In the far northwest of Greenland, one of the island’s largest glaciers is showing worrying signs of change. New cracks are spreading across Petermann Glacier, warm ocean water is rapidly melting it from below, and another large section of its floating ice tongue could soon break away.
A new study published in Science Advances warns that continued retreat could eventually weaken the natural support that slows the glacier’s movement towards the ocean.
Petermann is the largest glacier in northern Greenland that flows directly into the sea, measured by the size of the area that feeds it and the amount of ice it carries. Its drainage basin contains enough ice to raise global sea levels by about 0.38 metres if all of it were eventually lost.
Dominik Fahrner, a glaciologist at the Geological Survey of Denmark and Greenland and lead author of the study, explains that Petermann is better understood as a relatively fast-moving “ice stream” rather than simply a conventional glacier. At its end is a long section of ice that extends from land and floats on the ocean, behaving in some ways like the floating ice shelves surrounding Antarctica.
Silent melting from below
According to Fahrner, much of Petermann’s problem begins where it cannot easily be seen: underneath the ice.
Relatively warm Atlantic water enters Petermann Fjord hundreds of metres below the surface and reaches the underside of the floating ice tongue. There, the ocean steadily eats away at the glacier.
Recent ice loss is dominated by this melting from below, known as basal melting. According to the study, it accounts for about 75% of Petermann’s recent mass loss. Melting at the surface contributes about 20%, while the breaking away of icebergs accounts for only around 5%.
More importantly, the underwater melting is not spread evenly across the glacier.
Deep channels have been carved into the underside of the floating ice, concentrating the melting in particular areas. Close to the grounding zone — the area where ice resting on the land begins to float on the ocean — melting has recently averaged around 60 metres of ice per year. At some margins, it has exceeded 100 metres a year.
The atmosphere and ocean can also reinforce each other.
During warmer summers, more ice melts at the glacier’s surface. Some of that freshwater travels through the glacier and reaches the ocean underneath it. This can strengthen water circulation beneath the floating tongue, helping draw more warm seawater towards the bottom of the ice.
The result is that an unusually warm summer can attack Petermann from two directions: warmer air melts more ice from above, while changes in water circulation can increase melting from below.
New cracks bring the glacier closer to a sensitive zone
Petermann has already undergone dramatic changes. Over the past two decades, the glacier has lost about 40 kilometres from the length of its floating tongue, equivalent to roughly 40% of its area, through major break-up events.
Now, radar images from March 2026 reveal new fractures in the ice. One appears to have extended across the width of the glacier.
If the weakened section breaks away, the glacier’s front would retreat to its farthest inland position since 1923. It would also leave the edge of the floating ice less than 40 kilometres from the grounding zone.
That would not mean Petermann is about to collapse completely. But it would bring the glacier closer to a potentially important threshold.
Earlier modelling suggests that if the ice front retreats to within roughly 12 kilometres of the grounding zone, the floating tongue could become much less effective at holding back the ice farther inland.
This resistance is important because the floating tongue acts somewhat like a brace. Although it is already floating and therefore does not directly raise sea levels when it breaks apart, it can slow the movement of grounded ice behind it.
If that support weakens substantially, the ice upstream could accelerate towards the ocean. That would increase the amount of land-based ice entering the sea, contributing directly to sea-level rise.
The study therefore highlights a danger that extends beyond the loss of the floating ice itself. What happens at the front of Petermann could influence how quickly a much larger reservoir of ice stored inland is released.
However, significant uncertainties remain. Direct weather observations across the Petermann basin are limited and incomplete, and climate models can either underestimate or overestimate surface melting. Continuous measurements of ocean temperature and salinity inside the fjord are also lacking, even though ocean heat appears to be the main driver of melting beneath the glacier.
Scientists also do not yet fully understand the shape of the seabed beneath and around Petermann or the amount of heat rising from the Earth beneath the ice. Both could influence how the glacier responds to future warming.
For now, the new fractures do not signal an immediate collapse. But combined with rapid melting from below and decades of retreat, they show that one of northern Greenland’s most important glaciers is moving closer to a position where further ice loss could have much larger consequences.
