Quantitative analysis of firn meltwater retention on the Greenland ice sheet

More than half of the Greenland ice sheet mass loss and contribution to sea-level rise originates from surface melt and subsequent runoff. The perennial snow, or firn, that covers ∼80% of the ice sheet, retains a part of this surface melt and hence buffers the ice sheet’s sea-level contribution. Yet, the characteristics and processes involved in the meltwater retention in firn are poorly constrained to date.

First, this PhD project contributed to a better understanding of low permeability ice slabs, which are known to prevent meltwater from being retained in the firn. Greenlandwide radar data and repeated firn core observations allowed to map ice slabs in the Greenland firn and climate models were used to estimate their future contribution to global sea-level rise.

Firn modelling at an ice slab site using weather station data and a regional climate model also revealed the impact of ice slabs on local runoff and how to represent ice slabs in firn models. Data from nine weather stations in the Greenland firn area were used to force a firn-evolution model between 1998 and 2015.

Increasing summer air temperature and melt was found at all sites. Simulated firn densities, tightly constrained by firn-core observations, increased by about 10% at the two warmest sites and translated into an important (up to 18%) decrease in the volume available for meltwater retention in the firn.

Contrastingly, stable firn densities were found at four of the five coldest sites. Simultaneously, summer heat flux from the atmosphere to the firn increased at all sites but one. The subsequent firn warming, however, did not alter the capacity of the firn to refreeze meltwater. Preferential percolation of meltwater was also investigated for its impact on firn heat fluxes and meltwater production.

The analysis of 360 firn-core observations also showed increasing firn density and consequently a 23 ± 16% decrease of the near-surface firn air content in the warmest and driest 14% of the firn area between 1998 and 2017. The reduced air content indicates a decreasing meltwater retention capacity of the firn.

The dataset also describes stable firn densities and air content between 1953 and 2017 in the coldest 74% of the firn area. Finally, the data collected for this project will provide a baseline for future firn observations and models. The improved understanding of meltwater retention in firn will help to reduce uncertainties when simulating the future mass loss from the Greenland ice sheet and its sea-level contribution.