On the frontal ablation of Alaska tidewater glaciers

Abstract

Sea level rise is a major problem that society will face in the coming century. One of the largest unknown components of sea level rise is frontal ablation (the sum of mass loss through calving and subaqueous melting) from glaciers and ice sheets. Using estimates of ice thickness, rates of glacier length change, and glacier velocities, we present a record of frontal ablation over the period 1985-2012 for 20 Alaska tidewater glaciers. We also present a new method for estimating ice thickness by solving the continuity equation between adjacent flowlines. Because of the wealth of data available, we apply this method to Columbia Glacier, Alaska. The mean ice thickness and volume of Columbia Glacier were approximately halved over the period 1957-2007, from 281 m to 143 m, and from 294 km�_ to 134 km�_, respectively. Using bedrock slope and considering how waves of thickness change propagate through the glacier, we conclude that the rapid portion of this tidewater glacier's retreat is likely nearing an end. We present a 64 year record of length change for 50 Alaska tidewater glaciers, over the period 1948-2012. Most (31) glaciers retreated over the period. Examination of the onset of glacier retreats indicates a correlation between high summer sea surface temperature and the triggering of retreat. Finally, we present a 27 year record of surface velocity for 20 Alaska tidewater glaciers derived from Landsat imagery. Surface velocities vary by as much as 80% throughout the year, indicating that using measurements from one time of year may bias estimates of frontal ablation. The total mean rate of frontal ablation for these 20 glaciers over the period 1985-2012 is 16.2 � 6.5 Gt a����_. Extending this to the remaining 30 Alaska tidewater glaciers yields an estimate of frontal ablation of 18.3 � 7.3 Gt a����_, approximately 50% of the climatic mass balance of the region. This indicates the important, non-negligible role frontal ablation can play in regional mass balance, even where tidewater glaciers cover a small fraction of the total area.