Cryospheric and Polar Processes Seminar: William Colgan

New paradigm for attributing recent Greenland ice loss

by Dr. William Colgan - Lassonde School of Engineering at York University

Abstract: There is a growing interest in the partition of recent Greenland Ice Sheet mass loss between surface balance and ice dynamic components. While assessing the contemporary magnitude and spatial distribution of these transient mass balance components is critical to calculating recent mass loss, it is equally important to understand the magnitude and spatial distribution of these components during the 1961-1990 reference climatology period, against which contemporary components are conventionally differenced to quantify “recent change”. In the past year, however, two longer term mass balance processes have been documented for the first time. On centennial time scales, historical aerial photos and analytical modeling have been used to infer a highly negative post-Little Ice Age mass balance around the ice sheet periphery. On millennial time scales, radiostratigraphy and numerical modeling have been used to infer a subtle positive mass balance due to changes in ice rheology in the ice sheet interior. Together, these processes challenge the assumption of near-equilibrium surface balance and ice dynamic components during reference period that underlies conventional attributions of ice loss. A consequence of acknowledging longer term negative mass balance is that recent mass loss since reference period is substantially less than previously assumed. Partitioning recent ice loss within this alternate paradigm suggests that all recent ice loss can potentially be explained by surface balance alone, without invoking significant changes in ice dynamics since 1990. Examining the spatial distribution of mass loss and mass balance components, however, highlights the perils of solving the ice dynamic component of mass balance as a residual.