Mountain glaciers and small ice caps collectively constitute a significant fraction of global land ice (Alaska and adjacent Canadian glaciers comprise 13% of the world's glacier area, exclusive of Antarctica and Greenland). Despite poorly resolved mass balances, recent evidence shows that Alaskan glaciers, by virtue of their large collective area, rapid response characteristics, and high mass fluxes, contribute to modern sea level rise at a rate approximately seven times greater than previously thought. Much of the mass loss from Alaskan glaciers comes from tidewater glaciers, which advance and retreat episodically, with long periods of advance and stability punctuated by rapid, apparently irreversible retreat. During retreat, tidewater glacier mass balance is dominated by the poorly understood process of iceberg calving, rather than direct climate forcing, and the magnitude of ice flux into the ocean can be very large. Columbia Glacier is presently discharging icebergs into the ocean at a rate of approximately 22 km3 yr-1. Thus, tidewater calving is an important, but poorly understood element of modern glacier change and sea level rise. The Principal Investigators present work plan at Columbia Glacier with two points of focus, each dedicated to an increased understanding the role of calving in tidewater glacier retreat. First, they will continue their program of aerial photography as the glacier terminus retreats through a major channel constriction. Second, they will conduct a passive seismological investigation of calving processes. This work is motivated by the importance of small glaciers (especially Alaskan tidewater glaciers) to modern global glacier mass balance and sea level change and by the need to better understand the role of calving in tidewater glacier dynamics. Additionally, Columbia Glacier is the last of the Alaskan tidewater glaciers to retreat, and is partly through its retreat phase and at a critical location in its channel geometry. This opportunity to study tidewater calving will be gone in a few decades, so it is important to take advantage of it now.
Tidewater calving is an important part of glacier dynamics and ice/ocean interaction, but is still poorly understood. Progress toward a better understanding of calving is handicapped by a lack of relevant observations and lack of knowledge of what the relevant observations should be. This research will provide a continuation of a highly successful observational program of aerial photography that has yielded the longest and most detailed observational record of tidewater glacier kinematics ever assembled, and has formed the basis of much of what is presently known about tidewater dynamics. The Principal Investigators will start a new series of seismic observations on Columbia Glacier in an attempt to discover in detail the timing, location, and geometry of fracture events leading to calving. Limited seismic investigations have been previously made on tidewater glaciers, including on Columbia Glacier, but modern advances in data acquisition and processing will allow them to use seismology much more effectively and to look at fracture and calving in much greater detail.
Broader impacts resulting from the proposed activity: In addition to its role in modern sea level change, tidewater calving played a critical role in global ice sheet and climate dynamic in the past. Ample evidence from the North Atlantic Ocean suggests that passive episodic calving was essential in the rapid breakup of the Laurentide Ice Sheet (LIS), but current LIS reconstructions do not attempt to incorporate calving processes due to the lack of a mathematical calving relationship. However, ice sheet modelers are eager for input concerning calving processes to incorporate in LIS models. Additionally, an understanding of tidewater calving provides useful crossover knowledge for Antarctic and Greenland marine-ending ice masses where calving occurs much less frequently.
