Why Did a Manhattan-Sized Ice Island Break Off Greenland's Petermann Glacier?

In August 2010, a slab of ice roughly the size of Manhattan snapped off Greenland’s Petermann Glacier — the largest Arctic calving event in nearly fifty years — driven by warm ocean water silently eroding the ice from below.

What Happened at Petermann Glacier in 2010?

Petermann Glacier, located in northwestern Greenland, extends into the ocean as a long, flat “ice tongue” — a floating shelf of glacial ice that juts out into the fjord. For years before the 2010 event, relatively warm Atlantic ocean water had been creeping beneath this floating shelf, gradually melting it from the underside where no satellite could easily see. The weakening was invisible until it wasn’t. A crack ripped through the ice and tore away approximately 97 square miles of the floating shelf — roughly one quarter of Petermann’s entire ice tongue — setting it adrift as a colossal ice island.

Researcher Andreas Muenchow at the University of Delaware was among the first scientists to detect it, catching the newly calved island on satellite imagery as it drifted into the fjord. The scientific community described the event as extraordinary — the single largest calving from a Greenlandic glacier since 1962.

Why Didn’t Sea Levels Rise?

Here is the counterintuitive part: despite losing a chunk of ice the size of a major city, this event did not raise global sea levels by a single inch. The reason comes down to a fundamental principle of physics: the Petermann ice tongue was already floating on the ocean. Floating ice displaces exactly as much water as it will eventually contribute when it melts. Think of an ice cube in a glass of water — when it melts, the water level doesn’t change.

Only land-based ice raises sea levels. Glaciers and ice sheets sitting on bedrock, like the vast bulk of the Greenland and Antarctic ice sheets, add new water to the ocean when they melt. Floating ice shelves, while dramatic when they break apart, are already part of the ocean’s water budget.

The 2012 Strike — It Happened Again

Petermann’s troubles didn’t end in 2010. Just two years later, in July 2012, the glacier was struck again. Another roughly 50 square miles of ice broke free in a second major calving event. Back-to-back losses of this scale were stunning to glaciologists. The glacier had shed a staggering volume of frozen mass that had taken decades — possibly centuries — to accumulate, gone within a span of two years.

Together, the two events reshaped scientific understanding of how quickly Arctic glaciers can change, and focused attention on the role of ocean heat — not just atmospheric warming — in driving glacial retreat.

What Does This Tell Us About Arctic Change?

The Petermann events are a window into a quieter, harder-to-see mechanism of Arctic ice loss. While melting at the surface gets significant attention, submarine melting — warm water attacking ice from below — is increasingly recognized as one of the most powerful forces reshaping glaciers in Greenland. It operates out of sight, weakening ice shelves over years until the structural failure arrives suddenly and spectacularly.

Satellites have become the essential eyes for monitoring these remote systems. Miles from any human settlement, in one of the most inaccessible places on Earth, it was only a blinking sensor in orbit that bore witness to one of the most dramatic geological events of the 21st century’s first decade.

The Petermann calving events are a reminder that the Arctic is not a static, frozen world. It is dynamic, responsive, and changing — often in ways that are invisible until the ice itself gives way.