Abstract. Despite their importance for sea-level rise, seasonal water availability, and as a source of geohazards, mountain glaciers are 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 Glacier Model (OGGM), developed to provide a modular and open-source numerical model framework for simulating past and future change of any glacier in the world. The modeling chain comprises data downloading tools (glacier outlines, 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 glacier dynamics: 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 glaciers before 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 glacier volume 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 glacier models comes at a reasonable computational cost: several dozen glaciers can 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 millennial timescales to estimates of the contribution of glaciers to past and future sea-level change. It has the potential to become a self-sustained community-driven model for global and regional glacier evolution.

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<p>The glaciers in the Alps are currently shrinking, in some cases dramatically, due to progressive warming. At some glaciers this recession has made it possible to find tree remains and other organic material at or near the termini. At Pasterze Glacier, such findings have been made since about 1990, allowing new insights into the Holocene evolution and variability of this glacier. Initially, only relocated wood and peat boulders were collected, but around 2010 an in-situ locality became ice-free. Tree remains and other organic material from this site have mainly provided dates for a period of more than a thousand years in the middle Holocene (around 6 ka) proving a continuously smaller extent of this glacier during this period compared to today. Furthermore, a comparative interpretation of all available, some 80 radiocarbon and dendro dates suggests that Pasterze Glacier was probably at least from about 10.2 ka to about 3.5 ka continuously shorter compared to the extent around 2010 AD. For the last nearly 2800 years there is no similar evidence of comparable small glacier extents. Finally, after the early- to mid-Holocene retreat phase, a relatively delayed increase of Pasterze Glacier during the early Neoglacial (in the Alps after about 4 ka) can be deduced. Other glaciers almost reached or even exceeded the later LIA dimensions already during this period.</p><p>Moreover, Pasterze Glacier is also lagging behind the current climatic changes, i.e., its extent is not in equilibrium with the current warming. This circumstance is not only proven by the rapid recession during recent years, but also by simulations with the glacier model OGGM. The simulation results show on the one hand that Pasterze glacier has to melt back for several more kilometres to reach equilibrium with the climatic conditions of 1980-2010. On the other hand, this also documents that the recent climate conditions are already sufficient to allow a recession comparable to the early and middle Holocene stages of this glacier. Both the delayed increase in extent during the early Neoglacial and the considerably delayed current recession can be explained by the size of the glacier and the topographic conditions.</p>