Firn data compilation reveals widespread decrease of firn air content in western Greenland
2019
Abstract. A porous layer of multi-year snow known as
firncovers the
Greenland-ice-sheetinterior. The
firnlayer buffers the
ice-sheetcontribution to sea-level rise by retaining a fraction of summer melt as liquid water and refrozen ice. In this study we quantify the Greenland
ice-sheet
firnair content (FAC), an indicator of
meltwaterretention capacity, based on 360 point observations. We quantify FAC in both the uppermost 10 m and the entire
firncolumn before interpolating FAC over the entire
ice-sheet
firnarea as an empirical function of long-term mean air temperature ( T a ‾ ) and net snow accumulation ( c ˙ ‾ ) . We estimate a total
ice-sheet-wide FAC of 26 800±1840 km 3 , of which 6500±450 km 3 resides within the uppermost 10 m of
firn, for the 2010–2017 period. In the dry snow area ( T a ‾ ≤ - 19 ∘ C), FAC has not changed significantly since 1953. In the low-accumulation percolation area ( T a ‾ > - 19 ∘ C and c ˙ ‾ ≤ 600 mm w.e. yr −1 ) , FAC has decreased by 23±16 % between 1998–2008 and 2010–2017. This reflects a loss of
firnretention capacity of between 150±100 Gt and 540±440 Gt, respectively, from the top 10 m and entire
firncolumn. The top 10 m FACs simulated by three regional climate models (HIRHAM5, RACMO2.3p2, and MARv3.9) agree within 12 % with observations. However, model biases in the total FAC and marked regional differences highlight the need for caution when using models to quantify the current and future FAC and
firnretention capacity.
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