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 ($\stackrel{\mathrm{‾}}{T_{\text{a}}}$) and net snow accumulation ($\stackrel{\mathrm{‾}}{\dot{c}})$. We estimate a total ice-sheet-wide FAC of 26 800±1840 km³, of which 6500±450 km³ resides within the uppermost 10 m of firn, for the 2010–2017 period. In the dry snow area ($\stackrel{\mathrm{‾}}{T_{\text{a}}}\le -\mathrm{19}$ ^∘C), FAC has not changed significantly since 1953. In the low-accumulation percolation area ($\stackrel{\mathrm{‾}}{T_{\text{a}}}>-\mathrm{19}$ ^∘C and $\stackrel{\mathrm{‾}}{\dot{c}}\le \mathrm{600}$ mm w.e. yr⁻¹), FAC has decreased by 23±16 % between 1998–2008 and 2010–2017. This reflects a loss of firn retention capacity of between 150±100 Gt and 540±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.