Topographic controls on ice flow and recession for Juneau Icefield (Alaska/British Columbia)

Abstract

Globally, mountain glaciers and ice caps are losing dramatic volumes of ice. The resultant sea‐level rise is dominated by contributions from Alaska. Plateau icefields may be especially sensitive to climate change due to the non‐linear controls their topography imparts on their response to climate change. However, Alaskan plateau icefields have been subject to little structural glaciological or regional geomorphological assessment, which makes the controls on their present and former mass balance difficult to ascertain. We inventoried 1050 glaciers and 368 lakes in the Juneau Icefield region for the year 2019. We found that 63 glaciers had disappeared since the 2005 inventory, with a reduction in glacier area of 422 km2 (10.0%). We also present the first structural glaciological and geomorphological map for an entire icefield in Alaska. Glaciological mapping of >20 800 features included crevasses, debris cover, foliation, ogives, medial moraines and, importantly, areas of glacier fragmentation, where glaciers either separated from tributaries via lateral recession (n = 59), or disconnected within areas of former icefalls (n = 281). Geomorphological mapping of >10 200 landforms included glacial moraines, glacial lakes, trimlines, flutes and cirques. These landforms were generated by a temperate icefield during the Little Ice Age (LIA) neoglaciation. These data demonstrate that the present‐day outlet glaciers, which have a similar thermal and ice‐flow regime, have undergone largely continuous recession since the LIA. Importantly, disconnections occurring within glaciers can separate accumulation and ablation zones, increasing rates of glacier mass loss. We show that glacier disconnections are widespread across the icefield and should be critically taken into consideration when icefield vulnerability to climate change is considered.

Surface mass balance modelling of the Juneau Icefield highlights the potential for rapid ice loss by the mid-21st century

“…As well as triggering the ice-elevation and the ice-albedo feedbacks, the rise in temperatures also causes the ELA to rise above the elevation of many major icefalls. Davies et al (2022), identified 13 major outlet glaciers at risk of disconnection due to ice thinning intersecting their icefalls. By 2040, our model projects that all the major icefalls mapped by Davies et al (2022) around the peripheries of the plateau will have a negative annual mass balance.…”

“…Davies et al (2022), identified 13 major outlet glaciers at risk of disconnection due to ice thinning intersecting their icefalls. By 2040, our model projects that all the major icefalls mapped by Davies et al (2022) around the peripheries of the plateau will have a negative annual mass balance. Across the 2031-2060 time period, the multi-model mean of our future projections indicates a range of negative SMB at icefall locations, between -0.5 m w.e.…”

“…CC BY 4.0 License. majority of glacial ice is held within its several icefields and ice caps situated along the mountainous coastal regions (Davies et al, 2022). Though global-scale projections show a linear loss of ice mass with temperature increases (Rounce et al, 2023), the hypsometry of plateau icefields, such as Juneau Icefield (Fig.…”

“…Though global-scale projections show a linear loss of ice mass with temperature increases (Rounce et al, 2023), the hypsometry of plateau icefields, such as Juneau Icefield (Fig. 1), makes them susceptible to rapid ice loss through three processes: i) once the equilibrium line altitude (ELA) rises to the elevation of the plateau, small increases in ELA lead to a rapid decrease in accumulation area and widespread thinning (Böðvarssson, 1955;Bolibar et al, 2022); ii) ELA rises cause the exposure of bare ice, which has a lower albedo than snow, promoting melting through an albedo-feedback (Johnson and Rupper, 2020); iii) dynamic thinning across icefalls flowing over steep topography can lead to the disconnection of glacier tongues from the plateau, leaving them devoid of an accumulation area, with glacier tongues left only to ablate (Davies et al, 2022). Thus, reliable projections of the future mass balance of Alaskan icefields and ice caps are critical for projecting future sea-level rise.…”

“…1). Situated within the Coast Mountain Range, and on the border between Alaska and British Columbia, Juneau Icefield consists of a high elevation (>1200 m) plateau which is drained by numerous outlet glaciers (Sprenke et al, 1999;Davies et al, 2022). Numerous observational assessments of the mass-balance of the Juneau Icefield have been conducted.…”