Abstract
Abstract. Observed changes in the surface elevation of the Greenland Ice Sheet are caused by ice dynamics, basal elevation change, basal melt, surface mass balance (SMB) variability, and by compaction of the overlying firn. The last two contributions are quantified here using a firn model that includes compaction, meltwater percolation, and refreezing. The model is forced with surface mass fluxes and temperature from a regional climate model for the period 1960–2014. The model results agree with observations of surface density, density profiles from 62 firn cores, and altimetric observations from regions where ice-dynamical surface height changes are likely small. In areas with strong surface melt, the firn model overestimates density. We find that the firn layer in the high interior is generally thickening slowly (1–5 cm yr−1). In the percolation and ablation areas, firn and SMB processes account for a surface elevation lowering of up to 20–50 cm yr−1. Most of this firn-induced marginal thinning is caused by an increase in melt since the mid-1990s and partly compensated by an increase in the accumulation of fresh snow around most of the ice sheet. The total firn and ice volume change between 1980 and 2014 is estimated at −3295 ± 1030 km3 due to firn and SMB changes, corresponding to an ice-sheet average thinning of 1.96 ± 0.61 m. Most of this volume decrease occurred after 1995. The computed changes in surface elevation can be used to partition altimetrically observed volume change into surface mass balance and ice-dynamically related mass changes.
Highlights
The mass balance of the Greenland Ice Sheet has been negative over the last few decades (e.g. Zwally et al, 2011; Shepherd et al, 2012; Hurkmans et al, 2014)
Observed changes in the surface elevation of the Greenland Ice Sheet are caused by ice dynamics, basal elevation change, basal melt, surface mass balance (SMB) variability, and by compaction of the overlying firn
While RACMO2.3 itself contains a multi-layer snowpack with the same compaction and meltwater routines as the firn model, the rationale for using an offline firn model is the ability to spin up the firn layer with a reference climate until it is in equilibrium with that reference climate
Summary
The mass balance of the Greenland Ice Sheet has been negative over the last few decades (e.g. Zwally et al, 2011; Shepherd et al, 2012; Hurkmans et al, 2014). We present a time series of elevation changes over the Greenland Ice Sheet due to changes in depth and mass of the firn layer, i.e. variability and change in the surface mass balance and associated firn processes like compaction, percolation, and refreezing. These time series extend from 1960 up to and including 2014, at a horizontal resolution of 11 km × 11 km.
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