Owing to vertical land movement (VLM), Norway has long had falling or stable relative sea levels and is yet to feel the impacts of sea-level rise. The danger is that this can foster a false sense of security, where the long-term risks are not understood or ignored.

Results from a recent national assessment show that sea-level rise is starting to push up water levels in some parts of the coast, most notably in Western and Southern Norway. Owing to global warming, Norway is transitioning from a country with on average falling or stable relative sea level, to one with rising relative sea levels. Measured coastal average geocentric (the ocean surface) sea-level rise is 2.3 ± 0.3 mm/yr for the period 1960-2022, i.e., an increase of 14 ± 2 cm over that time.

IPCC AR6 sea-level projections are tailored to the Norwegian coast using the semi-empirical model NKG2016LU to estimate VLM and associated geoid changes. Although the broad pattern of regional VLM is caused by glacial isostatic adjustment, there is evidence of other processes driving changes, especially on local scales. Projections show Norway’s coastal average relative sea-level change for 2100, compared to the period 1995-2014, will range from 0.13 m (likely -0.12 to 0.41 m) for the very low emissions scenario (SSP1-1.9) to 0.46 m (likely 0.21 to 0.79 m) for the very high emissions scenario (SSP5-8.5). A rise between 40% and 70% lower than the projected global average. For scenarios with higher greenhouse gas emissions than SSP1-2.6, a majority of the coast will likely experience relative sea-level rise for 2100.

Sea-level rise will increase flood risk in Norway by pushing up the height of sea level extremes (the combination of tides, storm surges, and waves) which will reach higher and further inland. Sea-level rise will also drive sharp increases in flooding frequency. There are large differences in the timing and extent of flooding frequency changes that partly depend on projected sea level and the regional VLM signal. Western and Southern Norway will experience increases in flooding frequency first.

In summary, careful treatment of VLM and its uncertainties is important for assessing observed sea level and tailoring national sea-level projections for their eventual use in adaptation planning. VLM also has important implications for how sea level information is communicated to decision makers and stakeholders.

How to cite: Simpson, M. J. R., Bonaduce, A., Borck, H. S., Breili, K., Breivik, Ø., Ravndal, O. R., and Richter, K.: Sea-Level Rise and Extremes in Norway: Observations and Projections Based on IPCC AR6, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5229, https://doi.org/10.5194/egusphere-egu26-5229, 2026.

A glacial forebulge is a load-driven bending-related upheaval of the lithosphere outside a glaciated area. As a typical feature of the glacial isostatic adjustment process the forebulge forms contemporaneously to the depression of the lithosphere below the ice sheet. Forebulge development and collapse related to the last glaciation has led to significant topographic changes in the order of several tens of meters in North America and Europe. Furthermore, forebulge behaviour has a significant effect on the evolution of lithospheric stresses, which can induce intraplate earthquakes, even in areas that were not covered by an ice sheet. Therefore, quantifying the present-day position, amplitude and subsidence of the forebulge is crucial for the estimation of future sea-level changes, the evolution of fluvial networks and understanding the distribution of deglaciation seismicity. Though the forebulge of the last glaciation attracted attention over more than one century, quantitative descriptions on the geometry and position of the forebulge are still rare. Key controlling factors for the position, amplitude and dynamic behaviour of the forebulge are the flexural rigidity of the lithosphere, asthenospheric flow processes, as well as ice-sheet geometry and history. Numerical simulations indicate that a higher flexural rigidity of the lithosphere leads to a lower amplitude of the forebulge and a greater distance to the load. Forebulge formation is also supported by the flow of asthenospheric material, which can occur as channel-flow or deep flow. In case of channel-flow, the forebulge shows an outward migration during collapse, whereas deep-flow leads to an inward migration. A non-linear mantle rheology is seen as a reason for stationary forebulge collapse. The height of the glacial forebulge of the last glaciation was in a range of several tens of meters, with a greater height in North America than in Europe due to the larger Laurentide ice sheet. 

How to cite: Brandes, C., Steffen, H., Steffen, R., Li, T., and Wu, P.: Quantifying the forebulge of the last glaciation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2315, https://doi.org/10.5194/egusphere-egu26-2315, 2026.

Glacial isostatic adjustment (GIA) induces lithospheric bending that can strongly influence depositional systems located within ice-sheet forebulges. While previous studies have documented the role of GIA in river-network reorganization during the last glacial cycle, its impact on sedimentary systems earlier in the Quaternary remains poorly constrained.

Here, we investigate the pre-glacial Daumantai Formation, exposed in several outcrops beneath the oldest tills in the Baltija Highlands of eastern Lithuania. This fluvial succession was dated using combined ¹⁰Be–²⁶Al exposure–burial dating to ~0.95 ± 0.1 Ma, while the same approach indicates that the overlying tills were deposited only ~30–50 kyr later. Importantly, the succession records a distinct change in palaeocurrent directions, initially toward the southeast and subsequently toward the northwest, occurring prior to the first documented advance of the Fennoscandian Ice Sheet (FIS) into the region. This shift is interpreted as a reorganization of the river network rather than merely a modification of river planform geometry, as the palaeocurrent reorientation is consistently documented at several sites across distances exceeding 10 km.

GIA was simulated using the ICEAGE normal-mode modelling framework to assess the potential role of lithospheric bending and associated slope changes in river-network reorganization. Four ice-sheet configurations were tested: (1) a late Gauss and (2) an early Matuyama extent after Batchelor et al. (2019, https://doi.org/10.1038/s41467-019-11601-2), (3) an additional, larger ice sheet extending ~150 km northwest of the study area, and (4) a MIS 20–24 ice-sheet extent from Batchelor et al., which directly overlies the analysed succession. A 380 kyr modelling scenario included five glacial cycles comprising ice growth, deglaciation, and ice-free periods, with 40 kyr and 100 kyr periodicities and increasing amplitudes. The modelling results indicate that the study area was affected by forebulge development associated with all tested ice-sheet extents. The two smaller ice sheets induced southeastward surface tilting, whereas the larger configuration produced northwestward tilting, with maximum slope changes reaching ~0.002°.

The resulting time-dependent uplift and subsidence fields were subsequently used as inputs for landscape evolution modelling to investigate the impact of episodic glacial loading and unloading on surface processes. Erosion and sedimentation were simulated using a stream-power–law, finite-difference approach under imposed time-varying three-dimensional deformation. Preliminary results suggest that repeated north–south tilting associated with glacial cycles exerts a strong control on fluvial dynamics and can locally lead to drainage reversals.

The postdoctoral project CosmoLith was caried out under the “New Generation Lithuania” plan (Nr. 10-036-T-0008) financed under the European Union economic recovery and resilience facility instrument NextGenerationEU. The research was supported by the Slovak Research and Development Agency under the contract No. APVV-21-0281.

How to cite: Šujan, M., Bitinas, A., Steffen, H., Balázs, A., Gedminienė, L., Kováčová, M., Chyba, A., Damušytė, A., Pan, R., Aherwar, K., and Rózsová, B.: Testing glacial isostatic adjustment as a cause of Early Pleistocene river network reorganization in the area of Lithuania (NE Europe), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10025, https://doi.org/10.5194/egusphere-egu26-10025, 2026.

The deglacial forebulge along the Atlantic coasts of North America and Europe has been a key area for glacial isostatic adjustment (GIA) studies. Relative sea level (RSL) changes in this region are highly sensitive to the 3D Earth structure, and the area hosts abundant RSL data that can help constrain the 3D Earth structure. However, many previous studies either relied primarily on 1D Earth models or adopted 3D structures without systematically exploring the magnitude of lateral heterogeneity or the uncertainty associated with deglacial ice histories.

Here, we use the latest standardized deglacial RSL databases from the Atlantic coasts of North America and Europe for comparison with 3D GIA models coupled with two widely used ice models, ICE-6G_C and ANU-ICE. Our 3D Earth model consists of a 1D background viscosity model (η_o) and lateral viscosity variations; the latter are derived from shear velocity anomalies in a seismic tomography model and scaled by a factor (β) denoting the magnitude of lateral heterogeneity. We explore a range of η_o and β to assess the sensitivity of RSL predictions to both the background viscosity and the magnitude of lateral heterogeneity. The RSL databases include sea-level index points and limiting data, which we further classify by depositional setting (base of basal, basal, intercalated). We compare the RSL data to the GIA model predictions using a weighted misfit approach that reflects data type and interpretive uncertainty.

We find that 3D Earth structure has significant influence on RSL predictions, and the optimal 3D models substantially improve the fit to RSL data compared with 1D GIA models (e.g., ICE-6G_C VM5a). The Atlantic coast RSL datasets from North America and Europe favor different combinations of η_o and β, although the former provides stronger constraints owing to its higher spatial coverage and lower data uncertainty. Notably, despite differences in ice history, ICE-6G_C and ANU-ICE prefer similar 3D Earth structures. Ongoing work will quantify the uncertainty of the 3D model resolved by the available RSL data.

How to cite: Li, T. and Walker, J.: 3D Glacial Isostatic Adjustment along the deglacial forebulge of the Atlantic coasts of North America and Europe, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15680, https://doi.org/10.5194/egusphere-egu26-15680, 2026.

During the last deglaciation, the retreating Laurentide Ice Sheet made way for massive proglacial lakes to form and then drain. In a similar fashion, dramatic changes in climate over the deglaciation were reflected in changing groundwater storage through time. We evaluate the impacts of these long-term changes in water storage on Glacial Isostatic Adjustment (GIA) in North America. To do so, we couple the Water Table Model (WTM) – which simulates depth to water table – with a gravitationally self-consistent GIA model to find both changing lake and groundwater storage volumes, and the impacts that these have on changing GIA. 

Our WTM results show an evolving water table that includes proglacial and pluvial lakes consistent with the geological record. Lake and groundwater loading deflect topography by tens of metres at some locations. Because depth to water table is topography-dependent, we repeat our WTM simulation using updated topographic inputs and find that water table depth is modified by several metres at some locations. The results are highly heterogeneous, reflecting that GIA and hydroclimate together drive long-term water-table change. 

How to cite: Callaghan, K. L., Wickert, A. D., and Austermann, J.: Glacial isostatic adjustment under a changing groundwater load since the Last Glacial Maximum, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15241, https://doi.org/10.5194/egusphere-egu26-15241, 2026.

The hypothesis of the ISVOLC project is that retreat of Icelandic glaciers since the end of the 19th Century has the potential to impact both volcanic and seismic activity. As volcanic activity increased significantly during (and in the <2kyr after) the late Pleistocene deglaciation, it is expected that present day deglaciation will once again effect volcanic activity in Iceland. The unloading of glaciers and subsequent rebound response of the Earth can significantly alter the state of stress in the crust and mantle. Within the ISVOLC project (https://isvolc.is) we have developed a new generation of Finite Element Glacial Isostatic Adjustment (GIA) models, using the COMSOL Multiphysics software package, which employ spaciotemporal estimates of glacier mass balance. Utilizing this new detailed ice history, we simulate GIA numerically and, once best fit earth parameters are found by utilizing InSAR and GNSS measurements, produce revised estimates for stress changes in the crust and mantle. From these we can calculate mantle decompression melting increases and shallow crustal stressing which may already be affecting volcanism and seismicity. We find that rates of total magma production beneath Iceland are enhanced by up to a factor of ~3 due to the glacier retreat induced decompression melting. However, it is highly uncertain when this additional magma will reach the surface and in what volumes. Stress changes around magma bodies at shallow level in the crust can bring such magma bodies closer to or further away from failure, depending on their geometry. Our new models predict stressing rates in the shallow crust that are comparable to those from tectonic extension for volcanic systems, seismic zones, and fissure swarms that are near or underneath Vatnajökull (Bardarbunga, Grímsvötn, northeastern Volcanic Zone, south Northern Volcanic Zone), with a unique pattern for each system. Glacially induced stressing in these areas may significantly shorten the timeline to seismic, diking, or eruptive events and alter the preferred orientations of dike propagation. Stressing rates from glacial mass loss are an order of magnitude smaller for systems beneath smaller glaciers and those not beneath ice (e.g. Askja, Katla, South Iceland Seismic Zone, Tjornes Fracture Zone), but still significant enough to consider when assessing hazard. Efforts in further improving the GIA modeling include effects of more realistic non-linear mantle rheology leading overall to somewhat higher viscosity estimates and more subdued GIA response in the far-field, as well as increases in the rate of magma production predicted by our models. Near-future work will involve the projection of glacial unloading and the subsequent earth response to evaluate effects on long-term hazards.

How to cite: Givens, T., Bellagamba, G., Parks, M., Schmidt, P., Sigmundsson, F., Geirssson, H., O´Hara, C., Sturkell, E., Ófeigsson, B., Drouin, V., Frídriksdóttir, H., Greiner, S., Vallson, G., Halldórsson, H., Magnússon, E., Pálsson, F., and Hreinsdóttir, S.: ISVOLC: Deglaciation and GIA Affecting Crustal and Mantle Stresses in Iceland. Much More Magma?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1014, https://doi.org/10.5194/egusphere-egu26-1014, 2026.

Systematic subsidence of ~1 mm/yr is observed across the Pacific at GNSS sites on islands that lack significant local tectonic and volcanic processes (Altamimi et al., 2023; Ballu et al., 2019; Hammond et al., 2021).  However, the horizontal motion of these sites is well described by Pacific plate motion.  This suggests that the observed subsidence represents a deeply rooted geophysical signal, rather than just localized deformation. 

Both ongoing and past ice mass redistribution are known to produce global deformation. Previous models of recent global ice mass redistribution (Coulson et al., 2021; Riva et al., 2017) predict tenths of a mm/yr of subsidence in far field locations such as the Pacific.  In addition, post-LGM GIA models like ICE-6G also predict subsidence in the Pacific on the order of tenths of a mm/yr.

Here we evaluate three new models of the global deformation associated with present day ice mass redistribution. These models use the methods of the elastic loading model originally published by Coulson et al. (2021), utilizing new mass change estimates with increased spatial and temporal coverage as input. The three updated models all use Velicogna et al.’s (2020) mass change estimates for the Antarctic and Greenland ice sheets, paired with global glacier mass change estimates from either Ciraci et al. (2020), the Copernicus group (Dussaillant et al., 2024), or Hugonnet et al. (2021). These models are evaluated at selected GPS sites near field to glaciers in regions such as SE Alaska, Greenland, etc., as well as 27 Pacific GPS sites located far field from ice mass change. We also use these far field sites to evaluate 39 different long-term GIA models that predict the present-day viscoelastic response of the earth to past loading. 

We find that all three models of elastic deformation due to recent global ice mass change produce very similar results in the far field. The most significant differences in the models are seen in the near field in SE Alaska and Svalbard. Additionally, there is a set of 15 long-term GIA models that improve the fit of both the horizontal and vertical observations in the Pacific when used in combination with an ongoing cryospheric loading model to correct the GPS data. Overall, we find that the sum of the deformation due to ongoing ice mass changes and long-term GIA explains about half of the subsidence signal that we observe in the far field. 

Studies of the contribution of different components of barystatic sea level (BSL) suggest that though cryospheric melting is the largest contributor, non-cryospheric terrestrial water storage (TWS) could be responsible for ~17 – 25% of BSL over the past couple of decades (Nie et al., 2025; McGirr et al., 2024).  Since changes in TWS may represent a non-trivial global loading signal, we choose to also consider if deformation associated with TWS may explain part of the residual ~0.5 mm/yr subsidence signal that we see in the far field after our cryospheric loading corrections.

How to cite: Vance, K., Freymueller, J., and Coulson, S.: Evaluating global models of deformation from ongoing ice mass changes and long-term GIA, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13801, https://doi.org/10.5194/egusphere-egu26-13801, 2026.

The Greenland Ice Sheet (GrIS) is currently undergoing substantial mass loss, with major consequences for both the Earth system and human societies, including a significant contribution to the ongoing acceleration of global mean sea level rise. Accurately estimating the GrIS mass balance therefore represents a major focus of current research. However, it remains challenging and, to date, still imprecise.

One of the main reasons is Glacial Isostatic Adjustment (GIA) - the viscoelastic response of the solid Earth to the growth and decay of ice sheets at its surface. Because geodetic observations are among the most used tools to quantify ice mass changes, robust estimates of GIA corrections are essential for the accurate interpretation of these measurements.

This study focuses on deformations induced by ice mass loss since the Little Ice Age (LIA) and their impact on present-day vertical land motion inferred from GNSS observations. Using a reconstructed history of the GrIS and its peripheral glaciers, we model LIA-driven viscoelastic deformations assuming different Earth models, exploring a range of values for two rheological parameters: the lithosphere thickness and the upper mantle viscosity. These simulations, combined with corrections for GIA associated with the last glacial maximum and the elastic response to contemporary ice melting, are compared against GNSS observations. Our results explain the uplift rates at most of the GNSS stations and are consistent with existing literature, with LIA-induced vertical land motion best accounted for by a 160 km thick lithosphere and an upper mantle viscosity of 2.73 × 10¹⁹ Pa·s.

As we explore the rheological structure beneath Greenland, we pay particular attention to the southeastern region, where uplift rates are unusually high. Southeastern Greenland exhibits significant lateral variations in mantle viscosity and lithospheric thickness, likely related to the track of soft material left by the Iceland hotspot. Our simulations support the presence of a low viscosity/thin lithosphere zone in this region, and we further investigate its effects by adding to our modeling an asthenospheric layer within the upper mantle.

Overall, this study demonstrates that deformations induced by the LIA constitute a non-negligible contribution to present-day geodetic signals. Accounting for this component is therefore essential to reduce uncertainties in ice mass balance estimates and to better understand Greenland’s contribution to global sea level rise.

How to cite: Gourrion, E., Métivier, L., and Greff-Lefftz, M.: The influence of LIA-induced viscoelastic deformations on geodetic observations in Greenland, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12406, https://doi.org/10.5194/egusphere-egu26-12406, 2026.

Vertical land motion in the Great Lakes Basin (GLB) arises from the combined effects of ongoing Glacial Isostatic Adjustment (GIA) and shorter-term environmental and hydrological loadings. Because the present-day GIA hinge line crosses the region, even small errors in separating long-term uplift from elastic responses can strongly bias geophysical interpretations. Over the past two decades, the GLB has experienced pronounced lake-level fluctuations. An analysis of GRACE/FO data indicates minimal Total Water Storage (TWS) changes across the GLB during 2002-2012, a period during which lake levels were relatively stable and vertical motions should therefore reflect GIA alone. In contrast, from 2012 to 2019 lake levels rose to record highs, and since 2020 they have been falling at a comparable rate. The area is densely instrumented with continuous GNSS stations, providing an exceptional opportunity to investigate how long-term GIA and short-term hydrological forcing interact. Our goal is to develop a robust, precise and accurate estimate of the GIA signal so that we can accurately remove GIA from observations and constrain surface/groundwater storage changes.

We compared the predictions of many GIA models with the pre-2012 observations, which should reflect the GIA signal alone, but none of the existing models adequately reproduce the observed data. Despite differences in viscosity structure or ice history, every model produces the same systematic bias: the hinge line (zero uplift) is positioned too far south. However, the shape of the modelled profiles matches the GNSS curvature extremely well.  Therefore, we developed a spatial optimization framework to minimize geometric misalignments between GIA model predictions and GNSS vertical velocities across the Great Lakes (2002–2012.5). Seventy-two GIA realizations based on diverse ice histories (ICE-6G_C, ICE-6G_D, ICE-7G_NA, ANU-ICE, NAICE, etc.) and Earth rheologies were subjected to systematic horizontal translations (via a grid search with limits ranging from ±2° to ±8°), with and without allowing for small planar rotations, yielding 576 model-configuration combinations evaluated using RMS misfit, concordance correlation, and 5-fold cross-validation. The best-fitting models achieve the lowest misfits (approximately 0.40 mm/yr), and highest concordance (ccc > 0.90). The models that fit well give very consistent hinge line predictions across the core of our region but are more variable toward the edges of the model domain.

We introduced a hierarchical set of model ensembles constructed by ranking all 576 optimized configurations by post-alignment RMS and grouping them into four tiers: ELITE (RMS ≤ 0.49 mm/yr), GOOD (≤ 0.59 mm/yr), MEDIUM (≤0.79 mm/yr), and ALL (>0.80 mm/yr). These hybrid fields reveal a systematic progression, with the ELITE and GOOD ensembles capturing the GNSS-derived deformation shape with narrow uncertainty bands, while the MEDIUM and ALL ensembles exhibit progressively larger uncertainties that grow with ensemble size. The mean models of the ELITE and GOOD ensembles are nearly identical and provide the most stable uplift geometry and the smallest GPS-calibrated uncertainties, with representative values below 0.18 mm/yr (ELITE) and 0.26 mm/yr (GOOD) for Michigan, demonstrating that tightly constrained multi-model ensembles can outperform any individual GIA realization.

How to cite: Guerra Neto, H. L. and Freymueller, J.: An Empirical Estimate of GIA-Induced Vertical Motion in the Great Lakes Basin Derived from an Ensemble of GIA Models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1437, https://doi.org/10.5194/egusphere-egu26-1437, 2026.

The future stability of the Antarctic Ice Sheet is determined by the marine ice sheet instability (MISI), an amplifying feedback, leading to potentially irreversible retreat. Glacial isostatic adjustment (GIA) potentially provides a stabilizing feedback, yet its influence on the timing and nature of ice-sheet tipping dynamics remains poorly constrained. Using an ensemble of idealized simulations in a synthetic Antarctic-type ice-sheet–shelf system, we systematically investigate how interactions between ice dynamics and the visco-elastic solid Earth affect MISI tipping dynamics under increasing basal ice-shelf melt. We find that the critical thresholds for bifurcation tipping strongly depend on the timescale and spatial extent of Earth deformation, increasing substantially (order of magnitude) relative to a fixed-bed (rigid) case.

For half of the ensemble members, rate-induced tipping occurs when melt rates increase sufficiently rapidly, triggering MISI before the threshold for bifurcation tipping is reached and reducing the effective tipping threshold by up to 80%. Bed uplift cannot halt MISI once initiated, due to rapid grounding-line retreat. We further identify grounding-line overshoots and self-sustained oscillations driven solely by internal ice - Earth interactions.

We find similar dynamics in more realistic simulations of the Antarctic Ice Sheet with a coupled ice sheet - solid Earth and sea-level model considering a three-dimensional Earth structure. Our results highlight that both the magnitude and rate of future climate forcing critically influence Antarctic ice-sheet stability.

How to cite: Albrecht, T., Feldmann, J., Klose, A. K., Wullenweber, N. K., Mojtabavi, S., Klemann, V., and Winkelmann, R.: Rate-induced tipping of ice sheets interacting with the visco-elastic solid Earth, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9816, https://doi.org/10.5194/egusphere-egu26-9816, 2026.