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.
Also see the list of publications using PISM.
|Sensitivity of the Lambert-Amery glacial system to geothermal heat flux|
|investigators:||M. Pittard and others|
|journal:||Annals of Glaciology|
The temperature of an ice sheet is an important control on the deformation of ice, but it also controls basal melt. The geothermal flux (GHF) is a significant thermal boundary condition, and there is large uncertainty in its magnitude and local variation. On the other hand, a balance of heat conduction and advection—the latter dominates in the large—determines the 3D temperature field within the ice. Thus a thermomechanically-important input—it is a velocity boundary condition—is the rate of surface accumulation. The 3D conduction/advection balance determines sliding because the base is weak where it is well-lubricated; this is dominated by basal melt in a cold ice sheet like the Antarctic.
All of this is modeled by PISM—not perfectly but it is all there. However, understanding the sensitivity of this thermomechanical system to it inputs, in a particular glacier, is not easy. This paper studies the Lambert-Amery ice stream/shelf system in East Antarctica by using observed surface velocities and ice thicknesses as the major constraints. The authors conclude that the ice flow is most sensitive to spatial variation in GHF near the ice divides and under the edges of the ice streams. Their control simulation has temperate ice up to 150 m thick, an average basal melt of 1.3 mm per year, and maximum basal melting of about 0.5 m per year.
|East Antarctic ice sheet most vulnerable to Weddell Sea warming|
|investigators:||N. Golledge and others|
|journal:||Geophysical Research Letters|
In practice, ice sheet models need to be re-run many times with slightly-altered parameters. This short paper describes the results of an ensemble of 42 PISM runs for the entire Antarctic ice sheet (AIS), starting from present-day conditions, each for 10,000 model years, to see which sectors of the continent are most affected by warming. The runs vary only in atmospheric (air temperature anomaly) and oceanic (sea surface temperature anomaly) warming.
The results are clear and consistent when examined at drainage basin scale (Zwally et al 2012). The basins of the West Antarctic Ice Sheet (WAIS) show strong sensitivity to a modest amount of ocean warming, with a 0.5 C anomaly generating an essentially-complete collapse of WAIS, just as expected from other observational and simulation evidence. In the East (EAIS), however, the Recovery ice stream basin stands out as sensitive to both kinds of warming. Other EAIS basins such as Wilkes and Aurora are sensitive to atmospheric heat inputs but much less so to oceanic inputs.
|The role of ice stream dynamics in deglaciation|
|investigators:||A. Robel and E. Tziperman|
|journal:||JGR Earth Surface|
Does the presence of ice streams enhance deglaciation? If so, this should render certain large ice sheets, those with geometry and basal properties conducive to stream formation, more sensitive to changes in the climatic mass balance input. Based on an idealized PISM configuration, the simulations in this study show that when the ice sheet is large and ice streams are sufficiently developed an upward shift in equilibrium line altitude results in rapid deglaciation, while the same shift applied to an ice sheet without fully-formed ice streams results in continued ice sheet growth or slower deglaciation. Rapid deglaciation in ice sheets with significant streaming behavior is caused by ice stream acceleration and the attendant enhancement of calving and surface melting at low elevations. Ice stream acceleration is ultimately the result of steepening of the ice surface and increased driving stresses in ice stream onset zones, which come about due to the dependence of surface mass balance on elevation.
|Centennial-scale Holocene climate variations amplified by Antarctic Ice Sheet discharge|
|investigators:||P. Bakker and others|
Little is known about the dynamical system formed when a marine-based ice sheet interacts with the global ocean/atmosphere circulation. While some understanding of this dynamical system can come from coupling ice sheet models to earth system models, this needs validation from observations on the relevant timescales of the coupled system. These timescales are likely to be multi-century, millennial, and longer.
This paper describes coupled simulations using a PISM-modeled Antarctic Ice Sheet (AIS) with incomplete coupling to the global circulation. On the one hand, the AIS model is forced by Southern Ocean temperatures from the LOVECLIM Earth System model, while on the other the modeled AIS meltwater is used to force the UVic global climate model. The model results are compared to high-temporal-resolution records of iceberg-rafted debris for the last 8000 years from two sites in the Scotia Sea, which provide a spatially-integrated signal of ice sheet variability in the Holocene. The model and data share variability at centennial and millennial frequencies. The primary conclusion is that fluctuations in AIS discharge caused by relatively-small changes in subsurface ocean temperature can amplify multi-centennial climate variability regionally and globally. A dynamic AIS may have driven climate fluctuations during the Holocene.
|Last Glacial Maximum precipitation pattern in the Alps inferred from glacier modelling|
|investigators:||P. Becker and others|
During the Last Glacial Maximum (LGM), glaciers in the Alps reached a maximum extent well-beyond their current coverage (red line). This study models the ice cap using PISM and parameterized forms of the uncertain LGM precipitation pattern. Constraints on the model come from geomorphological reconstruction of ice extent based on interpretation of moraines, trimlines and erratic boulders. The model is forced using different temperature cooling and precipitation reduction factors. Use of the present-day precipitation pattern leads to a systematic overestimation of the ice cover on the northern part of the Alps relative to the southern part. A more severe decrease in precipitation in the north was required to reproduce the LGM extent. This result supports a southwesterly advection of atmospheric moisture to the Alps, sustained by a southward shift of the North Atlantic storm track during the LGM.
|Repeated large-scale retreat and advance of Totten Glacier indicated by inland bed erosion|
|investigators:||A. Aitken and others|
The East Antarctic Ice Sheet (EAIS) has been more stable than the other major ice sheets for at least a million years. However, for older warm periods there is evidence suggesting collapse of the Totten Glacier, one of several large, ocean-forced outlets of the EAIS, with retreat into interior basins. This paper shows that deep ice-sheet erosion has occurred in two regions, one at the present-day head of the Glacier and the other deep within the subglacial basin behind it. Aerogeophysical data suggest two distinct configurations, a “modern” ice sheet with a near-present-day margin, and a retreated configuration. The transitional region between these is less eroded, suggesting shorter-lived exposure to repeated retreat–advance events.
What is a role for PISM is such work? In this case, ice-sheet modelling indicates that the sector's retreat-driven global sea-level increase could 0.9 meters in modern-like configurations, with a jump up to 2 meters or more if there is a transition to the retreated configuration.
|The influence of continental shelf bathymetry on Antarctic Ice Sheet response to climate forcing|
|investigators:||P. Bart, D. Mullally, and N. Golledge|
|journal:||Global and Planetary Change|
Depth of water on the Antarctic continental shelf is one key factor determining the maximum possible contribution of ice shelf processes (calving and sub-shelf melting/freezing) to ice-sheet mass balance. This paper uses PISM to investigate how shelf-depth changes through geologic time might have affected Antarctic Ice Sheet (AIS) dynamics. Over-deepened, shallow, and intermediate versions of BEDMAP2 bathymetry were combined with unmodified land elevations. For climate forcing similiar to the last glacial cycle, a polar AIS surrounded by shallow and intermediate bathymetries experiences rapid grounding-line advance early during the transition from interglacial to glacial conditions. The corresponding increase in mass is primarily a result of lower calving fluxes from smaller-area ice shelves. In contrast, the currently over-deepened bathymetry in the same forcing generates the expected gradual advance of grounding lines.
|Modelled glacier dynamics over the last quarter of a century at Jakobshavn Isbræ|
|investigators:||I. S. Muresan, S. A. Khan and others|
This paper uses a regional PISM outlet glacier model to simulate the behaviour of Jakobshavn Isbrae (JI) in Greenland. Atmospheric and ocean parameterizations are applied and the model is tuned to reproduce observed frontal changes of JI during 1990–2014. The model simulates two major accelerations (1998,2003-2004) that are consistent with observations of changes in glacier terminus, including retreat, thinning and break-up. A major result is that most of the JI retreat during 1990–2014 is driven by the ocean mass balance, but the glacier's subsequent response is governed by bed geometry. The results also suggest that the variability in modelled horizontal velocities can be regarded as responses to variations in terminus position. This study demonstrates the significant progress that has occurred in modelling temporal variability of outlet glaciers.
|Organization of ice flow by localized regions of elevated geothermal heat flux|
|investigators:||M. L. Pittard, B. K. Galton-Fenzi, J. L. Roberts, C. S. Watson|
|journal:||Geophysical Research Letters|
Geothermal flux is one input to a thermo-mechanically coupled ice flow model such as PISM, with significant impact on both ice softness and basal lubrication. Maps of geothermal flux under present-day ice sheets come from nontrivial geophysical inversions, based on seismic and/or magnetic observations, which generate non-unique and (inevitably) smoothed maps. For example, solutions by Shapiro & Fitzwoller (2004) and Fox Maule et al (2005) are familiar to Antarctic ice sheet modelers. However, measurements on ice-free continents show geothermal flux has strong spatial variations including concentrated highs (hot spots).
A model like PISM can, at least, demonstrate the effects on ice flow of small-spatial-scale variations in geothermal flux. This paper studies the Lambert-Amery glacial system in East Antarctica, where a variety of evidence indicates high heat flux regions of at least 120 mW per square meter. Localized regions of elevated geothermal flux are tested in PISM simulations. The results show significant effects on slow-moving ice, with influence extending both upstream and downstream of the geothermal anomaly. Fast-moving ice is relatively unaffected. This contrast suggests that the effect of geothermal flux on ice softness may dominate the lubrication effect.
|Numerical simulations of the Cordilleran ice sheet through the last glacial cycle|
|investigators:||J. Seguinot, I. Rogozhina, A.P. Stroeven, M. Margold, and J. Kleman|
This paper uses PISM, calibrated against field-based evidence, to reconstruct the Cordilleran ice sheet's history through the last glacial cycle. Until now, geological studies of this major North American ice sheet have lacked ice-sheet-wide spatial reconstructions.
Simulations are driven by time-dependent temperature offsets from six proxy records located around the globe. Although model response to evolving climate forcing is variable, all simulations produce two major glaciations during marine oxygen isotope stages 4 (62.2–56.9 ka) and 2 (23.2–16.9 ka). The timing of glaciation is better reproduced using temperature reconstructions from Greenland and Antarctic ice cores than from regional oceanic sediment cores. During most of the cycle the modelled ice cover is discontinuous and restricted to high mountain areas. However, a central ice dome in the Skeena Mountains persists throughout, and it hosts the last remains of Cordilleran ice into the middle Holocene (6.7 ka).
|Complex Greenland outlet glacier flow captured|
|investigators:||A. Aschwanden, M. Fahnestock, and M. Truffer|
The paper is based on PISM simulations of 600 m grid resolution over the entire Greenland ice sheet. All parts of the ice sheet, and each outlet glacier in particular, see the same physics. The quality of this flow model for 29 major outlet glaciers is assessed by comparison with present-day-observed surface velocities at cross-flow near-ocean profiles, often called “flux gates”. The main result is that the majority of the outlet glaciers show strong correlation between modeled and observed velocity. The paper demonstrates that outlet glacier flow can be captured with high fidelity if ice thickness is well-constrained and if vertical shearing as well as membrane stresses are included in the model. While it is not clear that solving the full-stress configuration would improve the fit, it is clear that the shallow hybrid model can be applied at higher resolution and for longer-duration runs. Inversion of surface properties for individual glaciers is not essential to reproduce the overall flow pattern. Spatial variability in flow can be explained in large part by the spatial variability in ice thickness.
|Investigating uncertainty in the simulation of the Antarctic ice sheet during the mid-Piacenzian|
|investigators:||Q. Yan, Z. Zhang, H. Wang|
|journal:||Journal of Geophysical Research: Atmospheres|
The middle part of the Piacenzian stage of the Pliocene (3.264-3.025 Ma BP) is a recent warm period in Earth's history before Northern Hemisphere glaciation. It was characterized by global sea levels 10-40 m above present, which motivates a focus on the role of the Antarctic ice sheet (AIS). This paper investigates the influence of atmosphere and ocean forcings, topography, model parameters, and model resolution on the modeled AIS. The Norwegian Earth System Model is used to force the Parallel Ice Sheet Model at 15 km resolution. The result is a nearly-collapsed West AIS in the mid-Piacenzian, with no significant retreat of the East AIS. Increased air temperature plays the key role in the overall mass loss of the AIS, but its role is comparable to that of ocean warming in the West.
For the PISM user not already studying this geologic period, this paper is interesting because of its use of Latin hypercube sampling (LHS) of a particular five-dimensional parameter space, denoted (a_snow, a_ice, f_ssa, f_sia, F_melt) in Figure 6 which is reproduced at left. Some results about this parameter space are (see Figure 7):
|Modeling the evolution of the Juneau Icefield between 1971 and 2100 using the Parallel Ice Sheet Model (PISM)|
|investigators:||F. Ziemen, R. Hock, A. Aschwanden, C. Khroulev, C. Kienholz, A. Melkonian, and J. Zhang|
|journal:||Journal of Glaciology|
The large icefields of North America are geometrically-complex. They are exposed to heterogeneous climatic conditions. They are intermediate in size between the individual mountain glaciers and ice sheets which are common modeling targets. And thus they are a good test of the capability and effectiveness of ice dynamics models.
This paper studies the 4000 km^2 Juneau ice field straddling the USA/Canada border using PISM. Perhaps it is no surprise that the modeled outcome of a future warming scenario is (1) loss of much of the glacier volume, and (2) strong dependence of the outcome on the precipitation and surface mass balance inputs. Climatic observations are too sparse and unrepresentative to allow use of interpolated values for climate inputs, though this was attempted. Instead, atmospheric climate model (20 km Weather Research and Forecasting Model) output was used. Simulated and observed surface mass balance gave good agreement only after precipitation adjustments to account for unresolved orographic effects. Under a RCP6.0 emission scenario, the PISM results then project a decrease in ice volume by 58–68% by 2099 compared with 2010. If the modeled 2070–99 climate is held constant beyond 2099, the icefield is eliminated by 2200. With constant 1971–2010 climate, the icefield stabilizes at 86% of its present-day volume.
|Delaying future sea-level rise by storing water in Antarctica|
|investigator:||K. Frieler, M. Mengel, and A. Levermann|
|journal:||Earth System Dynamics|
This paper uses PISM to estimate the time-scale on which the East Antarctic ice sheet (EAIS) can be used as temporary storage of the ocean. The geoengineering goal of such an action would be to reduce sea level everywhere by reducing global ocean volume. The ice dynamics modeling aspect of this investigation suggests that the time-scale before the EAIS starts to “put back” the water, by accelerated flow into the ocean, is shorter than the pure advection result using present-day velocities would suggest. That is, under the schemes tested, a significant kinematic wave propagates faster than the interior ice flow speed. It alters flow rates at the margin through steepening, and this shortens the effective storage time. (The ice delivered at the margin is not the sea water put into the interior.) While ice flow modeling is part of the analysis here, engineering, economic, and ethical factors are also examined. The analysis suggests, for example, that a terawatt of (non-fossil-fuel!) electricity generation capacity–perhaps wind turbines–would be needed to drive pumps to lift the water kilometers vertically and hundreds of kilometers inland.
Geoengineering is a loaded term, of course. Once mentioned, effort is needed to separate the “should” from the “could” of geoengineering proposals. In any case, this paper shows ice sheet models surely contribute to answering science questions with societal and political impacts. See also press releases by the Potsdam Institute and Columbia University, as well as articles in Newsweek, the Washington Post, and the Christian Science Monitor, among other places.
|Collapse of the West Antarctic Ice Sheet after local destabilization of the Amundsen Basin|
|investigator:||J. Feldmann and A. Levermann|
|journal:||Proceedings of the National Academy of Sciences|
This prominent paper uses a PISM simulation to show how a localized destabilization in the Amundsen Sea sector of West Antarctica causes a complete disintegration of the marine ice in West Antarctica. In these 5-km horizontal resolution simulations, the region disequilibrates after 60 y of currently-observed sub-shelft melt rates. Thereafter the marine ice-sheet instability fully unfolds and is not halted by topographic features. In fact, the ice loss in Amundsen Sea sector shifts the catchment's ice divide toward the Filchner–Ronne and Ross ice shelves, which initiates grounding-line retreat there. Our simulations suggest that if a destabilization of Amundsen Sea sector has indeed been initiated, Antarctica will irrevocably contribute at least 3 m to global sea-level rise during the coming centuries to millennia.
See the videos linked in this news item.
Simulated ice extent and velocity in April (left) and November (right) of 1995.
|The Autumn of break-ups: When Jakobshavn Isbrae lost its floating tongue|
|investigators:||A. Aschwanden, M. Fahnestock, M. Truffer, and R. Motyka|
|venue:||2015 AGU Fall Meeting|
Jakobshavn Isbrae, Greenland's fastest-flowing outlet glacier, lost its floating tongue in 1995, an event which is often attributed to changes in ocean temperature. This poster and movie show the results of PISM simulations of this event, based on a step increase from 180 m/yr to 225 m/yr in sub-shelf melt rate during 1995 (Motyka et al. 2011). The simulations are started from reasonably-detailed observations of the 1985 state of the outlet glacier. A high-resolution HIRHAM5 reanalysis (Langen et al. 2015) is used for the atmospheric 1989–2011 climate. The results show that general patterns are simulated correctly, with ice speeds which almost double after break-up of the floating tongue. The timing of the break-up is too early and too fast, but these simulations do not include the “ice rumple” (Echelmeyer et al. 1991), which may add stability to the floating tongue.
|Reconciling the ICE-6G_C reconstruction of glacial chronology with ice sheet dynamics: The cases of Greenland and Antarctica|
|investigators:||G. Stuhne and W. Peltier|
|journal:||J. Geophys. Res.: Earth Surface|
ICE-6G_C ice thickness histories come from present-day uplift rates, exposure-age and radiocarbon dating, the theory of glacial isostatic adjustment (GIA), and a self-consistent theory of sea level. Such reconstructions are independent of ice dynamical approximations. This paper asks whether ICE-6G_C histories for the Greenland and Antarctic ice sheets are compatible with ice dynamics as represented by PISM models. They infer compatibility when uncertainties in mass balance history are taken fully into account. Uncertainties in atmospheric and sub-shelf mass balance since the Eemian (-122ka)—here represented by the SeaRISE paleo-modeling choices, along with simplifications in the PISM ice dynamics model, are carefully considered in a time-dependent inverse-modeling framework. Modeled Holocene shoreline evidence for relative sea level changes, present-day ice velocities, and present-day uplift rates (figure at left), are used to assess the agreement. The magnitudes of the mass balance modifications needed to “nudge” the thicknesses toward ICE-6G_C values, with several relaxation timescales considered, are evaluated as a measure of misfit between the reconstruction and the ice dynamical simulation.
|Linear sea-level response to abrupt ocean warming of major West Antarctic ice basin|
|investigator:||M. Mengel, J. Feldmann, and A. Levermann|
|journal:||Nature Climate Change|
This paper might best be understood as the second of three studies, by these authors, of three Antarctic ice sheet/shelf basins. These basins are among the biggest and, before studying their properties in detail, the most potentially unstable. But the PISM model results do not suggest all of these basins act the same.
The first of these papers, M. Mengel and A. Levermann (2014) "Ice plug prevents irreversible discharge from East Antarctica", suggests that the Wilkes basin is likely to destabilize under sufficient forcing to remove a certain (quantified) amount of near-ocean ice, but that the time scale of destabilization is long. The third of these papers, J. Feldmann and A. Levermann (2015) "Collapse of the West Antarctic Ice Sheet after local destabilization of the Amundsen Basin", which just appeared in November 2015, demonstrates the fast, and very large in magnitude, destabilization of the whole of WAIS from an Amundsen Sea basin forcing. The current paper suggests that, by contrast, the Filchner-Ronne basin is essentially stable in the sense that the forcing dominates its response.
Ocean models do indicate an abrupt intrusion of warm circumpolar deep water into the cavity below the Filchner–Ronne ice shelf within the next two centuries. The basin's retrograde bed slope would allow for an unstable ice-sheet retreat, but the buttressing of the large ice shelf and the narrow glacier troughs tend to inhibit such instability. This paper's main result, as shown in the graph at left, is that buttressing “wins”. Stronger forcing (“shelf melting”) generates greater ice loss, but there is no tipping point as with the other basins. The response is roughly linear.
|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.
|Combustion of available fossil fuel resources sufficient to eliminate the Antarctic Ice Sheet|
|investigator:||R. Winkelmann and others|
Ice sheet scientists have probably asked each other, over a beer or otherwise, how much of the Antarctic Ice Sheet would melt if all fossil fuels were burned up until they were gone. Co-author Ken Caldeira of this paper said in an interview that “I've been wondering about this question for 35 years but was never able to address it.” These authors think that ice sheet science has gotten sophisticated enough to take this question seriously.
Their PISM-based answer is that serious destruction of the ice sheet occurs in the first millenium, at about 3 m sea level rise per century. Actually, PISM is at the end of a chain of models: emission scenarios, CO2 concentrations, and global mean temperature pathways are first combined in an Earth system model (GENIE) and then downscaled to surface and ocean temperature anomalies for Antarctica using scaling factors also from Earth system modeling (ECHAM5/MPIOM). These regional warming scenarios are then used to force PISM. In particular, PISM's positive-degree-day scheme models surface melt and a three-equation model (BRIOS model; Timmerman et al. 2002) describes subshelf melting.
Losses come from a combination of marine-ice-sheet instability and surface elevation versus mass balance feedback. However, in the first century the simulations show the same relatively-modest AIS mass changes as seen in other recent (e.g. IPCC AR5) modeling work, because dynamic losses driven by increasing ocean temperatures are partly offset by increasing snowfall.
|Consistent evidence of increasing Antarctic accumulation with warming|
|investigator:||K. Frieler and others|
|journal:||Nature Climate Change|
The Antarctic ice sheet (AIS) will likely experience higher snow accumulation rates in a warmer climate because warmer air has a higher moisture-holding capacity. This paper quantifies the effect based on ice-core data and paleo-climate simulations, which together show a consistent continental-scale accumulation increase of 5 percent per degree Kelvin. (Note ice-core data and GCM-type modelling results agree for the last deglaciation.) However, some of the mass gain of the AIS is offset by dynamical losses induced by accumulation. This is where PISM plays a supporting role in the paper. PISM results were used to generate a response function allowing projections of sea-level fall in terms of continental-scale accumulation changes. In PISM the accumulation changes can and do compete with changes in surface melting and with dynamical losses induced by mechanisms like ocean interaction and sliding.
|Simulating the Antarctic ice sheet in the late-Pliocene warm period: PLISMIP-ANT, an ice-sheet model intercomparison project|
|investigator:||B. de Boer and others|
Understanding the behaviour of ice sheets during warm intervals in Earth history is of fundamental importance for understanding future climate change. The late Pliocene warm period (3.264–3.025 Ma BP) serves as an analogue for future climates. Although Pliocene ice locations and extents are still poorly constrained, a significant contribution to sea-level rise is expected from the Antarctic ice sheets based on sea-level reconstructions. All six included ice-sheet models in this paper, including PISM v0.6, used the shallow ice and shelf approximations for the complete Antarctic domain, including grounded and floating ice, for both modern control and Pliocene ice sheet runs, in five sensitivity experiments. The models simulate a comparable present-day ice sheet, considering the models use their own parameter settings. For the Pliocene, all six models have difficulty simulating significant retreat or re-advance of the East Antarctic ice grounding line, which is thought to have happened for the Wilkes and Aurora basins. The specific sea-level contribution of the Antarctic ice sheet at this point cannot be conclusively determined. Improved grounding line physics is apparently needed.
|Mass-conserving subglacial hydrology in the Parallel Ice Sheet Model v0.6|
|investigator:||E. Bueler and W. van Pelt|
|journal:||Geoscientific Model Development|
This paper describes and tests a major extension of PISM, introduced in v0.6 and fully-supported in v0.7, a two-horizontal-dimension subglacial hydrology model which combines till with a distributed system of water-filled, linked cavities. This sub-model accomplishes three specific goals: (1) conservation of the mass of water, (2) simulation of spatially- and temporally-variable basal shear stress from physical mechanisms based on a minimal number of free parameters, and (3) convergence under grid refinement. Besides a broad approach to the source of the model equations and detailed attention to the implementation and testing of the numerics, this paper demonstrates the model at scale by modeling the whole Greenland ice sheet at 2 km horizontal resolution, with one million nodes in the hydrology grid. But the model is far from complete. It both takes a very conservative approach to coupling hydrology to a model for basal shear, and it does not include the physics which determines the location and evolution of subglacial conduits.
|Selective erosion beneath the Antarctic Peninsula Ice Sheet during LGM retreat|
This paper uses PISM to investigate how the last glacial maximum Antarctic Peninsula Ice Sheet might have modified its bed both at maximum extent and during progressive grounding line retreat. The work exploits high-resolution whole-Antarctic modelling by the same author (Golledge et al 2013, Golledge et al 2014). PISM results are post-processed to compute an erosion potential which is proportional to the product of modeled basal shear stress and sliding velocity. The results show that peak subglacial erosion rates are preferentially located in areas of convergent flow and where horizontal strain rates are highest, leading to deepening of subglacial basins in such locations. Because the ice sheet selectively erodes its bed beneath outlets, over successive glacial cycles erosional deepening may accelerate the retreat of the ice sheet margin during periods of rising sea level.
|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.