Relationships between climate and geophysical processes: what climate histories can be inferred from glaciers, lakes, and ice streams?

Thesis (Ph.D.)--University of Washington, 2014This dissertation aims to characterize the present and future variability of the Earth's climate by putting it in the context of past variations in climate. Herein I explore how the spatial and temporal fluctuations of climate variables such as temperature, precipitation, evaporation, and sea level are filtered and integrated by the geophysical systems that they influence. I use relatively simple models to explore the scale over which a paleoclimate proxy record is relevant, the physics and parameters to which the system is most sensitive, and how one can distinguish a climate signal from noise. The three geophysical systems explored in this work are detailed below: 1. Glaciers: Glaciers integrate interannual variations in precipitation and temperature and respond with kilometer-scale, multi-decadal terminus fluctuations [Oerlemans, 2000, Reichert et al., 2002, Roe and O'Neal, 2009]. My work extends these studies, and uses reanalysis data and correlation analysis to establish how patterns in precipitation, temperature, and glacier geometry give rise to patterns in glacier advance and retreat. Using a linearized glacier model, I also derive analytic expressions to calculate the expected coherence of regional glacier advance and retreat, and to assess the sensitivity of these glaciers to temperature and precipitation changes. By focusing on how climatic and geometric heterogeneity affect patterns of regional glacier length variations, I isolate the parameters that exert the most influence on the timing and magnitude of glacier response to temporal variations in the climate. 2. Lakes: Like mountain glaciers, lakes integrate year-to-year climate fluctuations to produce large, persistent surface fluctuations on timescales of decades or longer. Using the Great Salt Lake as a case study, I model lake-level variability in response to perturbations in evaporation and precipitation. Though there already exists a body of work that has characterized persistence in observed lake- level variations [Mason et al., 1994, Lall and Mann, 1995, Abarbanel and Lall, 1996, Mohammed and Tarboton, 2011], my research shows that this persistence not only reflects any autocorrelation in the climate, but is also intrinsic to the dynamics of the lake system. My work also shows how the geometry of the lake influences the magnitude and persistence of lake level fluctuations. These results develop a null hypothesis in expected lake-level variability which can be compared to the magnitude and frequency of paleo lake-level variations. 3. Ice streams: Previous studies have used flowline models to understand the behavior of ice streams on idealized bed geometries [Schoof, 2007, Docquier et al., 2011]. This work applies the flowline model approach to a realistic basal topography beneath the West Antarctic Ice Sheet (WAIS), and evaluates changes in grounding line positions and upstream ice profiles in response to changes in model physics and environmental factors. These sensitivity studies demonstrate that the present positions of many Weddell Sea-sector grounding lines lie within an asymmetric trench, implying a strong stability to retreat, but also creating the potential for significant advance due to either sea-level lowering on the order of tens of meters, or conceivably, from precipitation increases of less than 10%. My evaluation reaffirms that the greatest concerns for WAIS retreat or collapse are locations of reverse slopes, muted basal topography, and limited lateral support. This dissertation uses models of low complexity, allowing for a complete understanding of the system, and providing a deeper and richer understanding of the temporal and spatial patterns of Earth's limitless complexity

2013), Radar-detected englacial stratigraphy in the Pensacola Mountains, Antarctica: Implications for recent changes in ice flow and accumulation

ABSTRACT. We used measurements of radar-detected stratigraphy, surface ice-flow velocities and accumulation rates to investigate relationships between local valley-glacier and regional ice-sheet dynamics in and around the Schmidt Hills, Pensacola Mountains, Antarctica. Ground-penetrating radar profiles were collected perpendicular to the long axis of the Schmidt Hills and the margin of Foundation Ice Stream (FIS). Within the valley confines, the glacier consists of blue ice, and profiles show internal stratigraphy dipping steeply toward the nunataks and truncated at the present-day ablation surface. Below the valley confines, the blue ice is overlain by firn. Data show that upward-progressing overlap of actively accumulating firn onto valley-glacier ice is slightly less than ice flow out of the valleys over the past $1200 years. The apparent slightly negative mass balance (-0.25 cm a -1 ) suggests that ice-margin elevations in the Schmidt Hills may have lowered over this time period, even without a change in the surface elevation of FIS. Results suggest that (1) mass-balance gradients between local valley glaciers and regional ice sheets should be considered when using local information to estimate regional ice surface elevation changes; and (2) interpretation of shallow ice structures imaged with radar can provide information about local ice elevation changes and stability