Last month, the Intergovernmental Panel on Climate Change published the third part of its Fifth Assessment Report. The first part of this report, published last year, cited over 9,000 peer-reviewed scientific publications and confirmed that climate change is taking place and that human activity is the dominant cause. Another of its conclusions is that glacier mass loss and ocean thermal expansion from warming together explain about 75% of the mean sea level rise that has taken place globally since the early 1970s. Earlier this year, an international team of scientists published a study in Nature Climate Change suggesting that one of Antarctica's biggest ice streams, the Pine Island Glacier, may be in uncontrollable retreat, with the potential risk of raising global sea levels by up to 1 cm by the year 2031.
The Pine Island Glacier accounts for around 20% of the total ice flow on the West Antarctic Ice Sheet and makes up around 10% of the total ice on Antarctica. Over the last 40 years, due to warm ocean currents, the melting rate of the glacier has been continually accelerating. According to the scientists, this makes it the largest single contributor to sea level rise in Antarctica. "We chose this specific glacier because it is the most unstable one on the continent," says Gael Durand of the University of Grenoble, France. "Its grounding line (the area underwater where the ice shelf meets land) has been retracting for at least 20 years now and due to the local bedrock topography, we envisaged the possibility of its instability."
Back in December 2013, research from the European Space Agency's CryoSat mission showed that the West Antarctic Ice Sheet is experiencing a loss of nearly 420 cubic kilometers of ice every year. The 2014 Pine Island Glacier study results shed even more light on the ice loss situation on the continent.
The glaciological community has undertaken serious efforts to develop and improve a new generation of ice-flow models. The results of these efforts were used in the Pine Island Glacier study. The team of researchers used three sophisticated computer models to assess the glacier's future based on its receding grounding line. They found that this line has waned by tens of kilometers over the past decade and is heading towards an unstable 40 km retreat. "As the ice is being drained faster towards the ocean, the main challenge of the modeling was to adequately describe the motion of grounding line," explains Durand.
The computation of the models was carried out using the supercomputing center of the University of Grenoble, the French National Supercomputing Center for Higher Education, and the University of Bristol's Blue Crystal Phase 2 supercomputer.
The simulations were carried out using the full-Stokes parallel finite-element model Elmer/Ice, the hybrid model BISICLES, and the Shallow Shelf Approximation Ua model. They are state-of-the-art models. In particular, Elmer/Ice and BISICLES rely heavily on general-purpose high-performance computing libraries and techniques, such as Trilinos, Hypre, and Chombo. "Without high-tech HPC facilities, simulations of that particular size would be impossible to perform and the desired accuracy would be absent," says Thomas Zwinger, a co-author of the paper.
The Elmer/Ice model was developed through the Partnership for Advanced Computing in Europe (PRACE). Based on the open-source multi-physics code Elmer, it offers scaling and performance improvements, while at the same time solving the complete set of dynamic equations of all stress components in the ice. "The huge area of the Pine Island Glacier implied the need for parallel computing," adds Zwinger. "The Finnish IT Center for Science (CSC) is mainly in charge of optimizing the open-source code of Elmer. For us, this means that the kernel of Elmer/Ice is optimized and ported to new platforms, such as Many Integrated Core."
"While the IPCC report makes the topic of marine ice sheet instability easier to understand, our results pinpoint that instability," concludes Durand. "The glacier has most probably crossed the point of no return and will not recover. We are facing massive and most probably permanent changes on human time scale."