| Summary: | Sea level is observed to be rising at an increasing rate. A significant contribution during the past decades has been from mountain glaciers, but the contribution from the Greenland Ice Sheet is anticipated to become dominant in the near future. This contribution is delivered to the ocean as both meltwater and icebergs that melt in the fjords and coastal ocean around Greenland. These contributions vary spatially. The proposed work will develop a model of Greenland's contribution to sea level rise, constrain the model using observed data, and estimate contributions based on scenarios of future climate. The project will contribute to STEM (science, technology, engineering, and mathematics) workforce development by providing support for the training of two graduate students. It will also provide support for a beginning investigator during the formative years of his career. It will contribute to the community resources by maintaining and enhancing the open source Parallel Ice Sheet Model (PISM) code for community use. Four possible controls on outlet glacier systems dynamics have been identified: 1) Warming subsurface ocean water and/or increased subglacial runoff may increase submarine ice melting at the glacier-fjord interface; 2) Rigid sea ice and ice mélange (a mixture of sea ice and icebergs) may suppress calving, allowing for terminus advance; 3) The terminus position relative to subglacial topography (e.g., over-deepenings or sills) influences rates of retreat; and 4) Changes in the resistive stress caused by contact with the fjord walls and/or glacier bed can lead to terminus advance, retreat, and/or thinning. Previous simulations of ice sheet contributions to sea level rise have been limited by the insufficient spatial resolution of models and observational data, which prevented whole-ice sheet simulations to faithfully capture outlet glacier flow. Results to date are either obtained from regional models or from highly idealized flow line models that were upscaled to ice-sheet scale. Recent advances in ice sheet modeling, and the availability of high-resolution subglacial topography, now allow one to resolve individual outlet glacier flow in ice sheet-wide simulations. This project will use the framework of the open-source Parallel Ice Sheet Model (PISM), uni-directionally coupled to new high-resolution hindcasts of the atmosphere and ocean. This will provide a test bed for assessing, on a glacier-by-glacier basis: 1) what is the relative present-day influence of the four controls on outlet glacier flow and ice discharge; 2) what is the potential for a substantial increase in 21st century ice discharge; 3) what conditions would precipitate large changes (e.g., spatio-temporal distribution of ocean warming, enhanced surface runoff); and 4) what observations are required in support of a Greenland Ice Sheet Ocean Observing System to capture the forcing or onset of large changes? Comparison to available remotely-sensed and in-situ observations, including, but not limited to, time-series of surface velocities, surface elevation, and mass changes will serve as metrics of success. Simulations of the 21st century evolution of the Greenland Ice Sheet will then be performed, forced by available atmosphere-ocean projections, to provide realistic estimates of future ice discharge.
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