Coming to glaciology through a passion for mountain sports, Fabien Gillet-Chaulet describes the Poles as "relatively deserted places that stimulate the imagination".
But the work produced by this researcher and his team is part of a much broader perspective, rooted in the collective interest. They help to feed the IPCC's analyses. Their research is proving essential for better understanding and anticipating changes in sea and ocean levels, at a time when the most recent news reports point to the possibility of a tipping point in Antarctica, or the cracks in "the glacier of the apocalypse".
Far from the dreamlike world of eternal snow evoked by Christmas, these scientists are methodically using observation and numerical simulation to understand these phenomena and better anticipate their consequences.
Ice flow: a major climate issue
The ice that makes up glaciers and polar ice caps behaves like a highly viscous fluid: it flows. Under its own weight, the snow that has become ice deforms and moves downwards. When it reaches the edges, it can melt or flow into the oceans to form icebergs. Ultimately, this loss of mass as a result of melting and ice flow will end up in the oceans. What will interest researchers here is the mass balance of these objects: how much is gained or lost each year, and to what extent this will lead to a change in volume.
A major challenge for scientific research then lies in the possibility of predicting the contribution of glaciers and polar caps to rising sea and ocean levels in a context of climate change.
There are, of course, special problems, with highly heterogeneous flows. For example, the center of the Antarctic and Greenland ice sheets is very flat, and despite ice thicknesses of up to 4km, the ice deforms very slowly, with surface flow velocities of just a few meters per year. However, in some areas, this phenomenon can be much more rapid, with displacements measured in kilometers, giving rise to "rivers of ice" that flow into the ocean and stimulate the development of icebergs. More worryingly, many glaciers have recently accelerated, flowing ever more rapidly into the ocean, and contributing significantly to the loss of mass of the ice caps. These singularities must imperatively be understood and taken into account.
Numerical simulation: an indispensable pillar of science to prepare for the future
Studying ice means, historically, working on a long time scale. First of all, numerical simulations have been developing for several decades. The first ice sheet models, in the late 1990s, were linked to the drilling of boreholes aimed at understanding the evolution of polar ice sheets over tens or even hundreds of thousands of years. As a counterpoint to the initial impression of slowness, satellite observation of flow velocities demonstrated that these ice caps were reacting much faster than expected to climate change. The IPCC reports of the early 2000s noted such developments, but were unable to forecast or even explain them. "That's when the research community in this field boomed," explains Fabien Gillet Chaulet. Particular attention was paid to the phenomenon of ice flow to reduce uncertainties. Its study is now carried out in every corner of the globe. The amount of data produced and processed has become very large.
The use of supercomputers, like OCCIGEN, is therefore proving indispensable, particularly depending on the "size of the objects" for which these resources are mobilized. "The larger the objects, the greater the computing resources required", explains Fabien Gillet-Chaulet. Flow situations in Antarctica and Greenland are particularly relevant here. It is also in these two areas that the interactions between the ocean and dynamic flow prove to be much more important for surface mass balance.
The success of this approach implies working with a widely heard scientific community, that of climate experts, in order, for example, to anticipate snowfalls, melts and hence, thanks to the resources made available, to "force ice flow models". Coupling between "ocean models" and "ice-cap models" is becoming increasingly common, not least because "the interaction between ocean and ice cap remains uncertain and difficult to observe", continues the scientist. Complementarity between observation and simulation is therefore becoming increasingly important in this area of research.
This collaborative spirit naturally transcends borders. While the project is part of the European and international Tipaccs and Protect initiatives in particular, the core of the simulation model used, Elmer/Ice, is being developed at the CSC in Finland.
Conclusion
Hundreds of thousands of computing hours granted under the DARI procedure and the support provided by the CINES teams are contributing to the key objective pursued by Fabien Gillet-Chaulet and his teams: to predict changes in the mass of the polar ice caps between now and the end of the century, and even between now and a hundred or two hundred years from now, in order to improve forecasts of rising sea and ocean levels. While "the tools are adapted to the needs", the volume of data and the complexity of the codes mean that we need to keep a watchful eye on technological developments, always with a view to being "more efficient in terms of resource consumption, particularly energy resources", concludes Fabien Gillet Chaulet. This wish will find a favorable echo in GENCI's acquisition of the Adastra supercomputer, soon to be installed, hosted and operated by CINES and its teams...
Find out more
Videos of simulation results:
- https://www.youtube.com/watch?v=etPmtGbAc54 (results published in Shannon, S.R., Payne, A.J., Bartholomew, I.D., van den Broeke, M.R., Edwards, T.L., Fettweis, X., Gagliardini, O., Gillet-Chaulet, F., Goelzer, H., Hoffman, M.J., Huybrechts, P., Mair, D.W.F., Nienow, P.W., Perego, M., Price, S.F., Smeets, C.J.P.P., Sole, A.J., van de Wal, R.S.W., Zwinger, T., 2013. Enhanced basal lubrication and the contribution of the Greenland ice sheet to future sea-level rise. Proceedings of the National Academy of Sciences 110, 14156-14161.https://doi.org/10.1073/pnas.1212647110)
- https://www.youtube.com/watch?v=LQbpAgRiDCk (results published in Favier, L., Durand, G., Cornford, S.L., Gudmundsson, G.H., Gagliardini, O., Gillet-Chaulet, F., Zwinger, T., Payne, A.J., Le Brocq, A.M., 2014. Retreat of Pine Island Glacier controlled by marine ice-sheet instability. Nature Climate Change 4, 117-121. https://doi.org/10.1038/nclimate2094)
References to recent articles:
- Sun et al., 2020. Antarctic ice sheet response to sudden and sustained ice-shelf collapse (ABUMIP). J. Glaciol. https ://doi.org/10.1017/jog.2020.67
- Brondex, J., Gillet-Chaulet, F., and Gagliardini, O., 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
- Seroussi et al., 2019. initMIP-Antarctica: an ice sheet model initialization experiment of ISMIP6. The Cryosphere, https ://doi.org/10.5194/tc-13-1441-2019
- Goelzer et al., 2018 Design and results of the ice sheet model initialization experiments initMIP-Greenland: an ISMIP6 intercomparison, The Cryosphere 12, 1433-1460. https ://doi.org/10.5194/tc-12-1433-2018
Photo credits: @66north - Unsplash - https://unsplash.com/photos/NaQMJ-xNDWI