ST2.3 Theme Clusters
- Mass balances
(I. Sasgen, F. Flechtner, O. Eisen)
The cluster “Mass balances” aims at determining the contemporary sea-level contributions of the ice sheets from satellite gravimetry (D2.17: Establish attribution systems of recent regional and global sea level change; 2025), along with their dynamic and surface-mass balance components using additional remote sensing and in situ data and models of various complexity (M2.15-2: Operating workflows for timely combination of monitoring systems [altimetry, tide gauges, GNSS, satellite gravimetry, magnetic field observations]; 2026). Further achievements will be made in the application, evaluation and improvement of corrections for glacial-isostatic adjustment (M2.15-3: Established consistent deformation model for loading correction in monitoring systems; 2026). Further enhancement of GRACE-FO data will result from the combination with satellite altimetry (M2.14-3: Combined analysis of GRACE-FO and altimetry with respect to ice mass balances; 2024). The dissemination of mass change data is realized through GFZ’s GravIS portal, while contextual and background information relevant for the public will be part of knowledge transfer within REKLIM.
- Coupled processes
a) Ocean heat and meltwater feedbacks near continental shelves and with the global ocean: Bridging spatial and temporal scales
(T. Kanzow, J. Klages, S. Schmidtko, T. Martin)
Ocean currents transport heat to ice shelves and marine terminating glaciers, which melts the ice from below (ice shelves) or at calving fronts (tide water glaciers). Increasing ocean heat supply may cause ice shelf and glacier retreat, thereby increasing ice sheet drainage and driving bathymetry-dependent instabilities of ice sheets, potentially massively increasing ice discharge and sea level rise. We want to understand present-day ocean-forced ice-shelf and glacier retreat on local and regional scales by employing in situ observations and coupled ocean-ice sheet numerical modelling. In order to put the present-day situation into a longer temporal context, paleo proxy-based evidence for the (ocean-driven) advance and retreat of selected ice shelves will be used. At the same time, we will investigate the impact of the increased freshwater supply from the Greenland and Antarctic ice sheet on the ocean in terms of ocean stratification, surface circulation and the formation of deep and bottom water formation which are vital for the global (deep) ocean circulation. Finally, it would be of huge importance to understand, whether (and if so, how) an increased freshwater runoff actually feeds back onto the ocean heat supply to the ice shelves.
M2.16-1: Determination of sensitivities of present-day ocean-driven melt to changing environmental conditions in key ocean-ice sheet systems (FRIS, 79NG, Thwaites); 2023.
b) Global climate modelling (long-term), tipping points, paleo benchmarks, calibration, sea level equation
(G. Lohmann, V. Klemann, J. Klages, T. Martin)
The major contribution to global mean sea level change under contemporary climate conditions involves thermal expansion of the ocean and outflow from the land ice, with the latter increasing more rapidly in percentage as a form of ice sheets. Current Earth system models can constrain thermal expansion with high confidence in projections. However, few of them have been successfully coupled to an ice sheet model and glacial isostatic adjustment to evaluate past and future evolution of ice sheets. Here, we want to establish a strong collaboration to estimate potential range of sea level changes over the next millennia and past climates to elaborate on potential tipping points in the system (West Antarctic Ice Sheet, Greenland, Feedbacks with the ocean etc.).
M2.17-2: Reconstruction, monitoring and modeling capacity of trajectories of key ocean-ice sheet systems toward tipping points of marine ice sheet instability; 2024.
c) High resolution ice sheet-ocean-solid earth coupling and local fingerprints
(A. Humbert, V. Klemann, R. Timmermann)
In a first step the high-resolution multi-physics ice sheet model ISSM will be coupled to the ocean model FESOM and applications for the Weddell Sea area (FRISio) and 79NG (GROCE2) are planned. The local fingerprinting will be retrieved using ISSM-SESAW. In a next step, VILMA is envisioned to be coupled to ISSM and a proposal for third party funding for this activity will be submitted. In general, these activities are targeted at processes on timescale of decades to centuries and to facilitate coupled projections 2100/2300 rather than long time scales. In addition, we will exploit strategies to employ fluid-structure interaction in the framework of coupled ice-ocean models.
Milestones:
M2c1: simplest coupling scheme between FESOM and ISSM accomplished; 2022.
M2c2: projections of Weddell Sea area accomplished; 2024.
M2.15-3: Established consistent deformation model for loading correction in monitoring systems; 2026.
M2.17-1: Incorporation of critical physical processes in the Earth system model (e.g., interactive ice sheets, interactions with the geosphere, loading, self-gravitation); 2023.
M2.17-3: Generate time-dependent fingerprints to recent ice mass loss; 2025.
- Consistent data sets, process understanding and initialization of model projections
a) empirical:
- Combination of remote sensing and in situ observations of ice sheets and oceans
(O. Eisen, T. Kanzow, T. Schöne, S. Schmidtko, I. Sasgen, C. Förste)
The empirical part of this cluster will aim at combining in-situ observations, such as in-situ mass balances, basal melting, ocean properties or sea level records, with those derived from remote sensing applications, e.g., ice sheet and shelf altimetry, sea surface height and oceanographic properties, or gravitational changes. The in-situ and remote sensing observations will be obtained from M2.14-1 (“COSMUS and ATWAICE expeditions to install infrastructure for year-round observations of warm water flow toward the FRIS and 79NG cavities"; 2022), M2.15-1 (”Established ground-truthing infrastructure to monitor stability and error budget of altimetry [tide gauges, GNSS profiling]“; 2024), M2.17-2 (“Reconstruction, monitoring and modeling capacity of trajectories of key ocean-ice sheet systems toward tipping points of marine ice sheet instability”; 2024) but also other ongoing observational projects. The outcome will be a combined and consistent in-situ/remote sensing data set for essential variables relevant for sea level change and process characterization to be delivered as part of D2.14 (”Established present state and improved paleo-dynamic history of key ice sheet systems [WAIS, NEGIS, 79NG]“; 2024), D2.15 (”Established capabilities for global to regional sea level monitoring“; 2026) and D2.16 (”Establish a system of spatio-temporal boundary conditions based on satellite, in situ monitoring and paleo proxy data for data-constrained modeling“; 2024). This will eventually provide initial and constraining values for model validation as well as initialisation of model projections, e.g., D2.17 (“Establish attribution systems of recent regional and global sea level change”; 2025), D2.18 (“Regional sea level change projections for the years 2030, 2050, 2100 for pathways into 1.5°C, 2°C and 4°C warmer worlds”; 2025). The data sets will also provide the observations to drive the data assimilation of this cluster.
- Consistent quantification of global, regional, and local sea level changes, fingerprinting (coop. with T4)
(T. Schöne, R. Weisse, J. Karstensen)
Various observational data bases contribute to studies on sea level changes on global, regional, and local scales. The radar altimetry observations (since 1992) complemented by a global tide gauge data (at some places starting in the late 18th century) provide the large scale reference. The global radar altimetry measurements constitute an operational backbone for studying and forecasting sea level rise. Consistent estimates of sea level change require inter-mission and inter-technology harmonization (M2.15-1: Established ground-truthing infrastructure to monitor stability and error budget of altimetry [tide gauges, GNSS profiling]; 2024) as well as novel background and correction models (e.g., M2.15-3: Established consistent deformation model for loading correction in monitoring systems; 2026). This will be ensured through existing monitoring infrastructure, e.g., in South East Asia and Lake Issyk Kul. The assimilation of consistent observations in ocean models enables us studying past and future sea level change. Fingerprint methods will assist detecting patterns of sea level change associated with their causes. Finally we will deliver products (M2.15-2: Operating workflows for timely combination of monitoring systems [altimetry, tide gauges, GNSS, satellite gravimetry, magnetic field observations]; 2026) describing the state and development of the sea level focusing on shelf seas in Western Europe and South East Asia
b) Data assimilation and inverse modelling (link to ST2.4)
(A. Humbert, M. Thomas/J. Saynisch-Wagner):
The main focus of this cluster is on the further development and application of data assimilation techniques and inverse modelling, such as Kalman filter or particle filter approaches, optimization techniques, level set methods, as well as tools of artificial intelligence, e.g., neural networks, in order to prove the physical consistency of both observations and model data as well as of different observations based on various monitoring systems and sensors, and representing different physical properties and spatiotemporal scales. By this, uncertainty estimates not only of model approaches but also of various observational products will be derived, eventually improving the initialization of local to global model simulations including the envisaged mid-term projections of sea level variations. The activities will mainly contribute to milestones M2.16-2 (“Provision of uncertainty estimates from model runs simultaneously constrained by complementary observations”; 2023), partly to M2.18-1 (“Establish initialization capacity that allows sea level change scenarios to capture aspects of observed/reconstructed trajectories”; 2023).
