Large ensemble modeling of the last deglacial retreat of the West Antarctic Ice Sheet: comparison of simple and advanced statistical techniques

Abstract

Abstract.A 3-D hybrid ice-sheet model is applied to the last deglacial retreat of the West Antarctic Ice Sheet over the last ∼ 20 000 yr. A large ensemble of 625 model runs is used to calibrate the model to modern and geologic data, including reconstructed grounding lines, relative sea-level records, elevation-age data and uplift rates, with an aggregate score computed for each run that measures overall model-data misfit. Two types of statistical methods are used to analyze the large-ensemble results: simple averaging weighted by the aggregate score, and more advanced Bayesian techniques involving Gaussian process-based emulation and calibration, and Markov chain Monte Carlo. The analyses provide sea-level-rise envelopes with well-defined parametric uncertainty bounds, but the simple averaging method only provides robust results with full-factorial parameter sampling in the large ensemble. Results for best-fit parameter ranges and envelopes of equivalent sea-level rise with the simple averaging method agree well with the more advanced techniques. Best-fit parameter ranges confirm earlier values expected from prior model tuning, including large basal sliding coefficients on modern ocean beds.

Variations of the Antarctic Ice Sheet in a Coupled Ice Sheet‐Earth‐Sea Level Model: Sensitivity to Viscoelastic Earth Properties

“…In comparison, the full range of other model parameters such as OCFAC and CSHELF produces a wide range of results from realistic to very unrealistic. Analogous behavior was seen in a similar large ensemble using a simple ELRA bedrock model, with the same model parameter variations as here except with EARTH replaced by the ELRA asthenospheric e ‐folding time (Pollard et al, ). The probability distribution functions and best fit values of OCFAC, CALV, and CSHELF are reasonably similar to the earlier results.…”

“…The data sets used are reconstructed past grounding‐line locations, relative sea level records, cosmogenic elevation‐age data, modern uplift rates, and modern ice distributions. Model misfits are averaged and combined into one score for each simulation using Gaussian error distribution concepts (Briggs & Tarasov, ; Pollard et al, ). In this study, a new parameter representing different viscoelastic Earth profiles is used in the LE to explore the sensitivity of the results to this input.…”

“…The figure is organized to show the scores in the 4-D space of parameter variations. The four model parameters, OCFAC, CALV, CSHELF, and EARTH are described briefly in the text; the first three parameters, and the algorithms used to score each run versus data, are discussed in more detail in Pollard et al (2016). Since each parameter only takes five values, the results are blocky but effectively show the behavior of the score over wide ranges of parameter values.…”

Glacial Isostatic Adjustment modelling: historical perspectives, recent advances, and future directions

“…Using data assimilation or statistical emulation, in combination with large ensemble modelling, it should be possible to determine a suite of GIA solutions that provide a reasonable fit to all available constraints. Such an approach has already 705 been pioneered for ice-sheet modelling Pollard et al, 2016), but it requires careful consideration of the probability distribution associated with each piece of constraining data and the method used to score each model run . When applied to GIA modelling, decisions will have to be made on: how to weight different data types, the length scale over which each is relevant, how to treat uncertainties in age and elevation, whether to use raw data or statistical reconstructions (e.g.…”

Progress in Numerical Modeling of Antarctic Ice-Sheet Dynamics

“…A crevasse penetration depth is calculated based on the pioneering work of Nye [90], and depends on the equilibrium between an opening term being longitudinal stretching, and a closing term being cryostatic pressure. Studies applying this criterion (and derivations of it) to Greenland [26,86,87] and Antarctica [32,100,101] could qualitatively reproduce coherent variations of the calving front. However, the approach is based on an instantaneous stress balance field, thus not considering accumulated weaknesses in the ice advected with the ice flow, nor the stress concentration at their tip on their vertical propagation.…”