Fabien Gillet-Chaulet

Enhanced basal lubrication and the contribution of the Greenland ice sheet to future sea-level rise

We assess the effect of enhanced basal sliding on the flow and mass budget of the Greenland ice sheet, using a newly developed parameterization of the relation between meltwater runoff and ice flow. A wide range of observations suggest that water generated by melt at the surface of the ice sheet reaches its bed by both fracture and drainage through moulins. Once at the bed, this water is likely to affect lubrication, although current observations are insufficient to determine whether changes in subglacial hydraulics will limit the potential for the speedup of flow. An uncertainty analysis based on our best-fit parameterization admits both possibilities: continuously increasing or bounded lubrication. We apply the parameterization to four higher-order ice-sheet models in a series of experiments forced by changes in both lubrication and surface mass budget and determine the additional mass loss brought about by lubrication in comparison with experiments forced only by changes in surface mass balance. We use forcing from a regional climate model, itself forced by output from the European Centre Hamburg Model (ECHAM5) global climate model run under scenario A1B. Although changes in lubrication generate widespread effects on the flow and form of the ice sheet, they do not affect substantial net mass loss; increase in the ice sheet’s contribution to sea-level rise from basal lubrication is projected by all models to be no more than 5% of the contribution from surface mass budget forcing alone.

Basal resistance for three of the largest Greenland outlet glaciers

Author(s): Shapero, DR; Joughin, IR; Poinar, K; Morlighem, M; Gillet-Chaulet, F | Abstract:

Full‐depth englacial vertical ice sheet velocities measured using phase‐sensitive radar

We describe a geophysical technique to measure englacial vertical velocities through to the beds of ice sheets without the need for borehole drilling. Using a ground-based phase-sensitive radio echo sounder (pRES) during seven Antarctic field seasons, we measure the temporal changes in the position of englacial reflectors within ice divides up to 900 m thick on Berkner Island, Roosevelt Island, Fletcher Promontory, and Adelaide Island. Recorded changes in reflector positions yield “full-depth” profiles of vertical ice velocity that we use to examine spatial variations in ice flow near the divides. We interpret these variations by comparing them to the results of a full-Stokes simulation of ice divide flow, qualitatively validating the model and demonstrating that we are directly detecting an ice-dynamical phenomenon called the Raymond Effect. Using pRES, englacial vertical ice velocities can be measured in higher spatial resolution than is possible using instruments installed within the ice. We discuss how these measurements could be used with inverse methods to measure ice rheology and to improve ice core dating by incorporating pRES-measured vertical velocities into age modeling.

Assimilation of surface velocities acquired between 1996 and 2010 to constrain the form of the basal friction law under Pine Island Glacier

In ice-sheet models, slip conditions at the base between the ice and the bed are parameterized by a friction law. The most common relation has two poorly constrained parameters, C and m. The basal slipperiness coefficient, C, depends on local unobserved quantities and is routinely inferred using inverse methods. While model results have shown that transient responses to external forcing are highly sensitive to the stress exponent m, no consensus value has emerged, with values commonly used ranging from 1 to ∞ depending on the slip processes. By assimilation of Pine Island Glacier surface velocities from 1996 to 2010, we show that observed accelerations are best reproduced with m≥5. We conclude that basal motion, in much of the fast flowing region, is governed by plastic deformation of the underlying sediments. This implies that the glacier bed in this area can not deliver resistive stresses higher than today, making the drainage basin potentially more sensitive to dynamical perturbations than predicted with models using standard values m = 1 or 3.

Simulations of changes to Glaciar Zongo, Bolivia (16° S), over the 21st century using a 3-D full-Stokes model and CMIP5 climate projections

Abstract Bolivian glaciers are an essential source of fresh water for the Altiplano, and any changes they may undergo in the near future due to ongoing climate change are of particular concern. Glaciar Zongo, Bolivia, located near the administrative capital La Paz, has been extensively monitored by the GLACIOCLIM observatory in the last two decades. Here we model the glacier dynamics using the 3-D full-Stokes model Elmer/Ice. The model was calibrated and validated over a recent period (1997–2010) using four independent datasets: available observations of surface velocities and surface mass balance were used for calibration, and changes in surface elevation and retreat of the glacier front were used for validation. Over the validation period, model outputs are in good agreement with observations (differences less than a small percentage). The future surface mass balance is assumed to depend on the equilibrium-line altitude (ELA) and temperature changes through the sensitivity of ELA to temperature. The model was then forced for the 21st century using temperature changes projected by nine Coupled Model Intercomparison Project phase 5 (CMIP5) models. Here we give results for three different representative concentration pathways (RCPs). The intermediate scenario RCP6.0 led to 69 ± 7% volume loss by 2100, while the two extreme scenarios, RCP2.6 and RCP8.5, led to 40 ± 7% and 89 ± 4% loss of volume, respectively.

Glaciology: Ice-sheet advance in Antarctica

Reliable forecasts of sea-level rise depend on accurately modelling the dynamics of polar ice sheets. A numerical framework that better reflects ice-sheet basal drag adds greater realism to such models.

Effect of uncertainty in surface mass balance–elevation feedback on projections of the future sea level contribution of the Greenland ice sheet – Part 1: Parameterisation

This manuscript provides a new parameterization of the surface mass balance – elevation feedback for the Greenland ice sheet, derived from the regional climate model MAR. They derive this parameterization using a set of 8 RCM experiments using different topographies, and different climatologies. Next to optimized values of the ‘SMB lapse rates’, they find plausible ranges for these values, using a Bayesian statistical approach, which enables them to constrain results (in a companion paper) with credibility intervals. This is an important subject that potentially bridges the gap between (regional) climate models and ice sheet models, circumventing synchronously coupled

Recent elevation and velocity changes of Astrolabe Glacier, Terre Adelie, Antarctica

SPOT and Pléiades images acquired since 2002 are used to describe velocity and elevation changes on Astrolabe Glacier, East Antarctica. Multi-temporal pairs of images are used to generate velocity fields by automatically tracking surfaces features. Stereo-pairs are used to create DEMs. Using three DEMs (2003, 2007 and 2013), we describe a surprising surface elevation increase since 2002, that reached a mean rate of 1.8 m/yr between 2003 and 2007. Conversely, the velocity fields did not reveal any major change in velocity so the origin of the strong increase in elevation remains non elucidated.