The aim of this paper is to characterise the internal structures and ice-flow history of representative valley glaciers in Svalbard and infer from them dynamic changes over centennial timescales. Three polythermal and one cold valley glacier are investigated using field- and laboratory-based techniques and remote sensing. Structures along flow-unit boundaries indicate that ice-flow configuration in three of the glaciers has remained stable spanning the residence time of the ice. Deformation of a flow-unit boundary in the fourth reveals an ice-flow instability, albeit one that has been maintained since its most recent advance. Macro-crystallographic, sedimentological and isotopic analyses indicate that basal ice is elevated to the glacier surface, as shown by entrained sediments and enrichment in heavy isotopes. In narrow zones of enhanced cumulative strain, new ice facies are generated through dynamic recrystallisation. The surface density of longitudinal foliation is shown to represent the relative magnitude of cumulative strain. Geometric similarities between flow-unit boundaries in Svalbard valley glaciers and larger scale longitudinal surface structures in ice sheets suggest that deformation mechanisms are common to both.

We present a 1000 km transect of phase-sensitive radar measurements of ice thickness, basal reflection strength, basal melting and ice-column deformation across the Ross Ice Shelf (RIS). Measurements were gathered at varying intervals in austral summer between 2015 and 2020, connecting the grounding line with the distant ice shelf front. We identified changing basal reflection strengths revealing a variety of basal conditions influenced by ice flow and by ice–ocean interaction at the ice base. Reflection strength is lower across the central RIS, while strong reflections in the near-front and near-grounding line regions correspond with higher basal melt rates, up to 0.47 ± 0.02 m a−1 in the north. Melting from atmospherically warmed surface water extends 150–170 km south of the RIS front. Melt rates up to 0.29 ± 0.03 m a−1 and 0.15 ± 0.03 m a−1 are observed near the grounding lines of the Whillans and Kamb Ice Stream, respectively. Although troublesome to compare directly, our surface-based observations generally agree with the basal melt pattern provided by satellite-based methods but provide a distinctly smoother pattern. Our work delivers a precise measurement of basal melt rates across the RIS, a rare insight that also provides an early 21st-century baseline.

The accumulation area ratio (AAR) of a glacier reflects its current state of equilibrium, or disequilibrium, with climate and its vulnerability to future climate change. Here, we present an inventory of glacier-specific annual accumulation areas and equilibrium line altitudes (ELAs) for over 3000 glaciers in Alaska and northwest Canada (88% of the regional glacier area) from 2018 to 2022 derived from Sentinel-2 imagery. We find that the 5 year average AAR of the entire study area is 0.41, with an inter-annual range of 0.25–0.49. More than 1000 glaciers, representing 8% of the investigated glacier area, were found to have effectively no accumulation area. Summer temperature and winter precipitation from ERA5-Land explained nearly 50% of the inter-annual ELA variability across the entire study region (${R}^2=0.47$). An analysis of future climate scenarios (SSP2-4.5) projects that ELAs will rise by ∼170 m on average by the end of the 21st century. Such changes would result in a loss of 25% of the modern accumulation area, leaving a total of 1900 glaciers (22% of the investigated area) with no accumulation area. These results highlight the current state of glacier disequilibrium with modern climate, as well as glacier vulnerability to projected future warming.