Greenland Ice Sheet Ice Slab Expansion and Thickening.

We use airborne accumulation radar data acquired over the Greenland Ice Sheet between 2002 and 2018 to identify changes in ice slab extent and thickness. We show that ice slabs several meters thick were already present at least as early as 2002. Between 2012 and 2018, they expanded by 13,400–17,600 km2 ${\mathrm{k}\mathrm{m}}^{2}$ inland, or 37%–44%. Our results document that the extremely warm summer of 2012 produced near‐surface ice layers at higher elevations, enabling ice slabs to develop with only moderate melting in the following summers. With repeat flights along a transect in southwest Greenland, we show that moderate melting primarily causes slab thickening through uniform accretion on top of the ice slabs, while large melting events can also trigger localized accretion below existing ice slabs. Plain Language Summary: Above the equilibrium line elevation, seasonal snow is not entirely removed by summer melting. As a result, firn—an interannual layer made of old snow and refrozen meltwater—builds up. Firn holds the potential to buffer sea level rise by trapping liquid water within its pore space. However, surface melting has increased in recent decades, making large quantities of water available to percolate into the firn where it refreezes, eventually creating meters‐thick ice slabs that hinder future percolation. We mapped ice slabs in the subsurface firn (0–20 m depth) by using airborne radar surveys and show that they have expanded inland and thickened from 2002 to 2018. Once formed, ice slabs continue to thicken, even under moderate melt conditions. Recent increases in the area drained by surface rivers on the ice sheet match well with the ice slabs extent, so we conclude that ice slabs will be an important control on the future runoff area of the ice sheet. Key Points: Ice slabs were already present in the early 2000s in southwest, central‐west and north GreenlandIce slabs expanded inland from 2002 to 2018 and thickened by accretion to both their tops and undersidesNear‐surface ice layers support subsequent ice slab development [ABSTRACT FROM AUTHOR]