Past and Recent Extreme Warm Events in Greenland derived from Firn Core Melt Layers

Over the past few decades, the Greenland Ice Sheet has experienced multiple widespread surface melt events (e.g. 2012 and 2019), affecting nearly its entire surface. While seasonal surface melt occurs regularly at the margins of the Ice Sheet, it is rare in the high-elevation, central-north area and thus an indicator for extreme warm events. However, due to sparse in situ observations, particularly prior to the instrumental period little is known about the historical occurrence of such melt events.

Signatures of surface melt are archived within the firn column of the ice sheet as layers of refrozen melt water (melt layers), visually distinguishable from the surrounding unaffected firn and bubbly ice, due to the higher density and absence of air bubbles. We here analyse 22 firn cores from 15 sites across north-central Greenland, covering the past ~1000 years (until 2018 CE), to identify melt layers using visual inspection and micro-computed tomography. For the first time, we present a derived Greenland melt feature database, comprising over 1000 melt features.

Interpreting the melt features as a proxy for past extreme warm events allows to reconstruct the spatial extent and frequency of past melt events. Initial analyses indicate that both, elevation and geographic location strongly influence melt occurrence: lower-elevation sites experience more melt than higher-elevation sites, and the north-eastern basin shows more frequent surface melt than the north-western basin. This new dataset also places recent surface-melt events into a long-term context, demonstrating that the 2012 melt event was the most intense event in north-central Greenland over the last millennium.