Thesis (Ph.D.)--University of Washington, 2022This dissertation describes three research projects investigating changes in polar climate and the ice sheets during the last deglaciation. The first project, Chapter 2, reconstructs the past 20,000 years of Greenland temperature and precipitation to learn about their relationship and influences on the ice sheet. The reconstruction method, paleoclimate data assimilation, uses oxygen-isotope ratios of ice and accumulation rates from long ice-core records and extends this information to all locations across Greenland using spatial relationships derived from a transient climate-model simulation. Evaluations against out-of-sample proxy records indicate that the reconstructions capture the climate history at locations without ice-core records. The reconstructions show that the relationship between precipitation and temperature is frequency dependent and spatially variable, suggesting that thermodynamic scaling methods commonly used in ice-sheet modeling are overly simplistic. Overall, the results demonstrate that paleoclimate data assimilation is a useful tool for reconstructing the spatial and temporal patterns of past climate on timescales relevant to ice sheets. To learn how these climate reconstructions relate to the behavior of the ice sheet, we must also reconstruct the history of the ice sheet. Most observational data of the past ice sheet geometry, however, is at the margins of the ice sheet, while the ice core climate records are located in the interior. The second project, Chapter 3, investigates a common paleoaltimetry method that reconstructs elevation from temperature records. This method suggests the climate and elevation signals contained within an ice-core temperature record can be disentangled by comparing two proxy locations with similar climates. The difference between the records is assumed to be due to elevation, which is estimated by scaling the temperature difference by a lapse rate. I investigate the errors associated with this approach using the Antarctic ice sheet during the Last Glacial Maximum as a case study. Using an ensemble of climate simulations from global circulation models (GCMs), I extract modeled temperatures at locations of real ice cores. The errors are on the order of hundreds of meters and result from spatial heterogeneity in non-adiabatic temperature change, which itself stems in part from elevation-induced atmospheric circulation change. These findings suggest that caution is needed when interpreting temperature-based paleoaltimetry results for ice sheets. The third project, Chapter 4, seeks to learn about the elevation and climate signals contained within the WAIS Divide ice core temperature record by investigating whether they are consistent with accumulation rate reconstructions and annual layer thickness data at the ice core site. The difference in temperature change between West and East Antarctic ice core sites during the last deglacial period is about 6 °C. If this were due to differential elevation change at the sites, then the WAIS Divide ice core site would have been about 400 m higher during the Last Glacial Maximum. Using an ice-flow model, I determine that this elevation change is not consistent with published accumulation rate reconstructions and the annual layer thickness data from the WAIS Divide ice core site. Three factors may explain this inconsistency: the spatial heterogeneity in non-adiabatic temperature changes during the deglaciation, assumptions in the accumulation rate reconstructions, and assumptions in the ice-flow model. Future investigations into these factors may lead to a more consistent understanding of Antarctic climate and interior ice sheet changes during the last deglaciation
