Glaciers terminating in the ocean experience mass loss through frontal ablation at the ice-ocean interface. This includes iceberg calving, submarine and subaerial melting, and sublimation. Quantifying frontal ablation is crucial for understanding glacier mass loss components, informing sea-level budgets, estimating freshwater contributions to the ocean, and assessing impacts on marine ecosystems and iceberg hazards. Previous model results suggested Northern Hemisphere frontal ablation rates of around 39 Gt a⁻¹ from 1980 to 1999 and projected a rate of 50.6 ± 23.8 Gt a⁻¹ for 2020–2040. However, observation-based estimates have been limited, often focusing solely on ice discharge without accounting for terminus changes. Complete observation-based estimates existed only for Alaska and Svalbard, but these were not updated since 2013. While regional-scale studies have determined total glacier mass changes using various methods, a comprehensive Northern Hemisphere frontal ablation rate had not been established due to the lack of consistent data on ice thickness, surface velocity, and terminus position. This study addresses this gap by identifying all marine-terminating glaciers in the Northern Hemisphere (excluding the Greenland Ice Sheet) and using measurements or estimates of ice thickness, surface velocity, and terminus position changes to estimate mean frontal ablation. The study period is divided into two decades: 2000–2010 and 2010–2020. Frontal ablation is estimated indirectly by calculating ice discharge and mass change due to terminus retreat or advance, while accounting for climatic mass balance. Submarine melting below floating tongues is excluded as virtually no floating glacier tongues remain in the Arctic.

Existing literature on glacier mass loss predominantly focuses on either regional studies or global models. Regional studies have offered insights into specific areas like Alaska and Svalbard, using geodetic and glaciological methods to quantify mass changes. However, these are often limited in scope and temporal coverage. Global models provide broader assessments of glacier mass balance, but often lack the detail needed for accurate quantification of frontal ablation processes. Model results, such as those indicating a Northern Hemisphere frontal ablation rate of around 39 Gt a⁻¹ from 1980 to 1999 and a projected 50.6 ± 23.8 Gt a⁻¹ for 2020–2040, rely on various assumptions and parameterizations, leaving room for uncertainty. The lack of comprehensive, observation-based data on frontal ablation, particularly considering the dynamic interplay of ice discharge and terminus retreat, has hindered the development of accurate and reliable global estimates. This study aims to bridge this gap by providing a comprehensive, observation-based assessment of frontal ablation for all Northern Hemisphere marine-terminating glaciers.

This study utilized a combination of remote sensing data, in situ measurements and modeling techniques to estimate frontal ablation of Northern Hemisphere marine-terminating glaciers. First, a comprehensive inventory of 1496 marine-terminating glaciers was compiled using the Randolph Glacier Inventory (RGI) version 6.0 and manual mapping of terminus positions in summer 2000, 2010, and 2020 from satellite imagery (Landsat 5, 7, 8, ASTER, Radarsat-1, ALOS PALSAR). Flux gates were manually drawn near the glacier terminus, preferentially in areas with available ice thickness observations. The total length of flux gates was 3802 km after correction for perpendicularity to flow. These flux gates were subdivided into 25m segments for data extraction. Ice surface velocities were primarily obtained from the ITS_LIVE dataset, with data gaps filled using MEASURES InSAR, Sentinel-1, Landsat, or Sentinel-2 data. Ice thickness was primarily determined from the Glacier Thickness Database (GlaThiDa) 3.0.3 and supplemented with data from Operation IceBridge, other radio-echo sounding observations and modeling results from Millan et al. (2022). A bias correction was applied to the modeled thickness data. Frontal ablation (Af) was estimated using the equation Af = Dice + Mterm, where Dice is the ice discharge rate and Mterm is the mass change due to terminus position change. Ice discharge was calculated using the equation Dice = ρ Σn=1N (VnHn dn) − (Sf − Bclim tm), where ρ is the ice density, N is the number of flux gate segments, Vn is the vertically averaged ice velocity, Hn is the ice thickness, dn is the flux gate segment width, Sf is the area below the flux gate, Bclim is the mean specific climatic mass balance rate and tm is the number of years. Mass change due to terminus change was calculated using Mterm = (ρ ΔSterm Hterm + ΔSterm/2 ⋅ Bclim tm) / Δt, where Hterm is the mean thickness of the terminus area, ΔSterm is the area change, and Δt is the time interval. Climatic mass balance was modeled using the Python Glacier Evolution Model (PyGEM), calibrated with geodetic mass balance estimates and monthly ERA5 air temperature and precipitation data. The uncertainty in frontal ablation estimates was determined considering uncertainties in each component of the calculations (velocity, thickness, terminus positions). The sea-level equivalent of frontal ablation was calculated considering the mass changes below sea level that do not contribute to sea-level change. Finally, an intensity index was developed to spatially aggregate frontal ablation from glaciers within 50km of any ocean point on a 10km grid.

The study found that the frontal ablation rate from all 1496 Northern Hemisphere marine-terminating glaciers was 44.47 ± 6.23 Gt a⁻¹ between 2000 and 2010 and 51.98 ± 4.62 Gt a⁻¹ between 2010 and 2020. Ice discharge accounted for 78-80% of the frontal ablation, with the remainder attributed to terminus retreat. The Russian Arctic experienced the highest frontal ablation rate, followed by Svalbard and Alaska. A small number of large glaciers (15 out of 1496 or 1%) accounted for 45% of the total Northern Hemisphere frontal ablation; 2% of the glaciers (30 glaciers) accounted for 55% of the frontal ablation. Most glaciers (87%) contributed less than 0.04 Gt a⁻¹ to the ocean. The intensity index analysis identified hotspots of frontal ablation, particularly around northeastern Svalbard, the strait between October Revolution and Komsomolets islands in Severnaya Zemlya, and near Hubbard and Columbia glaciers in Alaska. Terminus retreat was the most important contributor to frontal ablation in Arctic Canada South, Arctic Canada North, and Greenland Periphery, while ice discharge dominated in Alaska. Analysis of glaciers with advancing termini revealed several surge-type glaciers, which contribute to the frontal ablation. Comparison with regional mass balance estimates suggests that the Northern Hemisphere glacier net mass loss from 2000–2020 is currently underestimated by 2–4%. The total ice discharge from 2000 to 2020 was equivalent to 2.10 ± 0.22 mm of sea-level rise, with an additional 0.11 ± 0.11 mm from terminus mass loss above sea level.

The findings of this study provide the first comprehensive, observation-based estimate of frontal ablation for Northern Hemisphere marine-terminating glaciers. The results highlight the significant contribution of these glaciers to global glacier mass balance and sea-level rise, emphasizing the importance of including frontal ablation in assessments of glacier dynamics and climate change impacts. The uneven distribution of frontal ablation across regions, and within regions, underscores the need for targeted research efforts focused on the most significant contributors to mass loss. The identification of hotspots of frontal ablation informs risk assessments for coastal communities facing iceberg hazards and emphasizes the potential impacts on marine ecosystems. The quantification of the relative importance of ice discharge versus terminus retreat helps refine our understanding of mass loss mechanisms and guides future research priorities to reduce observational uncertainties. The comparison with regional mass balance estimates reveals discrepancies, highlighting the need for further investigation into the complexities of glacier mass balance.

This study presents the first comprehensive observation-based estimate of frontal ablation from Northern Hemisphere marine-terminating glaciers, revealing their substantial contribution to global glacier mass loss and sea level rise. The findings highlight the uneven spatial distribution of this mass loss, with a small number of glaciers accounting for a disproportionate share. The analysis of discharge versus terminus retreat helps elucidate dominant mass loss mechanisms. The identified hotspots of frontal ablation and the comparison with regional mass balance estimates point to future research directions for improving understanding of glacier dynamics and their impacts on coastal communities and marine ecosystems. Future work could focus on improving data quality and coverage, particularly in data-scarce regions, refining models to incorporate the complex interactions between ice dynamics and climate forcing, and developing advanced methodologies for monitoring and forecasting frontal ablation.

The study acknowledges several limitations. The accuracy of frontal ablation estimates depends on the quality and availability of input data (ice thickness, velocity, terminus positions, climatic mass balance). Data gaps were filled using various methods, which introduce uncertainties. The assumed U-shaped valley profile for ice thickness estimation may not accurately represent all glaciers. The exclusion of submarine melting below floating glacier tongues, though likely negligible, introduces a potential source of error. The assumption of independence between uncertainty components in the error analysis may be an oversimplification. The use of decadal mean values ignores potential seasonal variability in ice motion, which may influence frontal ablation estimates. Despite these limitations, the study provides the most comprehensive assessment of frontal ablation to date.