NASA images show new Alaskan island as Alsek Glacier retreats, agency says warming to blame
Prow Knob — long a nunatak within the Alsek Glacier — became surrounded by water between July and August after decades of glacial retreat and lake growth, NASA researchers say.
NASA satellite imagery shows a small mountain in southeastern Alaska has become an island after the Alsek Glacier retreated, a change the agency and glaciologists attribute to regional warming linked to human-caused climate change.
Images from the Landsat 8 satellite captured in August show the landmass, known locally as Prow Knob, completely cut off from the surrounding ice and encircled by water. NASA's Earth Observatory reported the transition likely occurred between July 13 and Aug. 6 and said "along the coastal plain of southeastern Alaska, water is rapidly replacing ice," in a statement that quoted science writer Lindsey Doermann.
Historical records and aerial photography show Prow Knob was long surrounded by the Alsek Glacier. Observations dating to 1894 and follow-up reports in 1907 described the glacier as covering what is now Alsek Lake and anchoring against a rocky outcrop — the nunatak later called Prow Knob — with an ice face as high as about 50 meters. Aerial photos from the 1960s still showed Prow Knob with ice on all sides.
Satellite and field data document a steady retreat of the Alsek Glacier through the late 20th and early 21st centuries. NASA imagery from 1984 taken by Landsat 5 showed the glacier's western edge converting to lake shoreline. In the 1990s, a northerly arm of the glacier detached from an island in the lake, a change that made the glacier more susceptible to calving — the process by which chunks of ice break off its front — and contributed to a faster loss of ice. Over subsequent decades the glacier's tributaries thinned and retreated to the point that they no longer supplied enough ice to sustain the glacier's terminus.
That longer-term retreat accelerated again after 2018, according to NASA, leaving Prow Knob exposed on all sides and transforming parts of the icefield into open water. The expansion of proglacial lakes has been substantial: Alsek Lake grew from roughly 17 square miles (45 square kilometers) in 1984 to about 30 square miles (75 square kilometers) by recent estimates. Combined with neighboring Harlequin and Grand Plateau lakes, the three bodies of water have more than doubled in area in roughly four decades.
Scientists say the pattern at Alsek is part of a broader trend across Alaska. Glaciologist Mauri Pelto has warned that the state is becoming a "new lake district" as meltwater fills depressions once occupied by glaciers. These proglacial lakes pose additional hazards: many are impounded by relatively fragile natural dams of rock and ice left by retreating glaciers. If those dams fail, they can trigger sudden glacial lake outburst floods that send waves of water and debris downstream at high speed.
A recent study cited by researchers warned that around 10 million people globally may be at risk from glacial lake outburst floods as glaciers and their lakes proliferate or destabilize. Scientists also note that shrinking mountain glaciers and the loss of ice sheets contribute to global sea-level rise. Researchers estimate glaciers worldwide currently release on the order of hundreds of billions of tonnes of ice into the oceans each year; major changes to Antarctic ice, such as long-term instability of West Antarctica's Thwaites Glacier, could have particularly large consequences for future sea levels.
The transformation of Prow Knob from an ice-bound nunatak to an island underscores how satellites have become central to monitoring rapid environmental change in remote regions. NASA and other agencies continue to use Landsat and other Earth-observing platforms to track glacier extent, lake growth and downstream hazards, work officials say is important for understanding local risks and informing communities and resource managers.
Scientists caution that while the formation of a single island does not by itself change global projections, it provides a visible example of how warming temperatures are reshaping landscapes that were long held in place by ice. Continued monitoring and assessment will be needed to evaluate downstream flood risks, changes to freshwater storage in mountain regions, and implications for coastal sea-level rise driven by ice loss at larger scales.