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

David Pollard,Patrick Applegate,Won Chang,Murali Haran,Robert Deconto

doi:10.5194/gmd-9-1697-2016

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.

Highlights

Modeling studies of future variability of the Antarctic Ice Sheet have focused to date on the Amundsen Sea Embayment (ASE) sector of West Antarctica, including the Pine Island and Thwaites Glacier basins
There is a danger of much more drastic grounding-line retreat and sea-level rise in the future, because bed elevations in the Pine Island and Thwaites Glacier basin interiors deepen to depths of a kilometer or more below sea level, potentially allowing marine ice-sheet instability (MISI) due to the strong dependence of ice flux on grounding-line depth (Weertman, 1974; Mercer, 1978; Schoof, 2007; Vaughan, 2008; Rignot et al, 2014; Joughin et al, 2014)
The simple averaging method, with quantities weighted by aggregate scores, produces results that are reasonably compatible with relatively sophisticated statistical techniques involving emulation, probability model/likelihood functions, and Markov chain Monte Carlo (MCMC) (Chang et al, 2015, 2016; Sect. 2.5)

Summary

Introduction

Modeling studies of future variability of the Antarctic Ice Sheet have focused to date on the Amundsen Sea Embayment (ASE) sector of West Antarctica, including the Pine Island and Thwaites Glacier basins These basins are currently undergoing rapid thinning and acceleration, producing the largest Antarctic contribution to sea-level rise (Shepherd et al, 2012; Rignot et al, 2014). Recent studies have mostly used high-resolution models and/or relatively detailed treatments of ice dynamics (higherorder or full Stokes dynamical equations; Morlighem et al, 2010; Gladstone et al, 2012; Cornford et al, 2013; Parizek et al, 2013; Docquier et al, 2014; Favier et al, 2014; Joughin et al, 2014). Because of this dynamical and topographic detail, models with two horizontal dimensions have been confined spatially to limited regions of the ASE and temporally

Objectives

Methods

Findings

Conclusion