Firn data compilation reveals widespread decrease of firn air content in western Greenland

A porous layer of multi-year snow known as firn covers the Greenland-ice-sheet interior. The firn layer buffers the ice-sheet contribution 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 firn air content (FAC), an indicator of meltwater retention capacity, based on 360 point observations. We quantify FAC in both the uppermost 10 m and the entire firn column before interpolating FAC over the entire ice-sheet firn area as an empirical function of long-term mean air temperature (inline-formula $M1inlinescrollmathml\stackrel{normal\; ‾}{T_{\text{a}}}$ 12pt16ptsvg-formulamathimg3f34f2e28fd131f1588d0a4a19d8cca4 tc-13-845-2019-ie00001.svg12pt16pttc-13-845-2019-ie00001.png ) and net snow accumulation (inline-formula $M2inlinescrollmathml\stackrel{normal\; ‾}{\stackrel{normal\; \dot{}}{c}})$ 12pt15ptsvg-formulamathimg2467b5ce9e85c5b99553be3356b76c30 tc-13-845-2019-ie00002.svg12pt15pttc-13-845-2019-ie00002.png . We estimate a total ice-sheet-wide FAC of inline-formula26 800±1840 kminline-formula³, of which inline-formula6500±450 kminline-formula³ resides within the uppermost 10 m of firn, for the 2010–2017 period. In the dry snow area (inline-formula $M7inlinescrollmathml\stackrel{normal\; ‾}{T_{\text{a}}}\le -\mathrm{normal\; 19}$ 45pt16ptsvg-formulamathimg6fe53f25472c89b27849cfe687f9f7b3 tc-13-845-2019-ie00003.svg45pt16pttc-13-845-2019-ie00003.png  inline-formula^∘C), FAC has not changed significantly since 1953. In the low-accumulation percolation area (inline-formula $M9inlinescrollmathml\stackrel{normal\; ‾}{T_{\text{a}}}>-\mathrm{normal\; 19}$ 45pt16ptsvg-formulamathimg42f9350ebdfedbd2e9ccdfba4e695aaa tc-13-845-2019-ie00004.svg45pt16pttc-13-845-2019-ie00004.png  inline-formula^∘C and inline-formula $M11inlinescrollmathml\stackrel{normal\; ‾}{\stackrel{normal\; \dot{}}{c}}\le \mathrm{normal\; 600}$ 39pt14ptsvg-formulamathimg3745663665b56693e2ecbf0a733ea7ad tc-13-845-2019-ie00005.svg39pt14pttc-13-845-2019-ie00005.png  mm w.e. yrinline-formula⁻¹), FAC has decreased by inline-formula23±16 % between 1998–2008 and 2010–2017. This reflects a loss of firn retention capacity of between inline-formula150±100 Gt and inline-formula540±440 Gt, respectively, from the top 10 m and entire firn column. 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 firn retention capacity.