Glacial Status Time Series

The Pemberton Icefield has undergone a profound cryospheric contraction over the 40-year study period, characterized by a sustained and accelerating decline in ice extent. From a 1985 baseline of 70,212 hectares, the total glacial area has diminished to 40,686 hectares by 2025. This represents a net loss of approximately 42% of the icefield’s area. While the retreat was relatively linear during the late 20th century, the data indicates a significant acceleration in mass loss following 2010; nearly half of the total observed area loss occurred in the final 15 years of the time series.

A notable statistical anomaly is observed in the year 2000, where the classified glacial area reaches a period maximum of 71,506 hectares. This value deviates from the multi-decadal downward trend and is attributed to extensive seasonal snowpack at the time of image acquisition. This spectral interference temporarily masked the underlying glacial recession, highlighting the challenges of differentiating perennial ice from late-season snow in automated classifications. Following this anomaly, the 2005 data (61,688 ha) confirms a return to the long-term recession trajectory.

Spatially, the time-series animation reveals a distinct pattern of marginal recession and increasing fragmentation. The most pronounced changes are concentrated at lower-elevation valley tongues and along the periphery of the central ice mass. Key spatial trends include:

• Terminal Retreat: Major outlet glaciers show consistent up-valley migration, with significant exposure of proglacial landscapes.
• Fragmentation: Large, contiguous ice bodies are increasingly bisected by emerging nunataks and rocky ridges, particularly in the central and eastern sectors.
• Peripheral Attrition: Smaller, isolated glacial patches on the outskirts of the main icefield have largely disappeared, transitioning entirely to "Non-Ice" status by 2025.
• Core Thinning: While the central core of the icefield remains the most resilient, its perimeter shows a "shrinking" effect, with the most rapid lateral retreat occurring between 2015 and 2025.

False Color Time Series

The false-color time series, spanning from 1985 to 2023, utilizes Landsat SWIR1-NIR-Red composites to provide a high-contrast visualization of cryospheric retreat. In this spectral rendering, snow and ice are characterized by a bright cyan to brilliant white signature. This distinct appearance is driven by the high reflectance of ice in the visible and near-infrared (NIR) spectra coupled with its strong absorption in the short-wave infrared (SWIR1) band. This contrast allows for the precise differentiation of glacial boundaries from the surrounding terrain, which appears in darker, muted tones of brown and grey representing exposed lithology and dormant vegetation.

The 38-year sequence reveals a significant and progressive reduction in total glacial area across the study site. Key observations include:

• 1985–2000: The initial period shows a relatively contiguous ice mass with several prominent glacial tongues extending deep into the southern and eastern valleys. The bright cyan signature dominates the central high-elevation plateau.
• 2000–2015: A period of accelerated marginal recession is evident. The glacial termini show noticeable longitudinal shortening, and the "thinning" of the ice is visually represented by the encroachment of darker pixels into previously solid cyan blocks, indicating the emergence of nunataks and subglacial topography.
• 2015–2023: The most recent frames show advanced fragmentation. The once-unified glacial system has decoupled into isolated ice patches. The bright cyan signature has retreated significantly toward the highest accumulation zones, leaving behind expansive proglacial zones characterized by exposed bedrock and morainal debris.

Spatially, the retreat is most pronounced in the lower-elevation valley glaciers, where the transition from clean ice (cyan) to debris-covered ice (pinkish-grey) precedes total ice loss. The geographic concentration of loss is highest along the southern aspects, likely due to increased solar radiation and thermal forcing. By 2023, the spatial extent of the ice cover represents a fraction of the 1985 baseline, illustrating a profound shift in the regional hydrological and cryospheric landscape.

5-Year Glacial Transitions

Analysis of the pixel-level transition matrices from 1985 to 2025 reveals a sustained and accelerating contraction of glacial extent. The Glacier-to-Glacier (G→G) stability—representing the persistent glacial core—exhibited a significant downward trend, declining from 23.15% in the 1985–1990 period to a terminal value of 13.89% by 2025. This represents a nearly 40% reduction in the stable ice core over four decades. While the core remained relatively resilient through 2010, the post-2010 epochs show a marked collapse in stability, with the G→G value dropping below the 20% threshold for the first time during the 2010–2015 interval.

The marginal dynamics are characterized by high volatility in the Glacier-to-Non-Ice (G→NI) transition, which serves as a primary indicator of ablation and terminal retreat. Two distinct periods of rapid melting are observed: * 2000–2005: A peak transition of 6.90% from Glacier to Non-Ice, the highest recorded loss in the study period. * 2010–2015: A secondary surge of 4.74%, which initiated a decade of consistent decline in total ice cover.

Conversely, the Non-Ice-to-Glacier (NI→G) transitions, representing potential glacial advance or seasonal snow persistence, remained negligible (typically <1.0%) for most of the study. A notable anomaly occurred during the 1995–2000 interval, where a 6.57% gain was recorded; however, this gain was transient, as it was immediately offset by the record losses in the subsequent five-year period.

By the 2020–2025 interval, the landscape reached a state of pronounced deglaciation. The Non-Ice-to-Non-Ice (NI→NI) stability reached a study-high of 82.51%, indicating that the vast majority of the study area is now permanently ice-free. The narrowing gap between the stable core and marginal loss suggests that the glacier is not only retreating at its edges but is also undergoing internal thinning, leading to a fragmented and increasingly vulnerable cryospheric footprint.