A new parameterisation of the Greenland ice sheet (GrIS) feedback between surface mass balance (SMB: the sum of surface accumulation and surface ablation) and surface elevation in the MAR regional climate model (Edwards et al., 2014) is applied to projections of future climate change using five ice sheet models (ISMs), among them Elmer/Ice. The MAR (Modèle Atmosphérique Régional: Fettweis, 2007) climate projections are for 2000–2199, forced by the ECHAM5 and HadCM3 global climate models (GCMs) under the SRES A1B emissions scenario.
In all results the elevation feedback is significantly positive, amplifying the GrIS sea level contribution relative to the MAR projections in which the ice sheet topography is fixed: the lower bounds of our 95% credibility intervals (CIs) for sea level contributions are larger than the "no feedback" case for all ISMs and GCMs.
Edwards, T. L., X. Fettweis, O. Gagliardini, F. Gillet-Chaulet, H. Goelzer, J.M. Gregory, M. Hoffman, P. Huybrechts, A.J. Payne, M. Perego, S. Price, A. Quiquet and C. Ritz, 2013. Effect of uncertainty in surface mass balance–elevation feedback on projections of the future sea level contribution of the Greenland ice sheet, The Cryosphere, 8, 195-208, doi:10.5194/tc-8-195-2014. [link to paper]
An international team of researchers led by the Laboratoire de Glaciologie et de Géophysique de l'Environment (LGGE) CNRS-Université Grenoble Alpes shows that Pine Island Glacier (PIG), the primary contributor to sea-level rise from Antarctica, has entered a period of self-sustained retreat and its discharge to the ocean will likely increase in comparison to observations from the last decade. The research is published today in Nature Climate Change.
The current imbalance of the West Antarctic ice sheet and its related net contribution to ongoing sea-level rise is now well established. In particular, PIG has receded by about ten kilometers during the last decade and alone contributes 25% of the total ice loss from West Antarctica. However, the future evolution of PIG remains poorly constrained with the possibility that it may lead to self-sustained retreat in what is known as the marine ice sheet instability.
As a result of the European Union’s four-year ice2sea project, and for the first time, three state-of-the-art ice-sheet models are tested against current observations and their simulations of PIG’s evolution over the next few decades compared. All of the models agreed that PIG has started a phase of self-sustained retreat and will irreversibly continue its recession over many tens of kilometers. This leads to a 3-5 fold increase in mass loss when compared to the current observations, equivalent to a 3.5-10 mm sea-level rise over the next 20 years.
Gaël Durand, CNRS - Université Grenoble-Alpes
Favier, L., G. Durand, S. L. Cornford, G. H. Gudmundsson, O. Gagliardini, F. Giller-Chaulet, T. Zwinger, A. J. Payne and A. M. Le Brocq, 2014. Retreat of Pine Island Glacier controlled by marine ice-sheet instability, Nature Climate Change, doi:10.1038/nclimate2094. [Link to paper]
In two recent papers the validity of the Shallow Ice Approximation (SIA) and its higher order extension, the Second-Order SIA (SOSIA), is analyzed. In Ahlkrona et al. (2013a) , the full Stokes equations are solved with the ice sheet/ice flow model Elmer/Ice for different aspect ratios (reflecting the shallowness of an ice sheet). By this approach it is possible to determine how the stresses, velocities and pressure depend on the aspect ratio. These dependencies, or scaling relations, are important to know correctly in order to make approximations such as the SIA. It is found that there is a thick boundary layer at the ice surface, altering the scaling relations in a way which is not considered in the classical derivation of the SIA and the SOSIA. These numerical results are consistent with existing boundary layer theory for ice sheets. In Ahlkrona et al. (2013b), we investigate, both by analysis and by numerical simulations using the ice sheet/ice flow model Elmer/Ice, how this boundary layer influences the accuracy of the SIA and SOSIA. We find that due to the boundary layer, the order of accuracy of the SIA is lower than usually assumed, and the SOSIA is in many cases not a significant improvement to the SIA and is also dependent on an ad-hoc auxiliary parameter introduced to evade singularities in the boundary layer.
Ahlkrona, J., N. Kirchner, and P. Lötstedt, 2013a. A Numerical Study of Scaling Relations for Non-Newtonian Thin-film Flows with Applications in Ice Sheet Modelling, Quarterly Journal Of Mechanics And Applied Mathematics, 66(4), 417-435, doi:10.1093/qjmam/hbt009. [link to paper]
Ahlkrona, J., N. Kirchner, and P. Lötstedt, 2013b. Accuracy of the zeroth- and second-order shallow-ice approximation – numerical and theoretical results, Geosci. Model Dev., 6, 2135-2152, doi:10.5194/gmd-6-2135-2013. [link to paper]