The Role of Ice Streams in Deglaciation
Abstract:Ice streams are regions of fast flow within ice sheets that can exhibit variability on time scales ranging from years to millennia. Observations and model reconstructions indicate that ice streams likely played a major role in the most recent deglaciation of the Laurentide Ice Sheet. The variability of these ice streams may have dictated the spatiotemporal progression of deglaciation, as some ice sheet domes may have been more sensitive to dynamic changes in ice stream activity.
We analyze the behavior of both idealized and realistic ice sheet configurations to determine the role of ice streams in setting ice sheet steady states and the spatiotemporal sequence of deglacial transitions. We also examine the way in which climate feedbacks could have amplified the response of ice streams to Milankovitch forcing, thereby initiating ice sheet collapse. We use the Parallel Ice Sheet Model (PISM), by itself and coupled to an energy balance atmospheric model. The presence of ice streams dramatically alters steady-state ice sheet configurations, allowing for the existence of thin, widely extended margins. There is a marked transition from binge-purge-like ice stream variability (that is believed to have occurred as part of Heinrich events during the last glacial period) to steady ice stream flow as climate (temperature, precipitation) is varied over a range corresponding to glacial variability. This qualitative transition in ice sheet dynamics amplifies small changes in external forcing (e.g. Milankovitch cycles) sufficiently to activate strong climate feedbacks. We discuss how the reorganization of ice sheet configuration, through changes in ice sheet extent and elevation, may favor certain climate feedbacks (e.g. ice-albedo and lapse rate feedbacks). We also relate these analyses to observations of Laurentide deglaciation and discuss implications for the future of the Greenland and Antarctic Ice Sheets.