Manhattan-Sized Ice Island Breaks Off Greenland: Explained
🕐 9 min read | 🌍 Natural Wonders
🔒 Key Takeaways
- Manhattan covers about 59 square kilometres — Petermann Glacier's August 2010 ice island was roughly 250–260 sq km, more than four times that area, and its July 2012 sibling was about 130 sq km.
- Petermann's floating tongue extended roughly 70 km seaward before 2010 and is about 150–200 metres thick near its calving front, so most of an ice island's bulk sits below the waterline.
- Because the tongue is already afloat, a calving event adds essentially no direct rise to global sea level; the risk lies in the lost buttressing of grounded ice behind it.
- Radar satellites tracked fragments of the 2010 Petermann ice island for more than two years as they drifted some 3,000 km down Nares Strait, through Baffin Bay and along the Labrador coast toward Newfoundland shipping lanes.
- Petermann Glacier drains a basin holding roughly 4% of the Greenland Ice Sheet, and satellite radar showed its grounding line retreating several kilometres — about 3–4 km — between 2016 and 2022.
Somewhere in the frozen dark of northwest Greenland, a rift that scientists had tracked by satellite for years finally ran its full length — and a Manhattan-sized ice island slid free into the fjord. Silent, slab-flat and thick enough to hide a 50-storey building beneath the waterline, it now drifts south on currents that will take months, sometimes years, to break it apart. The unsettling part isn't the ice that left. It's what its departure reveals about the ice that stayed behind.
What Is an Ice Island, and How Big Is Manhattan-Sized?
An ice island is not an ordinary iceberg. Ordinary bergs are chaotic, jagged chunks that tumble and roll off the steep fronts of tidewater glaciers such as Jakobshavn Isbræ. An ice island is a tabular slab — flat as a runway on top, sheared cleanly from a floating ice shelf or glacier tongue, and often only 40 to 60 metres above the waterline. Manhattan covers roughly 59 square kilometres, so the comparison describes an area you could walk across for hours without leaving the ice. Petermann Glacier's August 2010 ice island measured about 250–260 square kilometres, more than four Manhattans, while the July 2012 event released a slab near 130 square kilometres. Beneath the surface these slabs typically extend 150 to 200 metres deep, meaning roughly 85–90% of their mass is invisible from a ship's deck. That hidden draft is why an ice island behaves less like floating debris and more like a slow-moving landmass, capable of grounding on seafloor banks 100 metres or more below sea level.
Petermann Glacier: Greenland's Great Floating Tongue
Petermann Glacier sits at roughly 81°N in northwest Greenland, funnelling interior ice down a fjord about 20 kilometres wide that opens onto Nares Strait. What makes it unusual is its floating tongue: instead of ending at the coastline, Petermann's ice lifts off the bedrock and continues seaward, afloat, for tens of kilometres. Before the 2010 calving, that tongue stretched some 70 kilometres — among the longest floating ice tongues left in the Northern Hemisphere. It drains a catchment holding roughly 4% of the Greenland Ice Sheet, and the ice moves at about 1 kilometre per year near the grounding line. Each summer, meltwater channels carve dark braided rivers across its surface, while transverse rifts migrate down its length like slow-motion fault lines. When two of those fractures connect, an area the size of a major city detaches in a single event, as happened in 2010 and again in 2012. Together those two calvings removed roughly a third of the tongue's floating area, leaving a front many kilometres closer to the grounding line than it was in the 1990s.
🤔 Did You Know?
Some Arctic ice islands are thick and stable enough to live on: the U.S. Air Force ran a manned research station on Fletcher's Ice Island (T-3), which drifted around the Arctic Ocean as an occupied floating laboratory on and off from 1952 until the 1970s.
The Hidden Trigger: Warm Water Melting From Below
The popular image of Greenland melting involves sunshine and blue meltwater ponds, but Petermann's most persistent stress arrives from underneath. Relatively warm, salty Atlantic-origin water flows at depth through Nares Strait and into the fjord cavity beneath the floating tongue, sitting a few tenths of a degree above 0°C — which is roughly 2 to 3°C above the pressure-adjusted freezing point of ice at several hundred metres depth. That modest temperature difference is enough to drive basal melt rates measured at tens of metres per year near the grounding line, far exceeding surface melting. Airborne radar and autonomous underwater vehicles have mapped inverted canyons up to a few hundred metres tall carved into the tongue's underside, thinning the slab along preferred lines. Where the ice thins, it flexes further under tides that raise and lower the tongue by roughly a metre twice daily, and fractures propagate upward until they meet surface crevasses. Satellite interferometry published in the 2020s also showed seawater intruding kilometres inland of the grounding line with each tidal cycle, melting ice once assumed to be firmly anchored to bedrock. The calving visible from space is the final, loudest step in a process that began years earlier in the dark.
Where Does a Manhattan-Sized Ice Island Actually Go?
Once free, an ice island becomes a very large, very slow vessel steered by currents, wind and sea ice. Petermann's ice islands enter Nares Strait — about 530 kilometres long and as narrow as 35 kilometres — and ride the southward flow between Ellesmere Island and Greenland, frequently halting for an entire winter when the strait freezes over. In spring they push into Baffin Bay and join the Labrador Current, the same southward conveyor that delivers Greenland icebergs into North Atlantic shipping lanes. Fragments of the 2010 ice island were still being tracked more than two years later, having covered on the order of 3,000 kilometres, with pieces grounding off Baffin Island and later drifting near Newfoundland's offshore oil fields. Along the way the parent slab calves its own children, and even a fragment a few hundred metres across carries enough mass to hole a hull. The Canadian Ice Service and Danish authorities issue dedicated bulletins for these objects because a tabular berg does not quietly melt away — it disintegrates in stages that are hard to predict.
Does a Calving Ice Island Raise Sea Level?
Here is the counterintuitive part: the calving event itself adds almost nothing to global sea level. The tongue was already floating, already displacing its own weight in seawater, exactly like an ice cube in a full glass. Melting it changes the water line only fractionally, because freshwater is about 2.5% less dense than the seawater it replaces — a global effect measured in tiny fractions of a millimetre. The consequential story is mechanical. A floating tongue presses against fjord walls and seafloor pinning points, acting as a brake on the grounded ice upstream, and removing that brake can let the glacier behind it accelerate. Ice that was resting on bedrock and then enters the ocean does raise sea level. Greenland has been losing roughly 250 billion tonnes of ice per year on average in recent decades, contributing close to 0.7 millimetres per year to global sea-level rise, and buttressing loss is one mechanism that allows those losses to compound.
The Grounding Line: The Number Glaciologists Really Watch
The grounding line is the buried boundary where a glacier stops resting on bedrock and begins to float. It is invisible from the surface and is detected mainly by measuring how the ice flexes with the tide, using satellite radar interferometry from missions such as Sentinel-1 and TanDEM-X. For Petermann, that line retreated by roughly 3–4 kilometres between 2016 and 2022, and research published in PNAS in 2023 documented tidal seawater penetrating far inland of it, carving a cavity hundreds of metres tall. Retreat matters because Greenland's fjords often deepen inland: as the grounding line slips into deeper water, thicker ice — several hundred metres at Petermann — comes into contact with warm ocean and melts faster, driving further retreat. That feedback, known as marine ice-sheet instability, operates on timescales of decades rather than millennia. A Manhattan-sized ice island is the spectacle; a quiet few kilometres of grounding-line retreat is the measurement glaciologists actually lose sleep over.
How Satellites Catch a Berg the Size of a City
Northwest Greenland is in polar night for months and cloud-covered for many more, so optical cameras alone would miss most calving events. Synthetic aperture radar satellites — the European Space Agency's Sentinel-1 pair, operating since 2014 — image the region every few days regardless of darkness or cloud, revealing fresh rifts as bright linear scars. NASA's Landsat 8 and 9 add 30-metre-resolution colour context in summer, while ICESat-2, launched in September 2018, uses a laser altimeter to track ice-surface elevation change with centimetre-scale precision. NASA's Operation IceBridge flights (2009–2021) and, more recently, autonomous underwater vehicles mapped the cavity and channels beneath the tongue directly. Combining these datasets, researchers can watch a rift lengthen over several seasons and anticipate, within weeks to months, when a slab is likely to detach. Once it does, the break is usually documented from orbit within a day or two, and drift is then tracked continuously by ice services.
Final Thoughts
An ice island drifting south from Petermann is a messenger, not the message: the ice that left was already afloat, but the brake it removed sits on bedrock holding back about 4% of Greenland's ice. Open the free Copernicus Browser and pull up Sentinel-1 radar scenes over Nares Strait, or follow the Canadian Ice Service iceberg bulletins, and you can watch a city-sized slab break apart week by week yourself. Then ask the harder question — what is the warm water doing beneath the tongue that remains?
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Frequently Asked Questions
How big was the ice island that broke off Greenland?
Petermann Glacier's best-documented calving events produced ice islands of roughly 250–260 square kilometres in August 2010 and about 130 square kilometres in July 2012. Since Manhattan covers around 59 square kilometres, those slabs were roughly four times and twice its area, with ice about 150–200 metres thick.
Will the Greenland ice island raise sea levels?
Not directly. The ice tongue was already floating and displacing its own weight in seawater, so calving adds essentially nothing to global sea level. The concern is that the tongue buttressed grounded ice upstream, which can now flow faster into the ocean — and that ice does raise sea level.
Is Petermann Glacier calving caused by climate change?
Glaciers calve naturally, but the size of the 2010 and 2012 losses and the sustained shortening of Petermann's floating tongue are consistent with warmer Atlantic-origin water reaching the fjord cavity and thinning the ice from below. Peer-reviewed studies have also documented several kilometres of grounding-line retreat between 2016 and 2022, which is difficult to explain without ocean warming.
Where do Greenland ice islands drift to?
They typically travel south through Nares Strait into Baffin Bay, then join the Labrador Current along Canada's east coast. Fragments of the 2010 Petermann ice island were tracked for more than two years across roughly 3,000 kilometres, reaching waters near Newfoundland where they were treated as hazards to shipping and offshore platforms.
📚 Further Reading & Research Sources
The following journals and institutions publish peer-reviewed research on the topics covered in this article:
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NASA Earth Observatory / Landsat imagery, public domain
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