The Community Earth System Model, developed and maintained by NCAR, is the first completely open-source Earth System Model. In this document, I will refer to CESM 1.0.5, which corresponds to the last official release of CESM 1.0 (the more recent version is now CESM 1.2). CESM 1.0.5 includes atmosphere, land, ocean, sea-ice and a partially coupled ice-sheet model. Several grid resolutions have been developed for each component of CEMS and for present-day Earth’s topography/bathymetry. One of the major advantage of this model is that the procedure to run it is straightforward in its present-day configuration. However, the CESM is not flexible when changes in the topography and bathymetry have to be introduced in order to simulate near and deep past climates. To implement a different land-sea mask, as well as different surface conditions, such as vegetation cover, ice sheets etc…, each component of CESM requires substantial changes in its initial conditions files, which require an advanced knowledge of the model structure. In order to make those kind of changes accessible to new users, the following document aims at detailing a relatively simple procedure to modify the initial conditions files for the coupled atmosphere-land-ocean-sea-ice configuration of the CESM 1.0.5 (B compset). This procedure was developed at NCAR and is based on the script released by the Paleo-working Group. This procedure has been successfully tested on CMCC IBM supercomputing facilities. In this user guide, the procedure is applied to a glaciation case, i.e. when large ice sheets covered the Northern Hemisphere repeatedly in the past and when sea level drop of about 120 meters. This procedure is by far non automatic and requires substantial manual work at each stage described in the document. Finally, following this procedure does not provide any guarantees that the simulations will be successful.
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