Predicting the Demise of Sit' Tlein

Mt. St. Elias in the background and the lobe of Malaspina Glacier, the largest piedmont glacier in the world, in the foreground.

Sít’ Tlein is the traditional Yakutat Tlingit name for Malaspina Glacier, in Wrangell-St. Elias National Park and Preserve. It translates roughly as "Big Glacier" and for good reason: Sít’ Tlein is one of the ten largest mountain glaciers in the world, and it is the world's largest piedmont glacier—the name we use to describe a type of glacier that spills out of the mountains and spreads out in a low featureless plain. In the case of Sít’ Tlein, the piedmont lobe of the glacier flows down onto a coastal plain adjacent to the Pacific Ocean in the Gulf of Alaska. The terminus reaches almost to the coastline, and in some areas is protected from warm ocean water by only a thin (10s to 100s of meters wide) bouldery beach. Though the terminus has barely retreated in recent decades, laser altimetry shows that the surface of the piedmont lobe has been lowering as the glacier melts and thins. Prompted by that observation, and also by the recent discovery of a direct hydrologic connection between the ocean and some of Sít’ Tlein's proglacial lakes (see a report and maps), a team of scientists set out to forecast the future of this massive and complicated glacier.

The team, consisting of glaciologists from NPS and several universities and led by computer science Professor Doug Brinkerhoff from the University of Montana, used satellite data and diverse field measurements to constrain a numerical model that simulates glacier mass balance (snowfall and melt), ice dynamics (ice deformation, sliding, and periodic surging), and other parameters for the next few hundred years. Because the time interval is so long, Brinkerhoff chose not to force the model with climate scenarios that are themselves based on uncertain predictions of societal behavior and instead simulated two simple end member scenarios: one in which the climate remains forever steady at present levels, and another where the observed rate of temperature increase from 1915 to present continues unabated into the future.

The model predicts that under both the steady climate and the warming climate scenarios, the piedmont lobe of Sít’ Tlein continues to rapidly lose mass through the end of the simulation period (AD 2344). It of course shrinks faster in the continually warming climate, but even without further climatic warming, the piedmont lobe retreats significantly from the present terminus over the next 50 years. This initial retreat is important because the bed of the current piedmont lobe is mostly below sea level; as the ice retreats, it is replaced by a rapidly growing proglacial lagoon. This situation allows the glacier to begin calving icebergs directly into the ocean, and also hastens melt where the ice cliffs are in contact with warm ocean water. This happens in either climate scenario, leading to disappearance of most or all of the Sít’ Tlein piedmont lobe over the coming 150 years. Notably, the whole glacier doesn't disappear—there will remain a large tidewater glacier pouring out of the high coastal mountains. But importantly, the portion of the glacier that is replaced by ocean water will engender a revision of the boundaries of Wrangell-St. Elias National Park and Preserve as terrestrial glacier habitat (part of the park) is replaced by marine waters that are not subject to NPS jurisdiction. We predict this will constitute the largest removal of park lands in the history of the National Park System.

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Surficial Geology and Proglacial Lake Change at Sit' Tlein (Malaspina Glacier)

The demise of the world's largest piedmont glacier: A probabilistic forecast

Abstract

Sít' Tlein in Alaska's St. Elias Range (briefly known as Malaspina Glacier) is the world's largest piedmont glacier and has thinned considerably over 30 years of altimetry, yet it's low-elevation piedmont lobe has remained intact in contrast to the glaciers that once filled neighboring Icy and Disenchantment bays. In an effort to forecast changes to Síit' Tlein over decadal to centennial time scales, we take a data-constrained dynamical modelling approach, in which we constrain the parameters of a higher order model of ice flow – the bed elevation, basal traction, and surface mass balance – with a diverse but spatio-temporally sparse set of observations including satellite-derived time-varying velocity fields, radar-derived bed and surface elevation measurements, and in situ and remotely sensed observations of accumulation and ablation. Nonetheless, such data do not uniquely constrain model behavior, so we adopt an approximate Bayesian approach based on the Laplace approximation and facilitated by low-rank parametric representations to quantify uncertainty in the bed, traction, and mass balance fields alongside the induced uncertainty in model-based predictions of glacier change. We find that Sít' Tlein is considerably out of balance with contemporary (and presumably future) climate, and we expect its piedmont lobe to largely disappear over the coming 150 years. We forecast a total mass loss at Sít' Tlein of between 500 and 1000 km³ of ice, a range that represents not only uncertainty in model inputs, but also in future warming scenarios. The resulting retreat and subsequent replacement of glacier ice with a marine embayment or lake will yield a significant modification to the regional landscape and ecosystem.

Brinkerhoff, D., B. Tober, M. Daniel, V. Devaux-Chupin, M. Christoffersen, J. Holt, C. Larsen, M. Fahnestock, M. G. Loso, K. Timm, R. Mitchell, and M. Truffer. 2024. The demise of the world’s largest piedmont glacier: A probabilistic forecast. The Cryosphere.