Narwhals document atlantification of East Greenland.

To resolve temperature and salinity changes in remote East Greenland coastal waters, including deep glacier-front basins, we equipped narwhals (Monodon monoceros) with conductivity-temperature-depth archival instruments. These data, combined with the World Ocean Database, reveal long-term warming of Atlantic Intermediate Water (AIW) penetrating East Greenland's coastal system. In the Blosseville Basin, AIW dominates below ∼250 meters with little Polar Water influence. From 1960 to 2021, temperatures above 500 meters increased by 0.047°C per year. Atlantification of the Greenland Sea now reaches deep basins in front of marine-terminating glaciers up to 300 kilometers inland. Glaciers in Scoresby Sound and Kangerlussuaq are exposed to significantly warmer and more saline waters than those along the shallower Blosseville Coast. Elevated summer iceberg discharge coincides with higher temperatures and salinities below 200 meters, indicating an intensified AIW influence. These observations show that warming AIW delivers heat directly to glacier fronts, promoting submarine melt and accelerating glacier mass loss.