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 metres thick were already present at least as early as 2002. Between 2012 and 2018, they expanded 13,400-17,600  inland, or by 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 in locations with only moderate melting in the following summers. With repeated flights over a transect in southwest Greenland, we show that ice slabs can thicken from above and on their undersides. Moderate melting primarily thickens ice slabs by top-down accretion, while large melting events can also trigger ice accretion below the slabs.