Scaling and Performance Improvements in Elmer/Ice

By gaining and losing mass, glaciers and ice-sheets play a key role in sea level evolution. This is obvious when
considering the past 20000 years, during which the collapse of the large northern hemisphere ice-sheets after the
Last Glacial Maximum contributed to a 120m rise in sea level. This is particularly worrying when the future is
considered. Indeed, recent observations clearly indicate that important changes in the velocity structure of both
the Antarctic and Greenland ice-sheets are occurring, suggesting that large and irreversible changes may already
have been initiated. This was clearly emphasised in the last report published by the Intergovernmental Panel on
Climate Change (IPCC) [7]. The IPCC also asserted that current knowledge of key processes causing the
observed accelerations was poor, and concluded that reliable projections obtained with process-based models for
sea-level rise (SLR) are currently unavailable. Most of these uncertain key processes have in common that their
physical/numerical characteristics, such as shallow ice approximation (SIA), are not accordingly reflected or
even completely missing in the established simplified models that have been in use since decades. Whereas those
simplified models run on common PC systems, the new approaches require higher resolution and larger
computational models, which demand High Performance Computing (HPC) methods to be applied. In other
words, numerical glaciology, like climatology and oceanography decades ago, needs to be updated for HPC with
scalable codes, in order to deliver the prognostic simulations demanded by the IPCC. The DECI project
ElmerIce, and enabling work associated with it, improved simulations of key processes that lead to continental
ice loss. The project also developed new data assimilation methods. This was intended to decrease the degree of
uncertainty affecting future SLR scenarios and consequently contribute to on-going international debates
surrounding coastal adaptation and sea-defence planning. These results directly feed into existing projects, such
as the European FP7 project ice2sea [9], which has the objective of improving projections of the contribution of
continental ice to future sea-level rise and the French ANR ADAGe project [10], coordinated by O. Gagliardini,
which has the objective to develop data assimilation methods dedicated to ice flow studies. Results from these
projects will directly impact the upcoming IPCC assessment report (AR5).