Climate Science: Skyscraper-Sized Underwater Waves Eroding Greenland's Glaciers

The Unseen Titan: Skyscraper-Sized Underwater Waves Secretly Devouring Greenland's Glaciers

In the vast, frozen expanse of Greenland, a drama of immense proportions is unfolding, largely hidden from view. Beneath the still, icy surfaces of its majestic fjords, colossal underwater waves, some as tall as skyscrapers, are relentlessly eating away at the foundations of the island's mighty glaciers. This newly discovered phenomenon is a powerful and previously underestimated force in the rapid decay of the Greenland Ice Sheet, a process with profound implications for global sea levels and the planet's climate system.

For decades, scientists have documented the alarming retreat of Greenland's glaciers. They have pointed to rising air temperatures that melt the ice from above and the incursion of warmer ocean waters that attack the ice from below. But recent groundbreaking research has unveiled a dramatic and powerful new mechanism: a "calving multiplier effect," where the very act of a glacier shedding ice into the ocean triggers a violent, sub-surface process that dramatically accelerates its own demise. This discovery is forcing a recalculation of how we understand the delicate and dangerous dance between ice and ocean in a warming world.

The Anatomy of a Greenland Fjord: A Stage Set for Destruction

To comprehend this hidden erosion, one must first understand the unique environment of a Greenlandic fjord. These long, deep, and narrow inlets, carved by glaciers over millennia, are the primary conduits between the Greenland Ice Sheet and the Atlantic Ocean. They are the battlegrounds where the fate of much of the world's frozen freshwater is decided.

During the warmer summer months, a distinct layering, or stratification, occurs in the fjord's water column. At the surface, a layer of cold, fresh water forms from the melting ice sheet and the numerous rivers of meltwater that pour into the fjord. This fresh water is less dense than the water below. Beneath this surface layer lies a much warmer and saltier body of water, originating from the Atlantic Ocean. This warmer, denser Atlantic water settles in the deeper parts of the fjord, creating a sharp boundary between the two layers known as a pycnocline.

This stratification is the key to the formation of the destructive underwater waves. While the surface of the fjord might appear calm, this boundary between cold, fresh water and warm, salty water can be disturbed, creating what are known as internal waves. These waves are not visible on the surface but can propagate for hours, carrying immense energy through the depths. For years, scientists understood that tides and winds could generate internal waves, but they were about to discover a far more violent trigger.

The Calving Event: A Trigger for Chaos

The sheer face of a marine-terminating glacier is a place of constant tension and dramatic change. These towering ice cliffs, where the glacier meets the sea, are where iceberg calving occurs—the spectacular process of huge chunks of ice breaking free and crashing into the ocean. This can range from smaller pieces to colossal icebergs the size of city blocks.

When a massive iceberg detaches, the event is anything but quiet. The initial splash is gargantuan, generating surface waves, sometimes called "calving-induced tsunamis," that can surge through the fjord, posing a danger to any vessels or wildlife nearby. Inuit hunters have long known to listen for the distant rumble of a calving glacier, aware that destructive waves might follow minutes later.

However, the most significant impact happens beneath the surface. The immense energy released by the calving iceberg plunging into the water and then moving through the fjord acts like a giant paddle, violently disturbing the stratified water column. This is what gives birth to the skyscraper-sized internal waves.

A pioneering 2025 study published in Nature by an international team of scientists provided the first-ever detailed look at this process. The researchers found that long after the surface tsunami has subsided and the water appears still, these massive internal waves continue to roll through the depths of the fjord. They act as powerful mixers, churning the water column with incredible force.

Imagine a glass of iced tea on a hot day. Without stirring, a layer of cold water forms around the ice cubes, insulating them from the warmer liquid. But if you stir the drink, you disrupt that cold layer, and the ice melts much faster. The internal waves in a Greenlandic fjord are doing exactly the same thing, but on a monumental scale. They violently disrupt the protective layer of cold, fresh meltwater that would normally insulate the glacier's submerged face, or terminus. This brings a steady and renewed supply of the warmer, saltier Atlantic water into direct, sustained contact with the ice, dramatically accelerating underwater melting and erosion at the glacier's base.

This creates a vicious feedback loop. The increased melting at the base of the glacier undermines the ice cliff above, making it more unstable and prone to further calving. More calving means more and larger internal waves, which in turn leads to even more melting. This "calving multiplier effect" represents a significant, and until recently, unquantified amplifier of ice loss in Greenland.

Peering into the Depths: The Revolutionary Technology of Fiber Optics

Observing these powerful, invisible waves presented an enormous challenge. The front of a calving glacier is one of the most dangerous and inaccessible environments on Earth. Traditional instruments, such as mooring lines with temperature sensors, provide only point-in-time, single-location data and cannot be safely placed in the direct impact zone of falling ice.

To overcome this, scientists on the Swiss Polar Institute's GreenFjord project turned to a revolutionary technology: Distributed Acoustic Sensing (DAS). In essence, DAS technology turns a standard fiber-optic cable into a string of thousands of virtual microphones. An instrument called an "interrogator" sends pulses of laser light down the cable. Microscopic imperfections in the glass fiber cause a tiny fraction of this light, known as Rayleigh backscatter, to reflect to the source. When the cable is stretched or compressed by even the tiniest amount—down to the scale of atoms—due to vibrations from sound waves or seismic tremors, it changes the phase of this backscattered light. By analyzing these changes, scientists can detect and pinpoint the location, intensity, and frequency of vibrations all along the cable's length in real-time.

For the groundbreaking study at the Eqalorutsit Kangilliit Sermiat glacier in southern Greenland, researchers deployed a 10-kilometer-long fiber-optic cable on the seafloor of the fjord, right in front of the glacier's three-kilometer-wide calving face. This fast-flowing glacier releases a staggering 3.6 cubic kilometers of ice into the ocean each year. The DAS system allowed the science team to safely "listen" to the symphony of sounds and vibrations produced by the glacier, from the initial fracturing of the ice to the crash of the iceberg and, crucially, the subsequent turmoil in the water column. For the first time, they could visualize the propagation of the massive internal waves and quantify their impact. It was like having a thousand sensors directly beneath the chaotic glacier front.

The use of DAS in glaciology is a transformative leap forward, allowing for unprecedented spatial and temporal resolution in some of the planet's most hostile environments. The technology, which is also used for monitoring earthquakes, pipelines, and even whale calls, is now set to revolutionize our understanding of the cryosphere.

Greenland's Major Glaciers: A Story of Widespread Retreat

The discovery at Eqalorutsit Kangilliit Sermiat provides a key to understanding the behavior of many of Greenland's other major outlet glaciers, which are responsible for draining the vast interior ice sheet.

Jakobshavn Isbræ (Sermeq Kujalleq): Often dubbed Greenland's fastest-moving glacier, Jakobshavn is located on the west coast and has been a major contributor to sea-level rise, accounting for a global increase of roughly 0.9mm on its own between 2000 and 2010. Its dramatic speed-up and retreat, particularly the disintegration of its floating ice tongue after 2001, have been linked to the arrival of warm subsurface ocean waters into the fjord. While it experienced a brief period of slowing and readvancing around 2016, its long-term trend has been one of significant mass loss. The processes of internal wave generation are almost certainly at play in this highly active system. Kangerlussuaq Glacier: Located in East Greenland, Kangerlussuaq is another giant that has undergone periods of rapid retreat and acceleration. Along with Jakobshavn and Helheim, it is one of the "big three" that have historically dominated Greenland's ice discharge. Its behavior has also been linked to the presence of an "ice mélange"—a mix of sea ice and icebergs in the fjord that can buttress the glacier front—and the warming of Atlantic waters. A study found that this previously stable glacier retreated ~7 km and doubled its ice discharge between 2018 and 2021 after an intrusion of warm Atlantic deep water. Helheim Glacier: Also in the southeast, Helheim has experienced dramatic retreat and thinning. Scientists filming at Helheim in 2018 captured a calving event where a four-mile-long iceberg broke away, an event that viscerally demonstrates the immense scale of these processes.

The story is similar across Greenland. Of the more than 200 coastal glaciers, the vast majority are in retreat. They are losing mass through a combination of increased surface melt driven by a warming atmosphere and dynamic changes at the calving front, where the ocean is playing an ever-more aggressive role. The discovery of the calving-multiplier effect adds a critical new piece to this puzzle, suggesting that the dynamic losses at the glacier front are even greater than previously modeled.

The Global Ripple Effect: Sea Level Rise and Ocean Circulation

The fate of Greenland's ice is not a remote Arctic issue; it has profound global consequences. The Greenland Ice Sheet holds enough frozen water to raise global sea levels by more than 7 meters (23 feet) if it were to melt entirely. While a complete collapse is not imminent, the current rate of melt is a major cause for concern.

Since 2002, Greenland has lost about 5,900 billion tonnes of ice. This melt is currently responsible for about 20% of observed global sea-level rise. For every 360 gigatonnes of ice lost, the global mean sea level rises by one millimeter. The Intergovernmental Panel on Climate Change (IPCC) projects that Greenland could contribute between 8 and 27 centimeters (3.1 to 10.6 inches) to global sea level by 2100. However, discoveries like the calving-driven internal waves suggest that current models may not fully capture the speed of this process, and some studies warn that contributions could be even higher.

Beyond sea-level rise, the massive influx of cold, fresh water from Greenland's melting ice sheet poses a threat to the Atlantic Meridional Overturning Circulation (AMOC), a vast system of ocean currents often described as the planet's "great ocean conveyor belt." The AMOC, which includes the Gulf Stream, transports warm, salty water northward from the tropics and sends cold, deeper water southward. This circulation is crucial for regulating global climate patterns, distributing heat, and supporting marine ecosystems.

The circulation is driven by density differences: in the North Atlantic, the warm, salty water cools, becomes denser, and sinks, driving the deep southward return flow. The flood of fresh water from Greenland is less dense and does not sink as readily, potentially slowing or even disrupting this entire process. Scientists have observed that the AMOC is currently weaker than it has been in at least the last 1,000 years, and there are concerns that it could be approaching a tipping point where it could collapse, which would have devastating consequences for the climate in Europe and beyond.

A Glimpse into the Future: Tipping Points and Scenarios

The question is no longer whether the Greenland Ice Sheet will contribute to sea-level rise, but how much and how fast. Scientists are increasingly concerned about a potential tipping point, a threshold beyond which the melting of the ice sheet becomes irreversible, regardless of future efforts to curb warming.

One key feedback mechanism is the melt-elevation feedback. As the ice sheet melts, its surface lowers in altitude. Lower altitudes are exposed to warmer air, which in turn accelerates melting, further lowering the surface in a self-reinforcing cycle.

Climate models paint a stark picture of different futures for Greenland depending on the world's success in reducing greenhouse gas emissions.

Low-Emission Scenario: If warming is kept to low levels, in line with the most ambitious goals of the Paris Agreement, the rate of ice loss would slow, though melting would continue to contribute to sea-level rise. One study suggests a contribution of 5-34 cm by 2100 in this scenario.
High-Emission Scenario: Under a high-emissions, business-as-usual trajectory, the melting would accelerate dramatically. In this future, the ice sheet could become ice-free within a millennium, committing the world to the full 7-plus meters of sea-level rise. Some models predict a contribution of up to 162 cm (64 inches) from Greenland by 2200 in this scenario.

The discovery of the skyscraper-sized internal waves adds another layer of urgency to these projections. This powerful feedback mechanism, which has not been fully incorporated into older climate models, suggests that the ice sheet may be more sensitive to warming than previously understood. As research continues under initiatives like the GreenFjord project, which studies the interconnected cryosphere, ocean, and biosphere, a clearer but likely more alarming picture of Greenland's future will emerge.

The silent, invisible power of these underwater waves serves as a stark reminder of the complex and sometimes surprising ways in which the Earth's systems are responding to climate change. What happens in the hidden depths of Greenland's fjords will not stay there; it will be felt on coastlines around the world for centuries to come. The roaring crash of a calving iceberg is not just an end, but the beginning of a process that is hastening the melt of our planet's great northern ice sheet.