Heavy Rain Accelerates Alaska Glaciers, Impacting Landscape and Sea Levels

Glaciers, often perceived as slowly moving ice masses, can undergo dramatic surges, accelerating orders of magnitude faster than their typical pace. These rapid advances have profound environmental consequences, acting like bulldozers to erode valleys, fragmenting glacier surfaces with crevasses, blocking side valleys to form temporary ice-dammed lakes, and potentially causing catastrophic floods when these dams burst. Such increased movement, especially as glaciers approach the sea, contributes significantly to ice calving and melting, impacting rising sea levels.

Understanding the mechanisms behind these aggressive advances is crucial for scientists monitoring glacier instabilities and their broader implications for climate change. Parker Wilkerson of Boise State University and colleagues focused their research on the steep, highly crevassed Sít' Kusá, also known as Turner Glacier, in Alaska, to investigate how rainfall can induce these glacier surges.

The research team employed GPS measurements from instruments installed around the glacier, recording its position every 15 seconds to track short-term speed changes against its longer-term movement. These measurements were then correlated with rain estimates, validated by data from the weather station at the nearby Yakutat Airport, to establish a link between rainfall events and glacier surges. The findings, published in The Cryosphere, confirmed a clear correlation.

Comparing a surge period from 2020–2021 with a quiescent, slow-flow period from 2022–2024, the study revealed that over half of the rainfall events during the surge phase caused speed-ups. The maximum recorded acceleration reached 7.5 meters (24.6 feet) in a single day, representing a 116% increase in speed, effectively more than doubling its pace. This surge also persisted for 66 hours, twice as long as any occurring during the quiescent period, and typically commenced 12 hours after precipitation reached at least 20 mm (0.79 inches). The glacier returned to a stable state approximately three days later.

During the quiescent flow period, rain-induced speed-ups were less frequent but required a higher rainfall threshold of 60 mm (2.4 inches) since the start of the event. Despite these events causing a smaller absolute increase of 1.5 meters (4.9 feet) per day, they resulted in a proportionately larger speed increase of 251%, peaking on average 32 hours after the rainfall began. The researchers also observed that glacier acceleration was consistently greatest in the downhill sections of the glacier during both phases, likely influenced by a combination of landscape topography, gravity, and the lubricating effect of meltwater.

The surge phenomenon is explained by the efficiency of a glacier's internal drainage system. An efficient system features linked channels that effectively drain rainwater and meltwater from the surface, through, and beneath the glacier. This allows the glacier to remain in contact with its underlying bedrock, maintaining its characteristic slow, steady advance. Conversely, an inefficient drainage system leads to issues where water becomes trapped in isolated pockets beneath the ice or within the sediment, known as till, directly below the glacier.

This trapped water causes pressure to build, counteracting the immense weight of the overlying ice. This reduction in friction creates a lubricating layer, enabling the glacier to slide rapidly over the underlying ground, a process known as basal sliding. In other scenarios, basal sliding can also occur when the ice weight reaches a critical point, altering the melting temperature of the ice and creating a water layer between the glacier and bedrock.

These insights into the relationship between drainage efficiency and glacier speeds are vital for scientists to refine future model projections for ice loss. Shifting weather patterns, which could lead to increased rainfall, are identified as a potentially important factor driving the frequency and duration of surges in the years to come. Coupled with increased melting in a warming world, continued glacier ice loss is anticipated, necessitating a deeper understanding of its implications for sea levels, particularly for vulnerable coastal and island communities facing flood risks. While predictive models are useful, the dynamic and evolving nature of glacier drainage systems, changing on decadal, yearly, and even daily timescales, introduces complexities and elements of unpredictability.

Editor's note: Thorough and accurate summary of the research findings, methodology, and the scientific explanation of glacier surges.