Retreat of Hektoria Glacier

 Satellite data confirms the Antarctic glacier lost record ice so rapidly  

To say something moves at a glacial pace is to imply sluggish, unhurried change. But what transpired over the course of 15 months at Antarctica’s Hektoria Glacier was uncharacteristically quick. The glacier lost about 25 km's (15 miles) in length. That included a two-month period in which the terminus retreated more than 8 km's (5 miles)—the highest rate of grounded glacial ice loss observed in modern history. A team of scientists published an analysis of Hektoria’s collapse based on a suite of remote-sensing data, finding that its particular geometry enabled the rapid change. Like many glaciers on the Antarctic Peninsula, Hektoria starts on land and extends to the sea, with the last section being a thick, floating plate of ice, or “ice tongue.” The researchers determined Hektoria lost both its ice tongue and an area of grounded ice spread over a flat plain, the latter directly contributing to sea level rise. Although Hektoria is relatively small as Antarctic glaciers go, scientists say that similar events at larger glaciers could be much more consequential.

The scale of the loss of Hektoria’s grounded ice on the eastern Antarctic Peninsula is huge. Hektoria’s terminus remained relatively stable after the sudden loss, the study reported, though the neighboring Green Glacier continued to retreat. The chain of events culminating in Hektoria’s breakup goes back to early 2002. At that time, the Larsen B ice shelf, which served as a backstop for Hektoria and neighboring glaciers, splintered and collapsed in short order. The glaciers then thinned and retreated for several years. In 2011, landfast sea ice in the Larsen B embayment near Hektoria’s terminus filled in enough to allow the glacier to start advancing. But after several years, the new support for the glacier front was suddenly removed. Landfast ice in the embayment broke up in January 2022, likely due to large, destabilizing ocean swells. From that point, rapid change at Hektoria was again underway. Throughout the rest of the austral summer, the floating ice tongue disaggregated in a series of calvings, resulting in a loss of 16 km's. Hektoria Glacier’s sudden eight-kilometer collapse stunned scientists, marking the fastest modern ice retreat ever recorded in Antarctica. Its flat, below-sea-level ice plain allowed huge slabs of ice to detach rapidly once retreat began. Seismic activity confirmed this wasn’t just floating ice but grounded mass contributing to sea level rise. The event raises alarms that other fragile glaciers may be poised for similar, faster-than-expected collapses.

The glacier’s terminus stabilized during the 2022 austral winter. However, satellite-based laser altimetry data, including ice elevation measurements from NASA’s ICESat-2 (Ice, Cloud, and Land Elevation Satellite-2) mission, revealed that the ice continued to thin during that winter. The thinner remaining ice was still grounded during the 2022 austral spring, the study authors concluded, based on the detection of earthquakes occurring beneath the glacier. They determined the ice was spread out over a relatively flat area of bedrock, forming an ice plain. This geometry allows seawater to infiltrate the glacier’s bed during high tide and intermittently lift ice off the ground. When ice is thin enough, large areas can lift and break away at once. The process, called buoyancy-driven calving, is believed to have caused the second stage of Hektoria’s rapid retreat, resulting in an additional loss of 8 km's in length. Hektoria and Green, once glaciated, are now reduced to drifting ice rubble. A glacier on the Eastern Antarctic Peninsula has undergone the quickest ice loss documented in modern times, according to a major international study. Hektoria Glacier shortened by nearly half its length in only two months during 2023. The glacier shed ice in such a short period, it is comparable to the rapid withdrawals which marked the end of the last ice age. Led by the University of Colorado Boulder (CU Boulder), the global research team, which included Swansea glaciologist Professor Adrian Luckman, determined that the underlying landscape played a major role in accelerating the glacier's retreat.

Hektoria Glacier had been positioned on an ice plain, a level expanse of bedrock located below sea level. Once retreat began, this setting allowed large portions of ice to detach quickly and in sequence. The remarkable speed and scale of the glacier's collapse may now guide researchers as they work to identify other glaciers with similar vulnerabilities and determine which ones require the closest observation. Although Hektoria Glacier is modest in size by Antarctic standards, covering about 115 square miles (slightly smaller than the city of Austin, Texas), its abrupt retreat serves as a serious warning. Comparable events occurring on larger glaciers could significantly influence the rate of global sea level rise. New platforms, such as the NISAR and SWOT satellites developed by NASA and partners, may aid in understanding rapid changes in glaciers. Naomi Ochwat, a glaciologist at the University of Innsbruck and the study’s lead author, is now looking into other glaciers which may be at risk of destabilizing in a similar way. As the Antarctic Peninsula responds to warming, more of its glaciers are losing their ice tongues, and their termini are now resting on the seabed, as Hektoria's does. (Called tidewater glaciers, this type is common in Alaska and Greenland.) New technologies developed by NASA and partners can aid in understanding rapid glacial retreat, said Ochwat and study co-author Ted Scambos, a senior research scientist at the University of Colorado Boulder. 

Glaciers don't usually retreat this fast. The circumstances may be a little particular, but this scale of ice loss shows what may happen elsewhere in Antarctica, where glaciers are lightly grounded and sea ice loses its grip. Although the record indicates some very rapid retreats in the past, the pace of retreat of Hektoria Glacier and its neighbors is unprecedented in the observational record. This is the latest chapter in a sequence of events which started with the collapse of the Larsen B Ice Shelf 23 years ago, marking a landscape-changing event that offers insights into the potential future rates of glacier retreat elsewhere in Antarctica. The NISAR (NASA-ISRO Synthetic Aperture Radar) satellite, for example, can detect the movement of land and ice surfaces down to the centimeter. Its data will be “very useful for structural evaluations of Hektoria and other glaciers in the region,” Scambos said. “In addition to NISAR,” Ochwat added, “I'm particularly interested in learning what SWOT can tell us about rapid glacier changes.” The SWOT (Surface Water and Ocean Topography) satellite’s primary mission is to observe the fine details of Earth’s surface water height. But scientists are also exploring its applications to the cryosphere, such as measuring surfaces of ice shelves and sea ice.

The research team used satellite data and seismic measurements to examine the glacier's breakdown in detail. Their analysis revealed several grounding lines, the points where a glacier transitions from resting on solid rock to floating on seawater. These features confirmed the presence of the ice plain and highlighted how easily the glacier could retreat when exposed to ocean-driven forces. Seismic devices also detected glacier earthquakes, small tremors caused by abrupt ice shifts. These signals showed that the ice was still grounded at the time of retreat, meaning the loss directly contributed to global sea level rise. At Hektoria Glacier, the days of dramatic change are likely past, now to be replaced by slow retreat. Scambos said he would not be surprised to see the ice slowing down. “The glacier has lost so much elevation and mass that it simply can’t continue to maintain the same output,” he said. “It’s on its way to being a fjord, not a glacier.” This kind of lightning-fast retreat really changes what's possible for other, larger glaciers on the continent. If the same conditions are set up in some of the other areas, it could greatly speed up sea level rise from the continent. The authors emphasize the importance of ongoing monitoring efforts and international scientific cooperation to better track and understand changes unfolding across frozen regions of our world.