Modeling of past and future variations of the Antarctic Ice Sheet with Large Ensembles.

Abstract:

Recent observations of thinning and retreat of the Pine Island and Thwaites
Glaciers identify the Amundsen Sea Embayment (ASE) sector of West Antarctica as
particularly vulnerable to future climate change. To date, most future modeling
of these glaciers has been calibrated using recent and modern observations.
As an alternate approach, we apply a hybrid 3-D ice sheet-shelf model to the
last deglacial retreat of Antarctica, making use of geologic data from
~20,000 years BP to present, focusing on the ASE but including other sectors
of Antarctica.

Following several recent ice-sheet studies, we use Large Ensemble statistical
methods, performing sets of ~600 runs over the last 30,000 years with
systematically varying model parameters. Objective scores for each run
are calculated using modern data and past reconstructed grounding lines,
relative sea level records, cosmogenic elevation-age data and uplift rates.
Two types of statistical methods are used to analyze the Large-Ensemble
results: simple averaging weighted by the aggregate score, and more advanced
Bayesian emulation and calibration methods that rigorously account for
some of the uncertainties in the model and observations.

Results for best-fit parameter ranges and envelopes of equivalent sea-level
rise with the simple averaging method agree quite well with the more advanced
techniques, but only for a Large Ensemble with dense (Full Factorial)
parameter sampling. Runs are extended into the future using RCP scenarios,
with drastic retreat mechanisms of hydrofracturing and structural ice-cliff
failure. In most runs this produces grounding-line retreat into the
West Antarctic interior, and into East Antarctic basins for RCP8.5, and
the Large Ensemble analysis provides sea-level-rise envelopes with
well defined parametric uncertainty bounds.