Since September 2011 we have featured one PISM application per month, either a published article or a presented poster, on the home page. This is the archive. Please let us know if you would like your work to appear as a PISM Application of the Month.
Also see the list of publications using PISM.
|Interaction of marine ice-sheet instabilities in two drainage basins: simple scaling of geometry and transition time|
|investigators:||J. Feldmann and A. Levermann|
The marine ice-sheet instability generally comes from the ocean side of the ice sheet. Using a flow-line geometry in PISM, this paper investigates whether instability can be triggered from the direction of the ice divide. The authors find that the instability in one basin can induce a destabilization in the other. The underlying mechanism is dynamic thinning and consequent motion of the ice divide. They conclude that for the three-dimensional case, the possibility of drainage basin interaction on timescales on the order of 1 kyr or larger cannot be excluded and needs further investigation.
|Testing the sensitivity of the East Antarctic Ice Sheet to Southern Ocean dynamics: past changes and future implications|
|investigators:||C. Fogwill, C. Turney, K. Meissner, N. Golledge, P. Spence, J. Roberts, M. England, R. Jones, and L. Carter|
|journal:||Journal of Quaternary Science|
The stability of the Antarctic ice sheet and its contribution to sea level under projected future warming remains highly uncertain. The Last Interglacial (LI; 135–116 ka ago) is a potential analogue for the present period, with sea levels 6.6–9.4 m higher than present, and thus it deserves study. This paper examines a possible source of LI sea-level rise. These authors report on model simulations exploring the effects of migrating Southern Hemisphere Westerlies (SHWs) on Southern Ocean circulation and Antarctic ice-sheet dynamics. The effect on ice dynamics is modeled with PISM, which plays only a supporting role in this work. They conclude that southerly shifts in winds may have significantly impacted the sub-polar gyres, inducing pervasive warming of 0.2–0.8 °C in the upper 1200 m adjacent to sectors of the East Antarctic Ice Sheet (EAIS). Thus the EAIS potentially made a substantial, hitherto unsuspected, contribution to LI sea levels.
|Antarctic contribution to meltwater pulse 1A from reduced Southern Ocean overturning|
|investigators:||N. Golledge, L. Menviel, L. Carter, C. J. Fogwill, M. H. England, G. Cortese, and R. H. Levy|
In this paper, researchers at Victoria University and the University of New South Wales describe a model study of Antarctic ice sheet evolution over the last 25 kyr using PISM with ocean-forcing inputs from the Earth system model LOVECLIM. They show that when the ocean around Antarctica becomes more stratified, warm water at depth melts the ice sheet faster than when the ocean is less stratified.
The study used a large ensemble of 15 km PISM simulations in a data-constrained mode. In the simulations that best fit a variety of temporal and spatial observations, several episodes of accelerated ice-sheet recession occurred, with the timing of the largest being coincident with meltwater pulse 1A. This episode saw an abrupt rise in global sea level, with an Antarctic contribution of nearly three meters over just a few centuries.
|Coupled ice sheet–climate modeling under glacial and pre-industrial boundary conditions|
|investigators:||F. Ziemen and others|
|journal:||The Climate of the Past|
Modeling Northern Hemisphere glacial conditions using general circulation models (GCMs) in quasi-equilibrium with prescribed ice sheets can lead to inconsistencies between the modeled climate and ice sheets. To avoid this problem, this paper models the ice sheets explicitly, giving the first results from coupled ice sheet–climate simulations for pre-industrial times and the Last Glacial Maximum.
They use the atmosphere–ocean–vegetation GCM ECHAM5/MPIOM/LPJ bidirectionally-coupled with a modified version of PISM 0.3 on a 20 km grid covering the Northern Hemisphere. The model system adequately represents large, non-linear climate perturbations, and the results agree reasonably well with reconstructions and observations. A large part of the drainage of the ice sheets occurs in ice streams which show recurring surges as internal oscillations. The Hudson Strait Ice Stream surges with an ice volume equivalent to about 5 m sea level and a recurrence interval of about 7000 yr, in agreement with basic expectations for Heinrich events.
Surface velocity, calculated fracture density, and modeled flow results for Filchner Ice Shelf. Click the image to go to The Cryosphere article page.
|Fracture-induced softening for large-scale ice dynamics|
|investigators:||T. Albrecht and A. Levermann|
Fracture processes within ice shelfs have been observed to reduce the retentive forces of the shelves on the Antarctic ice sheet. This paper adds a continuum representation of fractures, and their evolution, to PISM, and applies it to several major ice shelves in Antarctica. A key addition is the introduction of a higher-order scheme for advecting the two-dimensional fracture density field. Fractures and ice flow are coupled through a reduction of modeled ice viscosity proportional to the fracture density, so fracture-induced softening can feed back to cause added shear and self-amplified fracturing. The results of the simulations are compared to observations. Observed sharp across-flow velocity gradients in fracture-weakened regions are reproduced. This fracture-softening model is a basis for a future model of enhanced fracture-based calving.
Three cross sections (north, center, south) through the modeled initial states, at a 5 km resolution. Click the image to go to Journal of Glaciology journal.
|Role of model initialization for projections of 21st-century Greenland ice sheet mass loss|
|investigators:||G. Adalgeirsdottir and 6 others|
This paper assesses the sensitivity of projections of Greenland ice sheet contribution to 21st-century sea-level rise to the model initial state. Four initialization methods are applied using PISM. The simulated contribution to sea-level rise by 2100 ranges from an equivalent of 0.2 to 6.8 cm. The largest uncertainties arise from different formulations of the regional climate models (0.8–3.9 cm) and applied scenarios (0.65–1.9 cm), but an important source of uncertainty is the initialization method (0.1–0.8 cm). These model simulations do not account for the recently observed acceleration of outlet glaciers and consequent thinning rates, ocean forcing, or the feedback occurring between ice-sheet elevation changes and climate forcing. These results should be considered a lower limit of Greenland ice sheet contributions to sea-level rise, until such processes have been integrated into large-scale ice-sheet models.
|A system of conservative regridding for ice–atmosphere coupling in a GCM|
|investigators:||R. Fischer, S. Nowicki, M. Kelley, and G. A. Schmidt|
|journal:||Geosci. Model Dev.|
This paper describes a conservative method using elevation classes to regrid surface mass balance fields between low-resolution GCMs and high-resolution ice sheet models. The proposed transformations are both mass and energy conserving, making them suitable for two-way coupling between climate and ice sheet models. These transformations are implemented in Glint2, a library used to couple atmosphere models with ice models.
|Ice plug prevents irreversible discharge from East Antarctica|
|investigators:||M. Mengel and A. Levermann|
|journal:||Nature Climate Change|
This paper uses PISM to define an “ice-plug” which, if removed from the coastal ice in the Wilkes Basin of East Antarctica, would initiate irreversible retreat of the grounded ice in that basin. The modeled retreats, which occur on a time scale of a few thousand years, generate 3–4 m of sea level rise from the region surrounding the basin. Thus this basin is a potential “tipping-point” ice sheet configuration, in additional to the better-known West Antarctica configurations. For the PISM user this paper shows its ability to model an ice sheet region (hashed in figure) at high resolution across a range of ice dynamics parameters and climate forcing choices.
|Changing basal conditions during the speed-up of Jakobshavn Isbræ, Greenland|
|investigators:||M. Habermann, M. Truffer, and D. Maxwell|
We use a Tikhonov inverse method, with PISM's SSA as a forward model, to invert for basal conditions from surface velocity data throughout a well-observed period (1985, 2000, 2005, 2006 and 2008) of rapid change. Ice-softness, model norm, and regularization parameter choices are justified using the data-model misfit metric and the L-curve method. The sensitivity of the inversion results to these parameter choices is explored. We find a lowering of effective basal yield stress in the first 7 km upstream from the 2008 grounding line and no significant changes higher upstream. The temporal evolution in the fast flow area is in broad agreement with a Mohr–Coulomb parameterization of basal shear stress, but with a till friction angle much lower than has been measured for till samples. The lowering of effective basal yield stress is significant within the uncertainties of the inversion, but it cannot be ruled out that there are other significant contributors to the acceleration of the glacier.
|The effect of climate forcing on numerical simulations of the Cordilleran ice sheet at the Last Glacial Maximum|
|investigators:||J. Seguinot, C. Khroulev, I. Rogozhina, A. P. Stroeven, and Q. Zhang|
An ensemble of numerical simulations of the Cordilleran ice sheet in western North America during the Last Glacial Maximum (LGM) using the Parallel Ice Sheet Model. Temperature offsets to the present-day climatologies are applied from five different data sets. Surface mass balance is computed from precipitation and temperature using a positive degree-day model. We assess the model against a geomorphological reconstruction of the ice margin at the LGM. Modelled ice sheet outlines and volumes appear highly sensitive to the choice of climate forcing. For three of the four reanalysis data sets used, differences in precipitation are the major source for discrepancies between model results. Part of the mismatch is due to unresolved orographic precipitation effects caused by the coarse resolution of reanalysis data.
|Spontaneous ice-front retreat caused by disintegration of adjacent ice shelf in Antarctica|
|investigators:||T. Albrecht and A. Levermann|
|journal:||Earth Planet. Sci. Lett.|
Floating ice shelves, fringing most of Antarctica, exert restraining forces on the ice flow. Though abrupt ice–shelf retreat has been observed, it is generally considered a localized phenomenon. This paper shows, by using PISM-PIK, that the disintegration of an ice shelf may induce the spontaneous retreat of its neighbor. The spontaneous but gradual retreat of the Larsen B ice front, as observed after the disintegration of the adjacent Larsen A ice shelf, is reproduced. The “A” collapse yields a change in spreading rate in “B”, via their connecting ice channels, and thereby causes a retreat of the ice front to its observed position of the year 2000. This reproduces the configuration of “B” prior to its collapse in 2002.
For the PISM user this paper illustrates what modeling becomes possible with the combined PIK mechanisms for ice shelf front modeling, including sub-grid mass conservation and “eigencalving”; see the references of the paper and Chapter 8 of the PISM User's Manual.
|Resolution-dependent performance of grounding line motion in a shallow model compared with a full-Stokes model according to the MISMIP3d intercomparison|
|investigators:||J. Feldmann, T. Albrecht, C. Khroulev, F. Pattyn, and A. Levermann|
By using MISMIP3d simulations across a range of resolutions, this paper shows that the SIA+SSA hybrid stress balance in PISM can model grounding line motion in a perturbed ice-sheet–shelf system. The key improvements, all included in pism0.6, are: linear interpolation of the grounding line, locally-interpolated basal friction, and an improved driving-stress computation across the grounding line. The reversibility of the grounding line, after a local perturbation of basal resistance comes and goes, is captured by the model even at medium and low horizontal resolutions (> 10 km). The transient model response is qualitatively-similar to that of higher-order models, though with higher sensitivity to perturbations on very short timescales. Our findings support the application of PISM to the Antarctic ice sheet from regional up to continental scales and even at relatively-low spatial resolutions.
|Paleo-glaciations of the Shaluli Shan, southeastern Tibetan Plateau|
|investigators:||Fu, P. and 7 others|
|conference:||EGU Annual Meeting, Vienna, Austria, April 07-12, 2013|
Geomorphological mapping, 10Be and 26Al exposure dating and glacial modeling are used to reconstruct the glacial history of the Shaluli Shan, southeastern Tibetan Plateau, and to understand the evolution of the glacial landscape. The Haizishan Plateau experienced multiple ice cap glaciations, and 10Be and 26Al exposure ages from bedrock, boulder and saprolite profile samples show limited glacial erosion on some parts of the plateau surface and more than 2 meters of bedrock erosion in other areas. This juxtaposition of high erosion and relict topography suggests that the paleo Haizishan ice cap had a complex basal thermal regime. A numerical glacier model (PISM) is now being used to investigate the thermal regime of the paleo ice cap and patterns of erosion potential. This work provides new insights into the paleoclimatic setting and glacial landscape evolution of the southeast Tibetan Plateau.
|Mountain building and the initiation of the Greenland Ice Sheet|
|investigators:||A. Solgaard, J. Bonow, P. Langen, P. Japsen, and C. Hvidberg|
|journal:||Palaeogeography, Palaeoclimatology, Palaeoecology|
In this paper, effects of a new hypothesis about mountain building in Greenland on ice sheet initiation are investigated using PISM in combination with a climate model. According to this hypothesis, low-relief landscapes near sea level characterized Greenland in the Miocene. Then two phases of km-scale uplift, beginning at 10 and ~5 Ma, respectively, initiated the formation of the present-day mountains. These results are consistent with the observed climatic variability superimposed on the general cooling trend in the late Cenozoic, and they indicate that the Greenland Ice Sheet of today is a relict formed under colder conditions. The late Cenozoic mountain building in Greenland augments the effects of the climatic deterioration leading to the Northern Hemisphere glaciations. Without the second phase of uplift, the Greenland Ice Sheet would have been more sensitive to the changes in climate over the past millions of years.
|Glaciology and geological signature of the Last Glacial Maximum Antarctic ice sheet|
|investigators:||N. Golledge and 12 others|
|journal:||Quaternary Science Reviews|
Continent-wide marine and terrestrial geological evidence constrains the dynamical configuration of the Antarctic ice sheet during the last, and possibly preceding, glacial maxima. This paper interprets results from a remarkably high-resolution (5 km) PISM model using this evidence, focussing on the basal thermal regime of the ice sheet, its flow pattern, variability in subglacial erosion and sediment transport, and how these characteristics evolve during glacial transitions. The results show that rapid basal sliding in discrete outlets eroded and advected sediment to the continental shelf primarily during the early stages of advance and retreat of the ice sheet. Sector-by-sector analysis of geologic constraints, exquisite figures showing sediment transport paths through tight geographic confinements, and careful consideration of peak erosion timing set a new standard for validating high-resolution time-dependent model results with extensive geophysical evidence.
|An open ocean region in Neoproterozoic glaciations would have to be narrow to allow equatorial ice sheets|
|investigators:||C. Rodehacke, A. Voigt, F. Ziemen, D. Abbot|
|journal:||Geophysical Research Letters|
A major goal of understanding Neoproterozoic glaciations and determining their effect on the evolution of life and Earth's atmosphere is establishing whether and how much open ocean there was during them. Geological evidence tells us that continental ice sheets had to flow into the ocean near the equator during these glaciations. Here we drive the PISM ice sheet model with output from four simulations of the ECHAM5/MPI-OM atmosphere-ocean general circulation model with successively narrower open ocean regions. We find that extensive equatorial ice sheets form on marine margins if sea ice extends to within about 20 degrees latitude of the equator or less (Jormungand-like and hard Snowball states), but do not form if there is more open ocean than this. Given uncertainty in topographical reconstruction and ice sheet ablation parameterizations, we perform extensive sensitivity tests to confirm the robustness of our main conclusions.
|Increasing the Scalability of PISM for High Resolution Ice Sheet Models|
|investigators:||P. Dickens and T. Morey|
|journal:||Proceedings of the 14th IEEE International Workshop on Parallel and Distributed Scientific and Engineering Computing, May 2013, Boston|
In this paper, authors discuss their work in evaluating and increasing the I/O performance of PISM on a state-of-the-art supercomputer by using a 1 km Greenland ice sheet setup. In particular, they found that the computation performed by PISM is highly scalable, but that the I/O demands of the higher-resolution model are a significant drag on overall performance. The paper describes a series of experiments to find the cause of the relatively-poor I/O performance and how such performance could be improved. By making simple changes to the PISM source code and one of the I/O libraries used by PISM authors were able to provide an 8-fold increase in I/O performance.
|An iterative inverse method to estimate basal topography and initialize ice flow models|
|investigators:||W. van Pelt and others|
A new inverse approach to reconstruct distributed bedrock topography and simultaneously initialize an ice flow model is proposed. The procedure runs PISM multiple times over a prescribed period, while being forced with space- and time-dependent climate input. After each iteration bed heights are adjusted using information of the remaining misfit between observed and modeled surface topography. Synthetic experiments with constant-climate forcing demonstrate convergence and robustness of the approach. Application to Nordenskiöldbreen, Svalbard, forced with height- and time-dependent climate input since 1300 AD show a high correlation against radar-observed thicknesses. Remaining uncertainties can be ascribed to inaccurate model physics, in particular, uncertainty in the description of sliding.
|Insights into spatial sensitivities of ice mass response to environmental change from the SeaRISE ice sheet modeling project II: Greenland|
|investigators:||S. Nowicki and others|
|journal:||J. Geophys. Res. (Earth Surface)|
This second paper explores Greenland climate scenarios and forcing experiments from the 31 member Sea-level Response to Ice Sheet Evolution (SeaRISE) project. Although the modeled responses are not always homogeneous, consistent spatial trends emerge from the ensemble analysis, indicating distinct vulnerabilities of the Greenland ice sheet. There are clear response patterns associated with each forcing (1. a change in oceanic condition, 2. a warmer atmospheric environment, and 3. enhanced basal lubrication). Similar mass loss at the whole ice sheet scale will result in different mass losses at the regional scale. All forcings lead to an increased mass loss for the coming centuries, with increased basal lubrication and warmer ocean conditions affecting mainly outlet glaciers, while the impacts of atmospheric forcings affect the whole ice sheet.
|Insights into spatial sensitivities of ice mass response to environmental change from the SeaRISE ice sheet modeling project I: Antarctica|
|investigators:||S. Nowicki and others|
|journal:||J. Geophys. Res. (Earth Surface)|
Antarctic climate scenarios and forcing experiments from the 31 member Sea-level Response to Ice Sheet Evolution (SeaRISE) project are applied to six three-dimensional thermomechanical ice-sheet models, including a PISM model lead by M. Martin at PIK. This paper assesses the century-scale model sensitivity revealed by these experiments. Results indicate (i) growth with warming, except within low-latitude basins (where inland thickening is outpaced by marginal thinning); (ii) mass loss with enhanced sliding (with basins dominated by high driving stresses affected more than basins with low-surface-slope streaming ice); and (iii) mass loss with enhanced ice shelf melting (with changes in West Antarctica dominating the signal due to its marine setting and extensive ice shelves). Ice loss due to dynamic changes associated with enhanced sliding and/or sub-shelf melting exceeds the gain due to increased precipitation. Remaining uncertainties include differences between basins and the impact of sub-shelf melting on ice dynamics.
|Hindcasting to measure ice sheet model sensitivity|
|investigators:||A. Aschwanden, G. Aðalgeirsdóttir, and C. Khroulev|
Validation and assessment of model performance is critical, but it is notoriously-challenging in ice sheet modeling. This paper couples PISM to the HIRHAM5 regional climate model for simulations of the Greenland ice sheet. The results are compared to observations in the 1989-2011 period (hindcasting), in which ice geometry, ice surface velocity, gravitationally-derived mass time-series, and surface elevation change observations are all available. The simulations reproduce the seasonal signal and decadal trends in mass loss but they show deficiencies compared to observed changes in ice discharge. The paper concludes that it is important to use multiple data sets for model validation, and it identifies rates of change of spatially-dense observations as preferred validation metrics.
|Grounding-line migration in plan-view marine ice-sheet models: results of the ice2sea MISMIP3d intercomparison|
|investigators:||F. Pattyn and others, including T. Albrecht and M. Huetten|
|journal:||Journal of Glaciology|
These are the results of a comparison between plan-view marine ice-sheet models, MISMIP3D. The major experiments use a spatially-varying perturbation in basal sliding parameters. The goal is to model the evolution of curved grounding lines and the corresponding generating buttressing effects. Steady-state grounding-line positions and the degree of reversibility are analyzed. PISM results from PIK authors Albrecht and Huetten, on a 1 km grid using a hybrid-SSA formulation, show the same quality of steady state positions and reversibility as models, often specially-designed for these grounding line geometries, with more complete stress balances.
Here, basal conditions for different years before and after the break-up of the tongue are inferred from surface velocity measurements to investigate the changes and to compare them with parameterizations of basal conditions commonly used in ice-sheet models.
All inversions reproduce the overall pattern of observed surface velocities, which shows that, in general, our data and model choices are capable of reproducing the observations by only adjusting basal yield stress. In the lower 5 km of the glacier a clear trend from higher to lower basal yield stress values is visible.
|Modelling the outlet glaciers terminating in Godthab fjord|
|investigators:||Antje Fitzner and Dorthe Dahl-Jensen, Centre for Ice and Climate, Copenhagen|
|conference:||IGS 2012 Fairbanks|
Can regional ice dynamics modeling help to understand the mass loss of the Greenland ice sheet through surface melting and flow into outlet glaciers (calving and basal melting), and estimate the fresh water flux into a fjord? This study considers an example, the outlet glaciers terminating in Godthab fjord, including glacier Kangiata Nunaata Sermia. surface mass balance and 2 m air temperature from RACMO and HIRHAM RCM output were
compared. The new PISM “regional” mode, the
pismo executable in
stable0.5, was applied. The model captures the high velocities near the terminus qualitatively, but even at high 2 km model resolution the distinct fast flowing arms are not well modelled, and the modelled velocities and fluxes are overall lower than than observed. The question remains: Are there are deep troughs in the bed topography where the surface velocity is very high?
|Are the simulated climatic and dynamic mass losses of the Greenland Ice Sheet decoupled during the next 100 years?|
|investigators:||Guðfinna Aðalgeirsdóttir and Andy Aschwanden|
Model simulations with the state-of-the-art ice sheet model PISM (Parallel Ice Sheet Model), that is forced with a number of climate forcings for the next century are presented. The climate forcings come from the EU FP7 project ice2sea where 3 regional climate models (HIRHAM5, MAR and HadRM3P) were used to dynamically downscale two scenario runs (A1B and E1) from two GCMs (ECHAM5 and HadCM3). These climate models are run with a constant ice sheet topography and therefore climate-elevation change feedback not included in the simulated mass changes.
To assess the sensitivity of the projections to the ice sheet model initial state, four initialisaton methods were used. Analyses of these 100 years simulations indicate that the mass changes due to climate forcing are decoupled from the changes due to dynamic response and the initialisation procedure. The simulated mass loss has a relatively large range, 0.5 to 6.5 cm sea level rise equivalent, which is to a large extent due to the range in the projected climate forcing from the regional climate models that were used to downscale the climate fields.
Two versions of PISM were among the ten ice sheet models used to study sensitivity of the Greenland and Antarctic ice sheets to prescribed changes of surface mass balance, sub-ice-shelf melting and basal sliding. Results exhibit a large range in projected contributions to sea level change. In most cases, the sea-level-relevant ice volume lost is linearly dependent on the strength of the forcing. Combinations of forcings can be closely approximated by linearly summing the contributions from single forcing experiments suggesting that non-linear feedbacks are modest.
Our models indicate that Greenland is more sensitive than Antarctica to likely atmospheric changes in temperature and precipitation, while Antarctica is most sensitive to increased ice-shelf basal melting. An experiment approximating the IPCC’s RCP8.5 scenario produces first century contributions to sea level of 22.3 and 8.1 cm from Greenland and Antarctica, respectively, with a range among models of 62 and 14 cm, respectively. By 200 years, projections increase to 53.2 and 26.7 cm, respectively, with ranges of 79 and 43 cm.
|LGM ice sheets simulated with a fully coupled ice sheet-climate model|
|investigators:||Florian Ziemen and others|
We interactively couple the atmosphere-ocean-vegetation general circulation model ECHAM5/MPIOM/LPJ with the ice sheet model mPISM, a modified version of the Parallel Ice Sheet Model, without flux correction or anomaly maps in our models. We run ECHAM5 in T31 resolution and mPISM on a 20 km grid covering most of the northern hemisphere. For comparison, we also perform an experiment using the PMIP2 protocol and the ICE-5G ice sheet reconstruction (Peltier, 2004) instead of mPISM. In runs using pre-industrial as well as LGM boundary conditions, the shape of the ice sheets has a strong influence on the wind systems and thereby on the global climate. Our model shows ice sheet collapses as regular part of the ice sheet behavior. These pulses create strong signals in the ocean.
|Increased future ice discharge from Antarctica owing to higher snowfall|
|investigators:||Ricarda Winkelmann and others|
Large uncertainties exist in the potential changes of dynamic ice discharge from Antarctica from precipitation. Here we show that snowfall and discharge are not independent, but that future ice discharge will increase by up to three times as a result of additional snowfall under global warming. Our results, based on PISM-PIK runs forced by climate simulations through to the end of 2500, show that enhanced discharge exceeds the effect of surface warming as well as that of basal ice-shelf melting. In an ensemble of simulations designed to capture ice-physics uncertainty, the additional dynamic ice loss along the coastline compensates for between 30 and 65 per cent of the ice gain due to enhanced snowfall over the entire continent. This results in a dynamic ice loss of up to 1.25 metres in the year 2500 for the strongest warming scenario.
|Self-inhibiting growth of the Greenland Ice Sheet|
|investigators:||Peter Langen and others|
|journal:||Geophysical Research Letters|
The build-up of the Greenland Ice Sheet from ice-free conditions is studied using PISM driven by fields from an atmospheric GCM. Experiments where the two are coupled off-line are augmented by one where an intermediate ice sheet configuration is coupled back to the GCM. The ice sheet regrows from the ice-free state but this is halted when the intermediate recoupling step is included. This inhibition of further growth is due to a Föhn effect of moist air parcels being lifted over the intermediate ice sheet and arriving in the low-lying Greenland interior with high temperatures. This demonstrates that two-way coupling between the atmosphere and the ice sheet is essential for understanding its dynamics. Conditions cooler than those of today may be necessary for the GrIS to regrow to the present volume.
|Dynamics of the Last Glacial Maximum Antarctic ice-sheet and its response to ocean forcing|
|investigators:||Nick Golledge and others|
|journal:||Proc. National Academy of Sciences|
Retreat of the Last Glacial Maximum (LGM) Antarctic ice sheet is thought to have been initiated by changes in ocean heat and eustatic sea level from northern ice sheets melted under rising atmospheric temperatures. The extent to which spatial variability in ice dynamics may have modulated the resultant pattern and timing of decay of the Antarctic ice sheet has so far received little attention, however. We use a 5-km resolution whole-continent PISM model to assess whether differences in the mechanisms governing ice sheet flow could account for discrepancies between geochronological studies in different parts of the continent. We simulate the geometry and flow characteristics of an equilibrium LGM ice sheet. Then we perturb the system with sea level and ocean heat flux increases to investigate ice-sheet vulnerability. We find that although ocean warming and sea-level rise bring about ocalized glacier acceleration, drawdown of ice from neighboring areas leads to widespread thinning of entire glacier catchments.
|Linear response functions to project contributions to future sea level|
|investigators:||Ricarda Winkelmann and A. Levermann|
Linear response functions can separately estimate the sea-level contributions of thermal expansion and solid ice discharge from Greenland and Antarctica. This formalism introduces a time-dependence which allows for future rates of sea-level rise to be influenced by past climate variations. The linear response function for the solid ice discharge is computed with the Potsdam Parallel Ice Sheet Model PISM-PIK (Winkelmann et al. 2011) under surface warming scenarios. Different from earlier studies we conclude that solid ice discharge from Greenland due to dynamic thinning is bounded by 0.42 m sea-level equivalent. Ice discharge induced by surface warming on Antarctica is best captured by a model which reflects the fact that ice loss increases with the cumulative amount of heat available for softening the ice in our model.
|Reconstruction of basal properties in ice sheets using iterative inverse methods|
|investigators:||Marijke Habermann and others|
|venue:||Journal of Glaciology|
Inverse methods are used to estimate model parameters from observations, here basal shear stress from the surface velocity of an ice sheet. One starts with an initial estimate of the model parameters and then updates them to improve the match to observations in an iterative process. Large-scale spatial features are adjusted first. A stopping criterion prevents the overfitting of data. In this paper, iterative inverse methods are applied to the shallow-shelf approximation forward model. A new incomplete Gauss–Newton method is introduced and compared to the steepest descent and nonlinear conjugate gradient methods. Two different stopping criteria, the discrepancy principle and a recent-improvement threshold, are compared. The IGN method shows faster convergence than the others. Though PISM is not mentioned by this paper, and the experiments were done in python, code supporting these inversion methods is already present in the PISM dev branch.
|Last Glacial Maximum climate in New Zealand inferred from a modelled Southern Alps icefield|
|ice sheet:||New Zealand (paleo)|
|investigators:||Nick Golledge and others|
|venue:||Quaternary Science Reviews|
In an attempt to constrain the climate of the Last Glacial Maximum period (LGM, c. 30–20 ka before present), a simulation of the New Zealand Southern Alps icefield is presented. PISM is applied at 500 m-resolution using empirical glaciological, climatological and geological data specific to the model domain, the entire icefield. An LGM cooling of at least 6–6.5 °C is necessary to bring about valley glaciers that extend beyond the mountains. However, climate–topography thresholds related to the elevation and hypsometry of individual catchments control the gradient of the rate of glacier expansion in the domain. In order to remain within geologically-reconstructed LGM limits we find that the LGM cooling was most likely associated with a precipitation regime up to 25% drier than today.
|Multistability of the Greenland ice sheet and the effects of an adaptive mass balance formulation|
|investigators:||Anne M. Solgaard and Peter L. Langen|
We use output from a general circulation model (CAM3+CLM) to construct adaptive temperature and precipitation patterns to force PISM off-line, taking into consideration that the patterns change in a non-uniform way, both spatially and temporally, as the geometry of the ice sheet evolves and as climate changes. In a series of experiments we investigate retreat from the present day configuration and build-up from ice free conditions during a warmer-than-present climate. We find that the ice sheet is able to survive and build up at higher temperatures using the more realistic adaptive patterns compared to the classic constant patterns. The ice sheet is multistable at least for certain temperature forcings, so it does not necessarily return to its initial configuration after a temperature excursion.
|Numerical simulations of cyclic behaviour in the Parallel Ice Sheet Model (PISM)|
|ice sheet:||simplified outlet glaciers|
|investigators:||Ward van Pelt and Johannes Oerlemans|
|venue:||Journal of Glaciology|
Numerical experiments are conducted on a synthetic topography with PISM, in which stress balance is connected to evolving thermodynamics and hydrology. The sensitivity of cyclic behaviour to changes in sliding-law parameters and the climate input is studied. Multiple types of oscillations were found, with strong variations in both amplitude and frequency. High-frequency oscillations (period 114–169 years), which are shown to have a major impact on ice velocities and a small effect on the ice volume, are related to variations in the water distribution at the base. Low-frequency cycles (period 1000+ years), which have a major impact on both velocities and ice volume, are linked to changes in the thermal regime.
|Kinematic first-order calving law implies potential for abrupt ice-shelf retreat|
|ice sheet:||Antarctic ice shelves|
|investigators:||Anders Levermann and others|
Observed large-scale disintegration of Antarctic ice shelves has moved their fronts closer towards grounded ice, accelerating ice-sheet discharge and contributing to global sea-level rise. Here we describe the first-order large-scale kinematic contribution to calving which is consistent with large-scale observation. This calving law depends only on local ice properties which are, however, determined by the full topography of the ice shelf. Simulations in PISM-PIK using the parameterization reproduces multiple stable fronts as observed for the Larsen A and B Ice Shelves, including abrupt transitions between them. We also ﬁnd multiple stable states of the Ross Ice Shelf.
|An enthalpy formulation for glaciers and ice sheets|
|ice sheet:||Greenland and other ice sheets|
|investigators:||Andreas Aschwanden and others|
|venue:||Journal of Glaciology|
Polythermal conditions are ubiquitous among glaciers and ice sheets. Fortunately, temperature and liquid water fraction are functions of a single enthalpy variable: a small enthalpy change in cold ice is a change in temperature, while a small enthalpy change in temperate ice is a change in liquid water fraction. The unified enthalpy formulation described in this paper models the mass and energy balance for the three-dimensional ice fluid, for the surface runoff layer and for the subglacial hydrology layer, together in a single energy-conserving theoretical framework. It is implemented in PISM. Results for the Greenland ice sheet are compared with those from a cold-ice scheme.
|Century-scale evolution of the Jakobshavn Isbræ with a high resolution regional model|
|lead investigator:||Daniella DellaGiustina|
|venue:||AGU Fall Meeting 2011|
A new regional mode in PISM is applied to the Jakobshavn outlet glacier. This mode is best suited for high spatial resolutions (< 1 km) and short timescales (< 1000 a). The first step is the identification of a drainage basin based on the surface gradient. Boundary conditions along the basin outline then partially-isolate the outlet glacier flow from the rest of the ice sheet. The ice dynamics model applied within the basin is the full enthalpy-based, SSA-sliding model. Both slow and fast ice flow are captured, as shown by a comparison to observations.
|Fracture ﬁeld for large-scale ice dynamics|
|ice sheet:||Antarctic ice sheet|
|investigators:||Torsten Albrecht and Anders Levermann|
|venue:||Journal of Glaciology|
A macroscopic fracture-density ﬁeld is introduced into PISM. Its evolution includes the initiation and growth of fractures as well as their advection with two-dimensional ice ﬂow. To ﬁrst approximation, fracture growth is assumed to depend on the spreading rate only, while fracture initiation is deﬁned in terms of principal stresses. The inferred fracture-density ﬁelds compare well with observed elongate surface structures. The aim of this study is to introduce the ﬁeld and investigate which of the observed surface structures can be reproduced by the simplest physically motivated fracture source terms.
|Fine-grid simulation of Antarctic ice stream dynamics at the Last Glacial Maximum|
|ice sheet:||Antarctic ice sheet (LGM)|
|lead investigator:||Nick Golledge|
|venue:||INQUA 2011 and SCAR International Symposium on Antarctic Earth Sciences, 2011|
The Antarctic Research Centre is using PISM to study Antarctic ice sheet behaviours during key periods of the past, particularly the LGM and the mid-Pliocene.
|When glacial giants roll over|
|ice sheet:||Antarctic ice sheet|
|investigators:||Anders Leverman, Potsdam Institute for Climate Impact Research (PIK)|
In a short News & Views article in Nature, Levermann reviews the potential for tsunami-genic iceberg calving as a phenomenon and a possible hazard. PISM-PIK Antarctic ice sheet simulations gave the frequency distribution of iceberg height and energy in discharge events per decade; see box. Iceberg discharge was computed from the vertical extent of the ice sheet and the marginal velocity distribution in a present-day equilibrium state. The results show a peak in the abundance of icebergs with a height of around 400 metres, and a distribution of energies up to several kilotonnes of TNT.
|Snapshots of the Greenland ice sheet configuration in the Pliocene to early Pleistocene|
|ice sheet:||Greenland (paleo) ice sheet|
|investigators:||Anne Solgaard, Centre for Ice and Climate, Denmark, and colleagues|
|venue:||Journal of Glaciology|
A study of the extent of the Greenland ice sheet during the Mid-Pliocene Warmth (3.3–3.0 Ma), its advance across the continental shelf during the late Pliocene to early Pleistocene glaciations (3.0–2.4 Ma) as implied by offshore geological studies, and the transition from glacial to interglacial conditions around 2.4 Ma as deduced from the deposits of the Kap København Formation, North Greenland.
|Numerical simulations of the Cordilleran ice sheet|
|ice sheet:||Cordilleran (paleo) ice sheet, North America|
This poster presents outcomes of step-cooling perturbation from current conditions using the Parallel Ice Sheet Model (PISM).
These simplified first experiments clearly demonstrate that ice sheet nucleation centers are consistent with the geological record. This as an encouraging start towards increased model complexity (ice shelves, lithospheric rebound) and transient model runs using past temperature and sea level reconstructions.
|The Potsdam Parallel Ice Sheet Model (PISM-PIK) – Part 2: Dynamic equilibrium simulation of the Antarctic ice sheet|
|ice sheet:||Antarctic ice sheet|
|investigators:||Maria Martin, Potsdam Institute for Climate Impact Research, and colleagues|
This paper presents a dynamic equilibrium simulation of the ice sheet-shelf system on Antarctica with the Potsdam Parallel Ice Sheet Model (PISM-PIK) with a focus on the Ronne-Filchner and Ross ice shelf areas as well as on the whole ice-sheet system.
PISM-PIK (and now PISM) allows free movement of grounding lines and calving fronts thanks to a physically-motivated calving law based on horizontal spreading rates and an implementation of a calving-front stress boundary condition.
The results support the approach of superposition of SIA and SSA for the representation of fast motion of grounded ice.