This project will reconstruct the size and shape of the Greenland Ice Sheet (GrIS) across the Last Interglacial period, a warm climate period 127,000-119,000 years ago. During this time, the GrIS melted substantially contributing several meters of sea level rise, but the exact amount of melting is not known. There is a need to define this more precisely because the GrIS is expected continue to melt in the coming years and this melting will accelerate under continued greenhouse gas emissions. Understanding the sensitivity of the GrIS to melting under warm climates can help constrain how much sea level rise could occur by 2100 and beyond. One outcome of this project will be interactive maps illustrating what the GrIS looked like through time, which can be used for teaching and science communication. This project will use a technique called data assimilation to achieve its goal. This technique is commonly used for weather forecasting but has not been applied yet to reconstruct the coupled evolution of climate and the GrIS. The researchers will combine transient simulations conducted with a new coupled climate-ice model (e.g., the Community Earth System Model v2 with the Community Ice Sheet Model v2) with geological proxy data for sea-surface temperature and conditions on the GrIS (i.e., ice core thickness and isotopes) in the data assimilation framework to produce a hybrid model-data product. The result will be a data-informed reconstruction of GrIS size and shape, which will allow for a more precise quantification of the contribution of the GrIS to global sea level rise. Broader Impacts associated with this award include the creation of an interactive visualization of GrIS evolution through geological time, support for a postdoctoral scholar and undergraduates at the University of Arizona, and partnership with the Southern Arizona Research, Science, and Engineering Foundation to develop climate learning and engagement activities for local K-12 students. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.