The shape of the ocean floor influences what happens at the glacier front
Beneath the Arctic's frozen surface, the shape of the seafloor has long been quietly deciding the fate of glaciers. In the summer of 2024, the Swedish icebreaker Oden became the first research vessel to survey Victoria Fjord in North Greenland, where scientists discovered that the absence of a protective underwater ridge — a sill — allows warm Atlantic water to flow unimpeded to the margin of C.H. Ostenfeld Glacier, explaining why it lost its floating ice tongue in the early 2000s while neighboring glaciers did not. The finding reminds us that the forces shaping our climate are often hidden from view, written in the contours of the ocean floor rather than the surface we can see.
- For over two decades, glaciologists could not explain why C.H. Ostenfeld Glacier lost its floating ice tongue while Ryder and Petermann glaciers, just nearby, kept theirs.
- Victoria Fjord was so remote and ice-choked that no research ship had ever collected scientific data there — leaving a critical blank in Arctic oceanography.
- Oden's 2024 expedition broke through, producing the first detailed bathymetric maps of the fjord and revealing a seafloor with no protective sill to block warm Atlantic water.
- Without that underwater barrier, warm water flows directly to the glacier's margin — a structural vulnerability that melted away its ice tongue while shielded neighbors survived.
- The findings have been added to the International Bathymetric Chart of the Arctic Ocean, and now point toward a new framework for identifying which glaciers worldwide face the greatest risk from ocean warming.
In the summer of 2024, the Swedish icebreaker Oden pushed into waters no research vessel had ever surveyed — Victoria Fjord in North Greenland, a remote and ice-choked body of water that had resisted scientific investigation for decades. The international team aboard was searching for an answer to a question that had puzzled glaciologists for more than twenty years: why had C.H. Ostenfeld Glacier lost its floating ice tongue in the early 2000s, while its two closest neighbors, Ryder and Petermann glaciers, still retained theirs?
The answer, it turned out, was written in the shape of the seafloor. Victoria Fjord lacks the shallow underwater ridge — called a sill — that exists in the fjords where Ryder and Petermann terminate. Those sills act as natural barriers, slowing the advance of warm Atlantic water toward the glacier fronts. Without one, warm water flows freely through Victoria Fjord and directly contacts C.H. Ostenfeld's margin. Over time, that sustained thermal exposure dissolved the glacier's ice tongue, while its neighbors, shielded by their seafloor geography, were spared.
Martin Jakobsson, a marine geology professor at Stockholm University and one of Oden's chief scientists, framed the discovery plainly: the seafloor is not passive geography. It channels currents, shapes temperature patterns, and ultimately determines what happens at the glacier front. The expedition also produced the first detailed bathymetric maps of Victoria Fjord, now incorporated into the International Bathymetric Chart of the Arctic Ocean — filling a meaningful gap in humanity's knowledge of the Arctic seafloor.
The loss of C.H. Ostenfeld's ice tongue is part of a broader pattern of retreat across Greenland and Antarctica. But the mechanisms differ from glacier to glacier, and Oden's findings suggest that underwater topography — not just the ice itself — is a key variable in determining vulnerability. That insight offers a new lens for anticipating which glaciers may be most at risk as ocean temperatures continue to rise.
In the summer of 2024, the Swedish icebreaker Oden—one of the world's most capable ice-breaking vessels—pushed into waters no research ship had ever surveyed before. The destination was Victoria Fjord in North Greenland, a remote corner of the Arctic that had remained largely unmapped and scientifically opaque. What the international team aboard Oden discovered there would answer a question glaciologists had puzzled over for more than two decades: why one glacier lost its floating ice tongue while its neighbors kept theirs.
C.H. Ostenfeld Glacier is one of three major outlet glaciers draining the northern Greenland ice sheet. Its two closest neighbors—Ryder Glacier and Petermann Glacier—both still possess floating ice tongues, those extensions of glacier ice that extend out over the ocean. But C.H. Ostenfeld lost its ice tongue sometime in the early 2000s, and the reason had remained a mystery. The glacier sits in Victoria Fjord, a body of water so isolated and ice-choked that no ship had ever successfully collected scientific data there until Oden arrived.
The expedition's findings, now published in peer-reviewed research, point to a simple but consequential difference in seafloor geography. Victoria Fjord lacks a shallow underwater ridge—what oceanographers call a sill—that exists in the fjords where Ryder and Petermann glaciers terminate. These protective sills act as natural barriers, slowing the intrusion of warm Atlantic water toward the glacier fronts. Without such a barrier, warm water flows freely into Victoria Fjord and directly contacts the margin of C.H. Ostenfeld Glacier. Over time, that thermal exposure melted away the glacier's floating ice tongue, while neighboring glaciers, shielded by their seafloor sills, retained theirs.
Martin Jakobsson, a marine geology professor at Stockholm University and one of the chief scientists aboard Oden, described the significance of the discovery in straightforward terms. For years, researchers had sought to understand why C.H. Ostenfeld evolved so differently from its neighbors. The expedition gave them the chance to investigate directly. What emerged was a powerful lesson: the shape of the ocean floor, hidden beneath the surface, exerts profound influence on what happens at the glacier front. The seafloor is not merely passive geography—it actively channels water, determines temperature patterns, and ultimately shapes the fate of glaciers.
The Oden expedition also produced the first detailed bathymetric maps of Victoria Fjord—precise measurements of seafloor depth and topography across the entire fjord. These data have since been incorporated into the International Bathymetric Chart of the Arctic Ocean, filling in a significant blank on the map of the Arctic seafloor. For Seabed 2030, the international initiative dedicated to mapping the world's ocean floor by the end of the decade, the expedition exemplifies why the work matters. Understanding how water moves through the ocean—how warm currents find their way to glacier margins, how seafloor features redirect or accelerate that flow—requires knowing the shape of the seafloor itself. Without maps, these processes remain invisible. With them, the hidden mechanisms that drive glacier change become legible.
The loss of C.H. Ostenfeld's ice tongue is not an isolated event. Across Greenland and Antarctica, floating ice tongues have been retreating and disappearing, a visible sign of warming oceans and changing polar climate. But the mechanism varies from glacier to glacier, and understanding those variations requires the kind of detailed oceanographic and bathymetric knowledge that Oden's expedition provided. The seafloor survey of Victoria Fjord suggests that some glaciers are more vulnerable to warm water intrusion than others—not because of differences in the ice itself, but because of differences in the underwater topography that surrounds them. That insight opens new avenues for predicting which glaciers might be at risk, and why.
Citações Notáveis
The shape of the seafloor influences how water moves through the ocean, including how warmer waters interact with glacier margins.— Martin Jakobsson, Stockholm University marine geology professor and chief scientist aboard Oden
A Conversa do Hearth Outra perspectiva sobre a história
Why does it matter that one glacier lost its ice tongue while others didn't? Aren't they all melting anyway?
The difference is in the mechanism. If all three glaciers were melting from above—from warmer air—we'd expect them to behave similarly. But they didn't. That tells us something specific is happening underwater, and understanding what that is helps us predict which glaciers are most vulnerable.
And the seafloor mapping is what revealed that difference?
Exactly. Until Oden reached Victoria Fjord, no one had mapped that seafloor. Without the map, you can't see why warm Atlantic water reaches C.H. Ostenfeld but not the others. The sill—that underwater ridge—acts like a gate. Victoria Fjord has no gate.
So the ice tongue didn't disappear because the glacier itself was weaker?
No. The glacier is fine. It's the water around it that changed its fate. The ice tongue was exposed to warmth it couldn't withstand. A few kilometers away, Ryder and Petermann glaciers sit behind their protective ridges, still holding their ice tongues.
Does this mean we can now predict which other glaciers will lose theirs?
It's a start. Now we know to look at the seafloor topology around other glaciers. If a glacier sits in a fjord without a protective sill, and warm Atlantic water can reach it, that's a warning sign. But every fjord is different, so we need more maps, more data.
How many other fjords in the Arctic are still unmapped like Victoria Fjord was?
Plenty. That's why Seabed 2030 exists. Most of the Arctic seafloor is still a blank space on our maps. Oden's expedition filled one small but crucial blank. There are many more.