Abstract

Repeat airborne or satellite measurements of surface elevation over ice caps are often used to calculate glacier-wide surface mass changes over time. However, these measurements typically do not account for vertical ice motion caused by firn densification and/or ice flow, so the effect of these factors for mass change measurements over an entire ice cap are currently poorly constrained. In this study, we update NASA Airborne Topographic Mapper (ATM) altimetry elevation changes across Penny Ice Cap (Baffin Island, Canada) to assess total changes in ice mass from 2005-2014, relative to 1995-2000. Dual-frequency GPS measurements and temporal changes in ice core density profiles are used to calculate firn densification and ice flow to isolate the component of elevation change due to surface mass change. Envisat satellite imagery is used to delineate the areas impacted by firn densification. These calculations, the first for a Canadian Arctic ice cap, indicate that accounting for firn densification may reduce the inferred surface mass loss by similar to 13-15%. Overall, there has been a fourfold increase in mass loss from Penny Ice Cap between 1995-2000 (-1.3 +/- 0.7 Gt a(-1)) and 2005-2013 (-5.4 +/- 1.9 Gt a(-1)). The rapid upglacier migration of the equilibrium line has left large areas of subsurface firn in the current ablation area and has far outpaced the ice flow response, illustrating that the ice cap is not in equilibrium and out of balance with the current climate.