Elmer/Ice News

A consensus estimate for the ice thickness distribution of all glaciers on Earth

farinotti2019Elmer/Ice was one of the five models participating in this exercice to improve the estimate of the ice thickness distribution of all glaciers on Earth.
Knowledge of the ice thickness distribution of the world’s glaciers is a fundamental prerequisite for a range of studies. Projections of future glacier change, estimates of the available freshwater resources or assessments of potential sea-level rise all need glacier ice thickness to be accurately constrained. Previous estimates of global glacier volumes are mostly based on scaling relations between glacier area and volume, and only one study provides global-scale information on the ice thickness distribution of individual glaciers. Here we use an ensemble of up to five models to provide a consensus estimate for the ice thickness distribution of all the about 215,000 glaciers outside the Greenland and Antarctic ice sheets. The models use prin- ciples of ice flow dynamics to invert for ice thickness from surface characteristics. We find a total volume of 158 ± 41 × 103 km3, which is equivalent to 0.32 ± 0.08 m of sea-level change when the fraction of ice located below present-day sea level (roughly 15%) is subtracted. Our results indicate that High Mountain Asia hosts about 27% less glacier ice than previously suggested, and imply that the timing by which the region is expected to lose half of its present-day glacier area has to be moved forward by about one decade.

Read more: Farinotti D., H. Matthias, J. Fürst, J. Landmann, H. Machguth, F. Maussion and A. Pandit, 2019. A consensus estimate for the ice thickness distribution of all glaciers on Earth, Nature Geoscience, doi:10.1038/s41561-019-0300-3

Sensitivity of centennial mass loss projections of the Amundsen basin to the friction law

julien2019Reliable projections of ice sheets’ future contributions to sea-level rise require models that are able to accurately simulate grounding-line dynamics, starting from initial states consistent with observations. In this new article, we use Elmer/Ice to simulate the centennial evolution of the Amundsen Sea Embayment (the most active drainage basin of the antarctic ice sheet) in response to a prescribed perturbation in order to assess the sensitivity of mass loss projections to the chosen friction law, depending on the initialisation strategy. To this end, we take advantage of inverse methods implemented in Elmer/Ice to construct three different model states by inferring both the initial basal shear stress and viscosity fields with various relative weights. Then, starting from each of these model states, prognostic simulations are carried out using a Weertman, a Schoof and a Budd friction law, with different parameter values. These experiments demonstrate that although the sensitivity of projections to the chosen friction law tends to decrease when more weight is put on viscosity during initialisation, it remains significant for the most physically acceptable of the constructed model states. In addition, it turns out that independently of the considered model state, the Weertman law systematically predicts the lowest mass losses. Finally, because of its particular dependence on effective pressure, the Budd friction law induces significantly different grounding-line retreat patterns than the other laws and predicts significantly higher mass losses. These conclusions urge the scientific community to undertake major efforts in order to get a better understanding of processes at the root of basal motion, with the aim of developing reliable friction laws that could be used in models to produce accurate estimates of future contribution of ice sheets to sea level rise.

Read more: Brondex J., F. Gillet-Chaulet and O. Gagliardini, 2019. Sensitivity of centennial mass loss projections of the Amundsen basin to the friction law, The Cryosphere, 13, 177-195, https://doi.org/10.5194/tc-13-177-2019

The de-neutralized equilibrium

Gladstone etal 2018 equilibriaPoor convergence with resolution of ice sheet models when simulating grounding line migration has been known about for over a decade. However, some of the associated numerical artefacts remain absent from the published literature.

Using Elmer/Ice on idealised grounding line evolution experiments, it is shown that with insufficiently fine model resolution, a region containing multiple steady-state grounding line positions exists, with one steady state per node of the model mesh. This has important implications for the design of perturbation experiments used to test convergence of grounding line behaviour with resolution. Specifically, the design of perturbation experiments can be under-constrained, potentially leading to a “false positive” result. In this context a false positive is an experiment that appears to achieve convergence when in fact the model configuration is not close to its converged state. We demonstrate a false positive: an apparently successful perturbation experiment (i.e. reversibility is shown) for a model configuration that is not close to a converged solution. If perturbation experiments are to be used in the future, experiment design should be modified to provide additional constraints to the initialisation and spin-up requirements.

This region of multiple locally stable steady-state grounding line positions has previously been mistakenly described as neutral equilibrium. This distinction has important implications for understanding the impacts of discretising a forcing feedback involving grounding line position and basal friction. This forcing feedback cannot, in general, exist in a region of neutral equilibrium and could be the main cause of poor convergence in grounding line modelling.

Read more: Gladstone, R. M., Xia, Y., and Moore, J.,2018. Neutral equilibrium and forcing feedbacks in marine ice sheet modelling. The Cryosphere, 12, 3605-3615. doi:10.5194/tc-12-3605-2018

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