The Open Global Glacier Model (OGGM) v1.1

Abstract. Despite their importance for sea-level rise, seasonal water availability, and as a source of geohazards, mountain glaciersare one of the few remaining subsystems of the global climate system for which no globally applicable, open source, community-driven model exists. Here we present the Open Global GlacierModel (OGGM), developed to provide a modular and open-source numerical model framework for simulating past and future change of any glacierin the world. The modeling chain comprises data downloading tools ( glacieroutlines, topography, climate, validation data), a preprocessing module, a mass-balance model, a distributed ice thickness estimation model, and an ice-flow model. The monthly mass balance is obtained from gridded climate data and a temperature index melt model. To our knowledge, OGGM is the first global model to explicitly simulate glacierdynamics: the model relies on the shallow-ice approximation to compute the depth-integrated flux of ice along multiple connected flow lines. In this paper, we describe and illustrate each processing step by applying the model to a selection of glaciersbefore running global simulations under idealized climate forcings. Even without an in-depth calibration, the model shows very realistic behavior. We are able to reproduce earlier estimates of global glaciervolume by varying the ice dynamical parameters within a range of plausible values. At the same time, the increased complexity of OGGM compared to other prevalent global glaciermodels comes at a reasonable computational cost: several dozen glacierscan be simulated on a personal computer, whereas global simulations realized in a supercomputing environment take up to a few hours per century. Thanks to the modular framework, modules of various complexity can be added to the code base, which allows for new kinds of model intercomparison studies in a controlled environment. Future developments will add new physical processes to the model as well as automated calibration tools. Extensions or alternative parameterizations can be easily added by the community thanks to comprehensive documentation. OGGM spans a wide range of applications, from ice–climate interaction studies at millennialtimescales to estimates of the contribution of glaciersto past and future sea-levelchange. It has the potential to become a self- sustained community-driven model for global and regional glacierevolution.