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Kangerlussuaq Glacier (KG) is a major contributor to central‐eastern Greenland mass loss, but existing estimates of its mass balance over the last century are inconsistent, and specific drivers of change remain poorly understood. We present a novel approach that combines numerical modeling and a 1933–2021 climate forcing to reconstruct its mass balance over the past century. The model's final state aligns remarkably well with present‐day observations. The model reveals a total ice mass loss of 285 Gt over the past century, equivalent to 0.68 mm global sea level rise, 51% of which occurred since 2003 alone. Dynamic thinning from ice front retreat is responsible for 88% of mass change since 1933, with short‐term ice front variations having minimal impact on centennial mass loss. Compared to earlier studies, our findings suggest that KG lost 59% (or 301 Gt) less mass over the century than previously thought. Plain Language Summary: Kangerlussuaq Glacier (KG) is a primary source of mass loss from the central‐eastern sector of the Greenland Ice Sheet. Despite various efforts, there is still disagreement on how much mass KG has lost over the past century and the reason for the mass loss. We used computer models and climate data to determine that the glacier has lost 285 billion metric tons of ice over the last 100 years. That is enough to raise global sea levels by about two‐thirds of a millimeter. Over half of this ice loss has happened in the last 20 years, and the rates of mass loss in the past two decades were significantly higher than for most of the last century. While the ice front of the glacier can retreat without losing mass, if the snowfall is significant enough, we found that most ice loss was related to the glacier retreating. This new estimate of ice loss is 59% lower than previous studies suggested, showing that the glacier's decline is less severe than we thought. Key Points: Dynamic thinning due to the retreat of the ice front explains 88% of the mass loss since 1933Short‐term variability in the ice front has a lesser impact on the mass balance on centennial time scalesKangerlussuaq Glacier lost 59% (or 301 Gt) less mass over the century than previously thought [ABSTRACT FROM AUTHOR]

Greenland's bedrock responds to ongoing ice loss with an elastic vertical land motion (VLM) that is measured by Greenland's Global Navigation Satellite System (GNSS) Network (GNET). The measured VLM also contains other contributions, including the long‐term viscoelastic response of the Earth to the deglaciation of the last glacial period. Greenland's ice sheet (GrIS) produces the most significant contribution to the total VLM. The contribution of peripheral glaciers (PGs) from both Greenland (GrPGs) and Arctic Canada (CanPGs) has not carefully been accounted for in previous GNSS analyses. This is a significant concern, since GNET stations are often closer to PGs than to the ice sheet. We find that, PGs produce significant elastic rebound, especially in North and East Greenland. Across these regions, the PGs produce up to 32% of the elastic rebound. For a few stations in the North, the VLM from PGs is larger than that due to the GrIS. Plain Language Summary: The solid Earth has long been compressed by the weight of overlying ice caps and ice sheets. As climate change causes ice to melt, and these loads diminish, the solid Earth relaxes, producing instantaneous elastic rebound and delayed viscoelastic rebound of the bedrock. Such displacements are recorded by the 58 permanent Global Navigation Satellite System (GNSS) stations that comprise the Greenland GNSS Network (GNET). So far, only the ice mass changes from the ice sheet have been considered in the analyses of deformation recorded by GNET. Here, we evaluate the contribution from the peripheral glaciers, which are often much closer to the stations than the ice sheet. We find that at many stations, the signal produced by the peripheral glaciers is non‐negligible, especially in North and East Greenland. This allows us to better understand the residual rebound signal produced by the end of the last ice age. Key Points: Elastic rebound due to ice loss from peripheral glaciers can exceed that due to ice sheet lossThis effect is most significant at Global Navigation Satellite System Network (GNET) sites in North and East GreenlandPeripheral glacier loss should be acknowledged when isolating glacial isostatic adjustment from GNET [ABSTRACT FROM AUTHOR]

The observed crustal uplift rates in Greenland are caused by the combined response of the solid Earth to both ongoing and past surface mass changes. Existing elastic Earth models and Maxwell linear viscoelastic GIA (glacial isostatic adjustment) models together underpredict the observed uplift rates. These models do not capture the ongoing mantle deformation induced by significant ice melting since the Little Ice Age. Using a simple Earth model within a Bayesian framework, we show that this recent mass loss can explain the data‐model misfits but only when a reduced mantle strength is considered. The inferred viscosity for sub‐centennial timescale mantle deformation is roughly one order of magnitude smaller than the upper mantle viscosity inferred from GIA analysis of geological data. Reconciliation of geological sea‐level and modern crustal motion data may require that the model effective viscosity be treated with greater sophistication than in the simple Maxwell rheological paradigm. Plain Language Summary: There are 57 permanent Global Navigation Satellite System (GNSS) stations on bedrock in Greenland. These stations provide point‐measurements of three‐dimensional crustal motion. We can model a large portion of the observed crustal uplift rates as elastic Earth response to ongoing rates of ice‐mass loss. We model the remaining part of the uplift rates as the ongoing viscous response of the solid Earth to past ice‐ocean mass exchange—a process termed glacial isostatic adjustment (GIA). Earth structure and deglaciation history in GIA models are usually constrained by geological data that record paleo sea level and past ice margins. To fully explain the GNSS‐measured uplift rates, we propose that these geologically constrained GIA models should additionally resolve: (a) ice‐mass changes during and after the Little Ice Age; and (b) broadband mantle relaxation processes. While such features are challenging to implement, they offer a more granular model paradigm appropriate to the improved temporal sampling that the collective geological and geodetic data sets now provide. Key Points: GIA models constrained by geological data underpredict the modern crustal uplift rates corrected for the elastic loading effectsMass loss since the Little Ice Age explains the residual uplift rates provided a reduced mantle viscosity on sub‐centennial timescalesReconciling geological and modern geodetic data requires a more sophisticated mantle rheology model than generally used in loading studies [ABSTRACT FROM AUTHOR]