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 localised accretion below existing ice slabs.