combination Service for Time-variable Gravity Fields (COST-G) is a Product Center of IAG’s International Gravity Field Service (IGFS). COST-G provides consolidated monthly global gravity fields in terms of spherical harmonic coefficients and thereof derived grids of surface mass changes by combining existing solutions or normal equations from COST-G analysis centers (ACs) and partner analysis centers (PCs). The COST-G ACs adopt different analysis methods but apply agreed-upon consistent processing standards to deliver time-variable gravity field models from GRACE/GRACE-FO low-low satellite-to-satellite tracking (ll-SST) and from Swarm GPS high-low satellite-to-satellite tracking (hl-SST). The poster presents the organizational structure of COST-G and results from the first releases of combined monthly GRACE and GRACE-FO solutions. In addition, the perspectives from the provision of deterministic signal models (DSM), allowing for a few months prediction and therefore application in operational precise orbit determination activities of low Earth satellites, are discussed.

The satellite altimetry observing system has proved since 1992 and the TopEx/Poseidon mission its capacity to monitor the sea level rise induced by climate change over the globe. On the basis of this accurate monitoring of sea level, space agencies developed climate data records that are now commonly used by scientists to study climate change and by stakeholders to support their decisions on climate change adaptation and mitigation strategies. For science users as for stakeholders it is essential to quantify the exact stability and accuracy of the altimetry-based climate indicators as they are at the basis of their analysis and assessments. In this context, we will present the latest released of the Global Mean Sea Level (GMSL) climate data record produced by the French space agency CNES and distributed on the AVISO+ website. This data record is based on reprocessed along-track data, so-called L2P21 data, of the reference missions TopEx-Poseidon, Jason-1/-2 and -3. It is delivered with an estimate of the associated uncertainties following the method developed in Ablain et al. (2019). Based on an updated version of the uncertainty budget of TopEx and Jasons altimetry missions, presented in Guerou et al. (2022, submitted), we will demonstrate that we achieve stability performances of ± 0.3 mm/yr at the 5-95% confidence level for the GMSL trend over the full data record and ± 0.06 mm/yr^2 (5-95% C.L) for the GMSL acceleration. In addition we will present the relative contribution of each uncertainty budget contributor (i.e., the altimeter, the radiometer, the orbit determination, the geophysical corrections) to the total GMSL uncertainty. Thanks to such analysis, we will identify the current limiting factors to the GMSL monitoring stability and accuracy and suggest key topics of investigations to help the altimetry community to improve the GMSL accuracy in the future.

Le changement climatique est déterminé par le déséquilibre au sommet de l’atmosphère entre l’énergie radiative entrante émise par le soleil et l’énergie radiative sortante émise par la Terre. Depuis le début de l’ère industrielle, les concentrations atmosphériques en gaz à effet de serre ont progressivement diminué les émissions radiatives sortantes et créé un déséquilibre radiatif au sommet de l’atmosphère. Du fait de ce déséquilibre, l’énergie s’est accumulée dans le système climatique, essentiellement sous forme de chaleur. Par sa très grande capacité calorifique, c’est l’océan qui stocke la quasi-totalité de cette énergie. En conséquence il se réchauffe, se dilate et le niveau de la mer monte.Au cours de mes recherches j’ai montré que les satellites géodésiques d’altimétrie radar et de gravimétrie permettent de mesurer la montée du niveau de la mer due à la dilatation thermique des océans et d’en déduire le déséquilibre énergétique de la planète. Ces nouvelles mesures, avec leur couverture globale et leur faible incertitude apportent une nouvelle contrainte observationnelle sur le bilan énergétique de la Terre. Combinées aux mesures radiative au sommet de l’atmosphère (par CERES) elles permettent en particulier de contraindre les termes du bilan d’énergie à la surface de la Terre ; combinées aux mesures de la température de surface de la Terre, elles permettent aussi de contraindre la réponse radiative de la Terre à un doublement de la concentration en CO2 de l’atmosphère ( i.e. la sensibilité climatique). En particulier mes travaux montrent que les sensibilités climatiques faibles, c’est à dire en dessous de 2.6K, ne sont pas compatibles avec les mesures de la dilatation thermique de l’océan confirmant ainsi les résultats des modèles de climat.

International audience; In this study, we compare the spatial patterns of simulated geocentric sea-level change to observations from satellite altimetry over the period 1993-2015 to assess whether a forced signal is detectable. This is challenging, as on these time scales internal variability plays an important role and may dominate the observed spatial patterns of regional sea-level change. Model simulations of regional sea-level change associated with sterodynamic sea level, atmospheric loading, glacier mass change, and ice-sheet surface mass balance changes are combined with observations of groundwater depletion, reservoir storage, and dynamic ice-sheet mass changes. The resulting total geocentric regional sea-level change is then compared to independent measurements from satellite altimeter observations. The detectability of the climate-forced signal is assessed by comparing the model ensemble mean of the 'historical' simulations with the characteristics of sea-level variability in pre-industrial control simulations. To further minimize the impact of internal variability, zonal averages were produced. We find that, in all ocean basins, zonally averaged simulated sea-level changes are consistent with observations within sampling uncertainties associated with simulated internal variability of the sterodynamic component. Furthermore, the simulated zonally averaged sea-level change cannot be explained by internal variability alone—thus we conclude that the observations include a forced contribution that is detectable at basin scales.