The Parallel Ice Sheet Model pism0.7 is an open source, parallel, high-resolution ice sheet model. Features:
|The multi-millennial Antarctic commitment to future sea-level rise|
|investigator:||N. Golledge and others|
The Antarctic ice sheet (AIS) contribution to sea-level rise under warming scenarios has been difficult to quantify. This paper uses 10km PISM simulations to show that if atmospheric warming exceeds 1.5 to 2 degrees Celsius above present then collapse of the major Antarctic ice shelves triggers a centennial- to millennial-scale response which is a long-term commitment (an unstoppable contribution) to sea-level rise. While another just-published AIS PISM paper considered a relatively extreme climate scenario, this one finds that substantial Antarctic ice loss can be prevented only by limiting greenhouse gas emissions to RCP 2.6 levels, a specific and worrysome conclusion. Higher-emissions scenarios lead to modeled ice loss from Antarctic that will raise sea level by 0.6–3 metres by the year 2300. Greenhouse gas emissions in the next few decades strongly influence the long-term modeled contribution of the AIS.
The PISM user should note that the first paragraph of the Methods section of this paper is a compact description of a canonical application of PISM. Later paragraphs describe more customized application, though largely through existing PISM code. Grounding line dynamical modeling is carefully done based on the parameterization derived from Feldmann et al (2014), which is part of PISM 0.6 and later. The RCPs are used to construct surface air temperature, precipitation, and ocean temperature, with PISM PDD and three-equation models used to determine (upper) surface mass balance and sub-shelf mass balance.
A new paper in the Proceedings of the National Academy of Sciences by J. Feldmann and A. Levermann, of the Potsdam Institute for Climate Impact Research, uses PISM simulations to show that nearly-complete WAIS collapse is triggered by present-day melt rates in the Amundsen Sea. Modeled WAIS deglaciation follows after relatively-short (60–200a) periods in which the present-day sub-shelf (i.e. ocean-caused) melt rates are sustained.
The simulations use conservative assumptions about, and (necessarily) modeling of, the interaction of the ice sheet with the ocean and atmosphere. In particular, subshelf melt rates for the present ice shelf geometry are taken from Finite Element Sea Ice-Ocean Model (FESOM) results. These are then extended to the evolving cavity geometry by a diffusive algorithm into regions below sea level, but with a pressure adjustment using the ice shelf base elevation. This leads to melt rates further inland that are similar to corresponding present-day-cavity-geometry-induced melt rates.
In most other ways this application of PISM is as expected, though at high (5km) resolution and using a full suite of marine ice sheet submodels: 50ka spinup, SIA+SSA model with plastic till, subgrid motion of the calving front, ocean-water stress boundary condition at the calving front, the "eigen-calving" calving law, and an interpolated grounding line.
The results of the simulations are most easily understood by seeing what happens:
This work appeared today, 2 November 2015. It is already featured in commentaries at the Washington Post, The Guardian, and Bloomberg Business News. It is also featured in Nature journal's "News:Explainer", and in Science magazine's "Latest News".
The last of these includes this high-level view from two well-known students of the behavior of Amundsen Sea-sector glaciers:
“This paper does confirm what we hypothesized, that knocking out the Pine Island Glacier and Thwaites takes down the rest of the West Antarctic Ice Sheet,” says Ian Joughin, a glaciologist at the University of Washington, Seattle, who co-authored last year’s Science paper. He adds, however, that the model’s weakness is its [temporal] resolution; it shows the destabilization of the glaciers occurring roughly 60 years from now, whereas present observations suggest that collapse is already underway. As a result, Joughin says, the model’s time scale for the collapse is probably too long. “It’s more likely measured in centuries rather than millennia.”
Indeed, “the jury is still out” on whether Feldmann and Levermann’s study got the time scale right, [Eric] Rignot [of the University of California, Irvine] says. The long-term evolution of an ice sheet “is a very complex modeling problem. Some of the variables controlling the models are not all that well known,” he adds, including forces such as winds, ocean circulation, and how icebergs calve. “There is not a model out there that is getting it right, because they all have caveats. I think the discussion is ongoing, and is only going to be more interesting with time.”
As a result of the buzz around Winkelmann et al. (2015)'s modeling of the effect of full conversion of available fossil fuels in the ground into atmospheric CO2, using PISM for determining ice dynamics/response timescale, on 5 October our local paper the Fairbanks Daily News Miner featured PISM. The content is a bit warped by scientist-to-journalist transmission issues, but we are happy to have local recognition of this UAF-lead project!
See the official position announcement here.
We are looking for a candidate who is interested in taking part in the research project “Modelling the ice flow in the western Alps during the last glacial cycle” which is a joint initiative between ETH Zurich and the University of Bern (Prof. Christoph Raible and Dr. Juan Jose Gomez-Navarro). The objective is to better understand the chronology of the last glaciation over the Alps via a modelling approach. The core of this doctoral research will be to model the ice flow and the glacial extent in the western Alps during the last glacial cycle. For that purpose, the PhD student will set up and run the Parallel Ice Sheet Model (PISM) on the clusters of the Swiss National Supercomputing Centre. The crucial step in setting up in PISM will be to include high-resolution climate simulation results, which will be conducted at the University of Bern. The combination of the two state-of-the-art models (ice flow and climate) will give a new insight of the ice flow field prevailing in the western Alpine region during some periods of interest like the last glacial maximum (22000 BP) and an earlier period (65000 BP). The final goal of the PhD will be to compare the new model results to the geomorphological evidence left on the Swiss landscape during the last glacial cycle (e.g. moraines, erratic boulders) in collaboration with quaternary geologists of EHT Zurich. The PhD student will be supervised by Dr. Guillaume Jouvet and Prof. Martin Funk.
The ideal candidate has a master degree either in geophysics, earth sciences, physics, applied mathematics, computer science, or a related field, and a keen interest in modelling of geophysical processes. Previous experience in computer modelling and scientific programming languages (C/C++, Python, Matlab) is an asset. Good writing and communication skills as well as the motivation to fruitfully collaborate within an interdisciplinary framework are essential, in particular with our climate modelling partners at the University of Bern.
For additional information please refer to www.glaciology.ethz.ch or contact Dr. Guillaume Jouvet, email@example.com (no applications).
A new open-access paper by Ricarda Winkelmann and others uses PISM to address an admittedly extreme question: If all currently-attainable fossil fuel resources are converted to atmospheric greenhouse gases, what happens to the Antarctic Ice Sheet?
This paper's model-based answer is that serious destruction of the ice sheet occurs in the first millenium, at about 3 m sea level rise per century. Such a large mass loss rate tails off in the two following millenia. The large losses come from a combination of marine-ice-sheet instability and surface elevation versus mass balance feedback, both of which are modeled effects in PISM. However, in the first century of the simulations there are the same relatively-modest AIS mass changes as seen in other recent modeling work, because dynamic losses driven by increasing ocean temperatures are partly offset by increasing snowfall.
Here is a quick methods summary, with more detail found in the paper and its supplementary material: Emission scenarios, CO2 concentrations, and global mean temperature pathways are combined in an Earth system model and then downscaled to surface and ocean temperature anomalies for Antarctica. These regional warming scenarios are then used to force PISM, in particular using its positive-degree-day scheme to model surface melt and a three-equation model for subshelf melting.
US National Public Radio featured the paper, including comments by co-author Ken Caldeira, on the 11 September edition of All Things Considered, as did the New York Times.
PISM is jointly developed at the University of Alaska, Fairbanks (UAF) and the Potsdam Institute for Climate Impact Research (PIK). For more about the team see the UAF Developers and PIK Developers pages.
UAF developers, who are in the Glaciers Group at the GI, are supported by NASA's Modeling, Analysis, and Prediction and Cryospheric Sciences Programs (grants NAG5-11371, NNX09AJ38C, NNX13AM16G, NNX13AK27G) and by the Arctic Region Supercomputing Center.