Temporal Evolution of Near-Surface Ice Slabs on the Greenland Ice Sheet: From Past Variability to Future Change

The temporal evolution of near-surface ice slabs (> 1m thick) on the Greenland Ice Sheet (GrIS) has significant implications for understanding both past variability and future changes in ice sheet mass balance. These ice slabs, formed through the refreezing of meltwater in the firn layer, modulate surface runoff dynamics and affect the ice sheet’s meltwater retention capacity, which is critical for understanding future surface mass balance (SMB) sensitivity to climate change. With ongoing climate warming, the mechanisms driving the formation and expansion of these ice slabs are likely to intensify. There is a need for coupled SMB and sub-surface firn models that can effectively simulate near-surface firn and ice slab evolution, allowing us to predict their long-term impact on Greenland's contribution to global sea-level rise.

In this study we present a 1-D physically based model with high vertical resolution (1 cm) to simulate surface melt, percolation, refreezing, ice slab evolution, runoff, and surface mass balance (SMB) across the GrIS. The high vertical grid resolution facilitates continuous simulation of the evolution of ice layers with thicknesses ranging from 1 cm to several metres. We applied a novel, laboratory defined, temperature-dependent ice layer permeability criterion whereby all ice layers are permeable above –0.15 °C but become impermeable beyond 1 m thickness. The model incorporates a novel parameterization of vertical snow and firn compaction, replacing frequently used theoretical relationships derived for dry snow compaction with a laboratory-derived, viscosity-based relationship for refrozen snow / firn more commonly found within percolation zones.

We applied the model to the GrIS from 1999 to the end of the 21st century at a spatial resolution of 0.11° and a temporal resolution of 15 minutes. The simulations reproduce the observed distribution of past ice slab occurrences across the GrIS and generate near-surface firn density profiles and proglacial discharge hydrographs that closely match available field measurements. The model also captures the observed interannual variability in the runoff limit, demonstrating strong consistency with satellite-derived estimates. We forced the model with future climate projections to investigate the projected evolution of near-surface ice slabs and their implications for future vertical firn densification, meltwater runoff and surface mass balance.