On the long-term memory of the Greenland Ice Sheet

In this study, the memory of the Greenland Ice Sheet (GIS) with respect to its past states is analyzed. According to ice core reconstructions, the present‐day GIS reflects former climatic conditions dating back to at least 250 thousand years before the present (kyr BP). This fact must be considered when initializing an ice sheet model. The common initialization techniques are paleoclimatic simulations driven by atmospheric forcing inferred from ice core records and steady state simulations driven by the present‐day or past climatic conditions. When paleoclimatic simulations are used, the information about the past climatic conditions is partly reflected in the resulting present‐day state of the GIS. However, there are several important questions that need to be clarified. First, for how long does the model remember its initial state? Second, it is generally acknowledged that, prior to 100 kyr BP, the longest Greenland ice core record (GRIP) is distorted by ice‐flow irregularities. The question arises as to what extent do the uncertainties inherent in the GRIP‐based forcing influence the resulting GIS? Finally, how is the modeled thermodynamic state affected by the choice of initialization technique (paleo or steady state)? To answer these questions, a series of paleoclimatic and steady state simulations is carried out. We conclude that (1) the choice of an ice‐covered initial configuration shortens the initialization simulation time to 100 kyr, (2) the uncertainties in the GRIP‐based forcing affect present‐day modeled ice‐surface topographies and temperatures only slightly, and (3) the GIS forced by present‐day climatic conditions is overall warmer than that resulting from a paleoclimatic simulation.