A floating ice tongue is already displacing its own weight in seawater, so the calving itself adds almost nothing to sea level. The concern is what the tongue was doing while it was attached: acting as a brake. A floating extension pressed against the fjord walls and seabed holds back the grounded ice behind it, and removing it lets that land ice accelerate seaward — and land ice does raise sea level as it enters the ocean. Petermann is one of Greenland's largest surviving ice tongues, and its flow has been accelerating in recent years. Scientists estimate that a complete collapse of the glacier would eventually raise global sea level by about 30 centimetres. The 2026 break also gives researchers a rare live subject: a tabular ice island whose formation, drift and disintegration can be tracked from the moment it was born.

Like most glaciers that discharge into the ocean, Petermann periodically sheds large icebergs, and large calvings have punctuated its recent record — the 2026 event is described as its biggest loss of floating ice since 2012, and the largest Arctic calving event of any kind since 2020. What distinguishes this episode is that it appears to be the first of a set. Satellite monitoring suggests two more slabs, of roughly 94 and 84 sq km, are poised to break away. If all three go, the floating tongue loses about 254 sq km — close to 22 per cent of its surface area — in a single season. That would leave a much shorter tongue with correspondingly less capacity to buttress the ice behind it.