Details

Autor(en) / Beteiligte

• King, Michalea D

Titel

Seasonal to Multidecadal Drivers of Variability at Greenland Outlet Glaciers

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2020

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• The Greenland Ice Sheet (GrIS) is losing mass at accelerated rates in the 21st century, due in part to faster flow at large outlet glaciers. Chapter 2 presents work published in The Cryosphere (King et al., 2018). Here, we sample rapid changes in thickness and velocity at all large outlet glaciers to derive the first continuous, GrIS-wide record of total ice sheet discharge, or the volume of ice glaciers export, for the 2000-2016 period. We resolve a distinct pattern of seasonal variability with an amplitude of 6%, and analyze how seasonal to annual variability in the discharge time series relates to both meltwater runoff and glacier front position changes over the same period. We find that the annual magnitude of discharge is closely related to cumulative front position change (r2 = 0.79), averaging over 2 km of retreat since 2000. We find that larger seasonal quantities of runoff do not relate to increased annual discharge, although seasonal acceleration of ice discharge does closely coincide with the onset of the melt season. These results suggest that changes in glacier front position drive secular trends in discharge, whereas the impact of runoff is likely limited to the summer months when observed seasonal variations are substantially controlled by the timing of meltwater input.In Chapter 3, we extend our 2000-2016 discharge time series to the period 1985-2018, combining more than three decades of GrIS-wide observational products of outlet glacier velocity, elevation, and front position changes, and compare decadal variability in discharge with calving front position. We find that the close relationship between frontal change and ice discharge identified over the 2000-2016 record holds true for the 34-year record, and that increased glacier discharge can be attributed almost entirely to the retreat of glacier fronts, rather than inland ice sheet processes, such as changes in meltwater runoff. Discharge sensitivity to retreat is remarkably consistent across the ice sheet, with all main regions showing an increase in discharge of 4-5% per km of retreat. Most importantly, we show that widespread retreat between 2000 and 2005 resulted in a step-increase in discharge and a switch to a new dynamic state of sustained mass loss. The increase in discharge was large enough to alone ensure persistent negative ice sheet mass balance, even before including more recent negative trends in surface mass balance. This study is published in Nature Communications Earth & Environment (King et al., 2020).Lastly, we revisit seasonal impacts of meltwater on short temporal variability in glacier dynamics. Chapter 4 work describes how, in lieu of widespread direct observations of subglacial systems, we derive a novel runoff index that quantifies the potential of a given melt season to sustain fast outlet glacier flow. This index, "Basal Pumping Index'', is based on the distribution and relative magnitude of individual runoff events throughout the summer, and reflects the current understanding of how subglacial drainage systems evolve over time to modulate water pressures at the ice bed. We also categorize glaciers based on several unique patterns of variability, and discuss how particular glacier properties impact seasonality and sensitivity to our index. We compare net GrIS discharge between low and highly effective runoff years, and find that particular runoff distributions can increase total GrIS dynamic ice losses by 3% in the late summer/autumnal months. In total, our results provide new insights into the large scale, ice sheet-wide drivers of variability in Greenland's ice discharge and mass balance on seasonal to decadal timescales. These insights will guide projections of future glacier response to ongoing climate change, improving our estimates of Greenland's contribution to global sea level rise and impacts on ocean salinity and circulation.