When the Ground and the Glacier Disagree: The Timing Mystery of Bedrock Uplift and Ice Discharge Peaks

Accurate partitioning of present-day Greenland Ice Sheet (GrIS) mass change is essential for closing the sea-level budget and constraining future projections. Vertical bedrock motion from the Greenland GNSS Network (GNET) has recently been used as a virtual instrument for GrIS mass change, but interpretations diverge. At Jakobshavn Isbræ, GNSS uplift has been linked both to dynamic thinning that leads changes in ice discharge by about 0.87 years, implying predictive power for future ice flux, and to seasonal uplift peaks that precede ice-mass loss by 4.5–9 weeks, interpreted as evidence for substantial transient meltwater storage within the ice sheet. Here we reconcile these seemingly contradictory results by jointly analysing GNET observations, mass-balance products, and a numerical ice-sheet model of Greenland’s major outlet glaciers. We show that there is neither a phase shift between bedrock uplift and ice mass-change signals nor any substantial seasonal missing mass. Instead, we find that the two earlier results stem from an incorrect physical interpretation of the GNSS signal. From our analysis, the variability in bedrock uplift is primarily driven by the advance and retreat of the ice front within roughly 10 kilometres of the glacier termini, a zone that is often poorly captured by input–output methods and coarse-resolution mass-balance products. Our results clarify the physical origin and timing of vertical bedrock shifts in Greenland and provide tighter constraints on the contemporary GrIS mass budget.