The glacial systems model (GSM) Version 25G

Abstract. We document the glacial system model (GSM), which is designed for large ensemble ice sheet modelling in glacial cycle contexts. A distinguishing feature is the extent to which it addresses relevant forcing and process uncertainties. The GSM has evolved from three decades of effort to constrain the last glacial cycle evolution of each ice sheet that was present (North American, Greenlandic, Icelandic, Eurasian, Patagonian, and Antarctic, and soon Tibetan). The core ice dynamics uses a hybrid shallow-shelf and shallow-ice approximation with full thermo-mechanical coupling. It also includes one of the largest range of relevant processes for the above context of any model to date, ranging from visco-elastic glacial isostatic adjustment with 0-order geoidal deflection to state-of-the-art subglacial sediment production, transport, and deposition. Furthermore, the GSM is to date the only model to have all of the above processes bidirectionally coupled with each other. Other relevant distinguishing features include: permafrost resolving bed-thermodynamics, a fast diagnostic solution of down-slope surface drainage and lake filling, subgrid hypsometric surface mass balance and ice flow, simple thermodynamic lake and sea ice representations, subglacial hydrology with dynamically evolving partitioning between distributed and channelized flow, and surface melt that physically accounts for insolation changes via a novel insolation above freezing scheme. To address the most challenging part of paleo ice sheet modelling, the GSM includes both a 2D energy balance climate model and variants of traditional input time series weighted interpolation (aka “glacial indexing”) of fields from General Circulation Model (GCM) simulations, all under ensemble parametric specification. It also includes options for one and two way scripted coupling with climate models. We demonstrate the significant errors that can ensue in the glacial cycle simulation of a single ice sheet when three aspects of glacial isostatic adjustment are ignored (as is typical). These are geoidal deformation, global ice load input, and correction of initial topography for present-day isostatic disequilibrium. We also draw attention to the relatively high sensitivity of the GSM (and presumably other ice sheet models) to the specification of the temperature dependence for basal sliding activation. The associated code archive includes configuration options for all major last glacial cycle ice sheets as well as idealized geometries and validation test setups.