Alaska tidewater glacier velocities and frontal ablation, 1985-2012

Despite their potential importance to sea level rise, little is known about the proportion of mass loss due to frontal ablation (the sum of ice loss through calving and submarine melt) from tidewater glaciers outside of the Greenland and Antarctic ice sheets. Frontal ablation contributes about half of the mass loss from the ice sheets, and lack of both understanding of and data on these important processes has been cited as a major hindrance to accurate predictions of global sea level rise. First, we present a 64 year record of length change for 50 Alaska tidewater glaciers, derived from manually digitizing aerial mapping photographs and Landsat scenery. Then, we present a 27 year record of surface velocity and frontal ablation for 20 Alaska tidewater glaciers (representing 80% of the total tidewater glacier area in the region), derived using a feature tracking algorithm, all available cloud-free Landsat 5 and 7 scenes, and estimates of glacier ice thickness derived from an inversion of surface topography. In general, the 50 glaciers studied retreated over the period 1948-2012, though these changes are not constant - several glaciers both retreated and advanced over the time period. We find the total mean rate of frontal ablation for these 20 glaciers over the period 1985-2012 is 16.2×6.5$ Gt a-1; scaling this result by area to the remaining 30 tidewater glaciers in Alaska, we estimate a mean rate of frontal ablation of 18.3×7.3 Gt a-1 over the period 1985-2012. Two glaciers in particular, Hubbard and Columbia, account for over 50% of the frontal ablation signal of the set of 20 glaciers. Seasonal changes in surface velocity match well with seasonal changes in length, indicating that rates of frontal ablation do not remain constant throughout the year. Despite coming from 15% of the glacierized area in the region, frontal ablation is a significant contributor to the regional mass budget. We estimate a specific mass loss through frontal ablation for all Alaska glaciers of 0.21 m w.e. a-1, equivalent to estimates from Svalbard, and over three times the rate for Greenland.