Understanding the Greenland Ice Sheet's (GIS) flow is crucial for predicting its future evolution under climate change. The GIS's current state is a consequence of its response to past climate perturbations. However, high-quality ice velocity and elevation measurements before GPS technology are scarce, limiting our understanding of multi-decadal trends. Most recent mass-balance assessments assume no ice dynamic changes in the high-elevation interior, and satellite-derived velocities struggle to detect summer acceleration above 1250 m elevation in Central West Greenland due to poor signal-to-noise ratios. The GIS is currently declining due to surface mass balance changes and ice dynamics, with warming temperatures increasing surface melt, runoff, and ice discharge from marine-terminating glaciers. Jakobshavn Isbræ, Greenland's fastest-flowing ice stream, has been extensively studied, with its floating ice tongue disintegration in 1997 triggering acceleration and thinning. Similar dynamic changes are observed in other outlet glaciers. Therefore, exploring the internal mechanisms driving this enhanced dynamic mass loss is essential for accurately predicting future sea-level changes. The French EGIG and NASA PARCA expeditions conducted overland traverses in Central West Greenland in the 1950s-1960s and 1990s, respectively, providing a unique baseline for comparison with contemporary measurements. This study reanalyzes and resurveys selected EGIG and PARCA measurements to identify multi-decadal trends in ice-sheet flow.

Previous research has highlighted the challenges in accurately measuring and modeling ice sheet dynamics, particularly in the high-elevation interior. Studies using satellite data often face limitations in detecting subtle changes in velocity, especially during summer months. The assumption of static interior ice flow in many mass balance assessments has been questioned, as various studies have shown evidence of dynamic changes even in areas previously thought to be stable. The role of basal sliding, meltwater availability, and changes in ice geometry in driving acceleration has been debated, with some studies attributing acceleration to changes in subglacial hydrology and others pointing to the impact of changes in driving stress due to ice thinning and steepening. The impact of the Jakobshavn Isbræ's floating ice tongue disintegration on its acceleration and the propagation of these effects inland has also been a topic of extensive investigation. Previous studies around Jakobshavn Isbræ have reported acceleration, but they have attributed the changes to different mechanisms, some linking it to enhanced basal sliding while others to geometric changes. Discrepancies exist in how these changes are interpreted and whether the changes are attributed to local processes or to upstream effects from faster-flowing regions. The exact mechanism(s) sustaining this enhanced velocity are still under debate.

This study re-analyzed historical data from the EGIG (1959-1967) and PARCA (1993-1995) expeditions, which involved stake surveys using theodolites and GPS, respectively, and resurveyed a subset of these locations using GPS in 2020-2022. The historical EGIG data were re-analyzed to reproduce the original velocity and azimuth values, accounting for uncertainties. The conversion of coordinates from the Hayford ellipsoid (used in the original EGIG surveys) to the WGS84 ellipsoid was addressed. The velocity and azimuth calculations for both historical and modern datasets were described in detail. A Monte-Carlo simulation was employed to estimate uncertainties in the historical velocity and azimuth values. The modern resurvey used 11 GPS receivers spanning 1800 km², with observation periods ranging from a few days to over 400 days. The derived velocities and azimuths from the change in the logged positions were carefully calculated and reported, considering uncertainties introduced by the measurement methods and the observation time spans. The ice thickness at each observation point was estimated from the BedMachine ice thickness map to ensure that the new and historic points were sufficiently close for valid comparison. The altimetry data from multiple campaigns, processed by previous studies, covering overlapping periods from 1995-2020, were used to assess changes in ice geometry and the migration of the knickpoint. The kinematic wave speed was estimated based on the knickpoint migration. Longitudinal coupling stresses were calculated to assess the potential influence of enhanced sliding downstream on the observed acceleration. A simple shear model of ice deformation was used to explore the effects of changes in ice thickness and surface slope on driving stress and ice velocity. The sensitivity of surface velocity to changes in ice thickness and surface slope was explored using a sensitivity plot. Finally, a simple case study examining the effect of changes in the temperate layer height on velocity at site T4 was presented based on temperature profiles from a thermomechanical ice-sheet simulation and calculations of the velocity profiles.

The resurvey revealed a considerable increase in ice velocity across all 11 sites (5.5-14.6%), with the largest increases observed at sites furthest from the terminus of Jakobshavn Isbræ. A consistent increase in azimuth (3-4.5°) was observed at 8 of the resurveyed EGIG sites, indicating a northward deflection of ice flow, well beyond the uncertainty limits. No significant azimuth change was observed at the two PARCA sites. Altimetry data showed marginal thinning and slight thickening in the central region, with considerable thinning (~3-8 m) at the EGIG sites (T1-T5) from 1995-2020, and a clear migration of the knickpoint inland (>100km) after 2005. This indicates a regional steepening of the surface slope and suggests the arrival of a kinematic wave at the observation sites. The estimated kinematic wave-speed was 4.3-9.1 km/yr, slower than those observed in alpine glaciers. The observed increase in velocity is not attributable to changes in basal sliding or sediment deformation, as the sites are far from the terminus of Jakobshavn Isbræ and above the meltwater runoff limit. There was no evidence of seasonal velocity signals. The calculated longitudinal coupling stresses from enhanced sliding were insufficient to explain the observed acceleration. Changes in ice geometry (thickness and surface slope) alone were also insufficient to account for the magnitude of the observed acceleration, as shown by the sensitivity analysis, particularly when comparing our findings with previous studies examining site cd38. The appearance of large transverse crevasses in the area suggests a fundamental shift in ice dynamics, possibly related to the arrival of the kinematic wave. A limited form of creep instability, affecting the lower part of the ice column, is proposed as a mechanism to explain the observed coincident acceleration and rotation.

The findings challenge the assumption of stable interior ice flow in the GIS and show that terminus perturbations can have a significant impact on ice flow far inland. The observed acceleration and rotation of the ice flow are not easily explained by previously suggested mechanisms such as changes in basal sliding due to enhanced surface melt or changes in driving stress due to changes in ice geometry. The absence of a seasonal signal and the distance from the terminus suggest other mechanisms at play. The hypothesis of creep instability provides a more plausible explanation for the observed acceleration and rotation. The creep instability may be triggered by dynamic thinning at the front of Jakobshavn Isbræ, which in turn could instigate a positive feedback between ice temperature, effective viscosity, and deformation. The appearance of crevasses further supports the notion that a fundamental shift in ice dynamics is underway in the study area. The inland migration of a kinematic wave is a significant observation, and its relatively slow speed highlights the complex interplay of factors influencing the dynamics of ice sheets.

This study demonstrates accelerating ice flow more than 100 km inland from the terminus of Jakobshavn Isbræ, challenging the assumption of stable interior ice flow. The observed acceleration and rotation, likely initiated by terminus perturbations about 25 years ago, are not fully explained by changes in basal sliding or ice geometry but are consistent with a form of creep instability affecting the lower ice column. The findings highlight the importance of in-situ measurements for detecting subtle changes in ice flow and the potential consequences of using outdated data in mass balance studies and other models. Further research should investigate the mechanisms driving creep instability and its potential impact on the future evolution of the GIS.

The study's findings are based on a limited number of observation sites. While the use of both historical and modern in situ measurements strengthens the findings, the potential influence of spatial variability in ice thickness and temperature on the observed acceleration is not fully addressed. The relatively short observation period of some of the GPS stations introduces uncertainties in velocity calculations. The lack of direct measurements of ice temperature and subglacial hydrology at the observation sites limits the ability to fully constrain the proposed creep instability mechanism. The uncertainty around the precise boundaries of Jakobshavn Isbræ's catchment area needs to be considered when interpreting the spatial extent of the observed effects.