PISM Applications of the Month
Since September 2011 we have featured one PISM application per month, either a published article or a presented poster, on the .
This is the archive.
Also see the .
investigators:
and others
Glaciologists know little about the conditions at the beds of glaciers because these places are so hard to observe directly.
Nonetheless most PISM runs need and use a model of the basal conditions.
This includes a model of the basal topography, derived from ice-penetrating radar (though necessarily more than the “raw” reflections), and a model of the relation between basal sliding velocity and shear stress (e.g. a power law with uncertain parameters).
Inverse modeling can examine the relationship between available surface observations and the conditions at the base.
This paper assesses the intermediate quantities in an inverse modeling exercise.
Such a calculation generates a (forward) model result for which a norm of the difference between the observations and the model result is minimized in some sense.
This difference itself, the residual, contains information which is lost in computing its norm.
These authors examine the residual, and other quantities as well, for meaning.
There are several types of errors in glaciological inverse modeling which generates basal shear stress.
These include observation errors (in the surface elevation and velocity), geometry errors, ice temperature errors, and “model errors”.
The last are particularly interesting in the PISM a question addressed by this paper is “what are the manifestations of the simplifications in PISM in the sense of residual features?”
A particular focus is on the difference between upstream and near-terminus regions of a deep-trough outlet glacier in the Greenland ice sheet.
& Ed Bueler
investigators:
When thermodynamics are coupled to basal sliding through the production of liquid water in a modeled basal till layer, PISM may exhibit cyclic, surge-like behavior.
This paper studies such ice-discharge instabilities in PISM for a buttressed ice-sheet-stream-shelf system.
When present the simulated surge cycles, which are believed to be fundamentally physical, have 1000 to 5000 year periods in the studied parameter ranges.
This work extends prior work on cyclic PISM behavior (, ) to marine ice sheet conditions, but the 1 km resolution model here is significantly more complete than in those studies.
It includes stable0.7 version implementations of enthalpy-based thermodynamics, an improved till-layer hydrology model (), a sub-grid scheme at the grounding line (), and a buttressed ice shelf.
The surface conditions are all constant, independent even of surface elevation, so the cyclic behavior arises purely internally in the ice and the basal layer.
This paper includes a new and careful analysis of cyclic behavior, identifying three competing time-scales controlling such cycles.
Results suggest that ice sheets of medium thickness may be more susceptible to surging than thin or thick ones.
The mechanisms studied here may play a role in Heinrich events of the late-LGM Laurentide Ice Sheet and/or in ice-stream shutdown and reactivation observed in West Antarctica.
This is the definitive study of the PISM parameter space for modeled ice stream-shelf systems.
& Ed Bueler
investigators:
and others
Whether a “tipping point” will be reached in the next few centuries, in the dynamics of various parts of the Antarctic ice sheet (AIS) as it responds to a warming world, is the most difficult prediction question.
It is an extreme challenge to models.
How complete a model must be, so as to resolve the relevant behavior of the (real) dynamical system, is unknown.
A negative result from modeling (i.e. no tipping points seen in model runs) has minimal value because a more-complete, or merely different, model may show a tipping point.
Insisting that a model include all possibly-relevant mechanisms is unfeasible to the point of silliness.
Tipping-point questions can be asked about the past states of the AIS, though using existing observations to resolve these questions is extraordinarily difficult.
This paper uses GCM/RCM and ice-sheet modeling to recreate conditions of the early to mid-Pliocene when atmospheric CO2 concentrations were similar to present day.
The focus of PISM application is on modeling grounding-line motion, ice shelf basal melt near the grounding line, and basal sliding.
A novel method here, relative to other AIS modeling literature, is an autocorrelation analysis of time series for (modeled) ice mass to find early-warning signs of impending tipping points.
A precursor to tipping-point-type destabilization, proximately by classic marine-based ice-sheet retreat, appears to be prolonged surface lowering in response to warm air temperatures.
& Ed Bueler
investigators:
and others
After the surface slope and the ice thickness, which determine the effect of the body force of gravity (the driving stress), the basal thermal state of an ice sheet is the most important boundary condition determining the stresses on a flowing ice sheet.
The basal thermal regime is, however, the integrated effect of flow history.
To model it one must track viscous dissipation of flow, sliding friction, and geothermal input, and compute the balance.
Whether the balance is just above (thawed, wet, and weak) or just below (frozen and strong) the freezing point is then critical to dynamical boundary condition.
The thermal state can only be observed directly at boreholes, which are few and biased toward cold, near-divide locations.
To determine the basal temperature and melt rates for the Greenland Ice Sheet (GrIS), this paper combines a survey of observations, including borehole and less-direct observations (surface velocity, surface texture, and radiostratigraphy), with analysis of 3D thermomechanically-coupled model results from eight different models, including PISM, which participated in the SeaRISE assessment process.
Such a synthesis has not been attempted before.
Its conclusions about the distribution of melting conditions, such as the map at left, are much more likely to withstand scientific scrutiny than any individual model results or point measurements.
The paper also delimits a specific large portion of GrIS, about one third by area, where additional observations would most improve knowledge of the basal thermal state.
& Ed Bueler
investigators:
Fluid dynamics experiments are often only possible because theory allows rescaling a flow to fit into a laboratory.
The target flow and its laboratory analogue must be dynamically-similar in the sense of dimensional analysis.
Engineers study rivers and air flow around airframes, and we can generate laboratory models of ice flow, by such “similitude”.
For example, the
includes a simulation of a , one which is similar to a grounded ice sheet.
This paper gives a framework for dynamical scaling of marine ice sheets.
For the SSA dynamical model they derive scaling conditions between the vertical and horizontal lengths,
the relevant physical parameters, and the response time.
Timing of grounding-line changes is a particular focus.
These relationships give parameter sets that leave the ice-sheet geometry unchanged, effectively rules for the design of numerical and laboratory experiments.
& Ed Bueler
investigators:
and others
Glacial erratic boulders in the Rhone Valley north of the Alps, which have intrigued scientists for two centuries, have characteristic lithologies which can be matched to source regions in the mountains. This study uses their locations, plus the maximum extent of the ice sheet (e.g. moraine mapping), to constrain a PISM model () of the ice flow around the time of the Last Glacial Maximum (LGM, ~24ka b.p.).
The 1 km grid, physically-comprehensive model, which includes membrane-stress-balanced sliding, polythermal thermodynamics, and earth deformation, generates the evolving ice geometry and velocity fields.
The boulders are modeled as tracers within the time-dependent flow.
The dominant flow fields in the Valley, including the distinctive erratics signature of the northeastern Solothurn ice sheet lobe, turn out to be insensitive to sliding parameters and bed elevation in the model.
Instead, the authors are able to infer the paleo-climatic conditions responsible for the transport routes of the erratics.
They show that only one tested precipitation pattern allows boulders to be (correctly) diverted into the Solothurn lobe during the LGM. This precipitation pattern supports, but suggests additional detail in, an existing theory of preferential moisture advection during the LGM.
A spectacular video of a full simulation of the ice sheet, over the entire 120ka ice age cycle, .
& Ed Bueler
investigators:
and others
Sometimes what you need from an ice sheet model is a context for your measurements.
In this paper ice microstructure (crystal orientation, grain size/shape), visual stratigraphy, and borehole tilt were measured in the EDML core drilled in Dronning Maud Land, Antarctica.
Microstructures reflect the dynamic conditions of the location as well as its thermodynamic history, and determine rheological effects.
PISM was used to generate the flow field and temperatures at depth at the ice core site from the large-scale geometry of the ice sheet and surface inputs.
The details of structural observations were evaluated using principal strain rates from PISM, with comparison to with borehole logging data. This comparison confirms the division of the core into five depth regions (figure) and in turn provides a wider view of the ice sheet.
& Ed Bueler
investigators:
and others
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- 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.
& Ed Bueler
investigators:
and others
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 .
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.
& Ed Bueler
investigators:
and E. Tziperman
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.
& Ed Bueler
investigators:
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 , while on the other the modeled AIS meltwater is used to force the .
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.
& Ed Bueler
investigators:
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.
& Ed Bueler
investigators:
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.
& Ed Bueler
investigators:
, D. Mullally, and N. Golledge
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.
& Ed Bueler
investigators:
, 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 .
The model simulates two major accelerations (04) 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
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.
& Ed Bueler
investigators:
, B. K. Galton-Fenzi, J. L. Roberts, C. S. Watson
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.
& Ed Bueler
investigators:
, 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).
& Ed Bueler
investigators:
, 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.
& Ed Bueler
investigators:
, Z. Zhang, H. Wang
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):
Modeled total ice volume is strongly controlled by the enhancement factor for the SIA-modeled part of the flow (f_sia).
Total ice area is significantly controlled by a_snow, a_ice, and f_ssa.
Other correlations in ice volume and area are hard to detect from the LHS results.
& Ed Bueler
investigators:
, R. Hock, A. Aschwanden, C. Khroulev, C. Kienholz, A. Melkonian, and J. Zhang
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
climate, the icefield stabilizes at 86% of its present-day volume.
& Ed Bueler
investigator:
, M. Mengel, and A. Levermann
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
and , as well as articles in , the , and the , among other places.
& Ed Bueler
investigator:
and A. Levermann
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 .
& Ed Bueler
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:
, M. Fahnestock, M. Truffer, and R. Motyka
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.
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
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.
& Ed Bueler
Simulated uplift rates compared with GPS observations at 42 Antarctic stations.
investigators:
and W. Peltier
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.
& Ed Bueler
investigator:
, J. Feldmann, and A. Levermann
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, , 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, , which , 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.
& Ed Bueler
investigator:
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.
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 , 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.
& Ed Bueler
investigator:
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
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 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.
& Ed Bueler
investigator:
and others
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.
& Ed Bueler
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.
& Ed Bueler
Thickness of subglacial water in “routing” model.
investigator:
and W. van Pelt
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.
& Ed Bueler
Erosion potential shown in color.
investigator:
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 (, ).
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.
& Ed Bueler
Click the image to go to The Cryosphere page.
Interaction of marine ice-sheet instabilities in two drainage basins: simple scaling of geometry and transition time
investigators:
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.
& Ed Bueler
Click the image to go to Journal of Quaternary Science page.
Testing the sensitivity of the East Antarctic Ice Sheet to Southern Ocean dynamics: past changes and future implications
investigators:
, C. Turney, K. Meissner, N. Golledge, P. Spence, J. Roberts, M. England, R. Jones, and L. Carter
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.
& Ed Bueler
Click the image to go to Nature Communications journal page.
Antarctic contribution to meltwater pulse 1A from reduced Southern Ocean overturning
investigators:
, L. Menviel, L. Carter, C. J. Fogwill, M. H. England, G. Cortese, and R.
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.
& Andy Aschwanden
Click the image to go to The Climate of the Past article page.
Coupled ice sheet–climate modeling under glacial and pre-industrial boundary conditions
investigators:
and others
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.
& Ed Bueler
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:
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.
& Ed Bueler
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:
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.
& Ed Bueler
Click the image to go to Geoscientific Model Development journal page.
A system of conservative regridding for ice–atmosphere coupling in a GCM
investigators:
, S. Nowicki, M. Kelley, and G. A. Schmidt
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.
& Constantine Khroulev
Click the image to go to Nature Climate Change journal page.
Ice plug prevents irreversible discharge from East Antarctica
investigators:
and A. Levermann
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.
& Ed Bueler
Click the image to go to The Cryosphere journal page.
Changing basal conditions during the speed-up of Jakobshavn Isbrae, Greenland
investigators:
, 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 (,
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.
& Ed Bueler
Click the image to go to The Cryosphere journal page.
The effect of climate forcing on numerical simulations of the Cordilleran ice sheet at the Last Glacial Maximum
investigators:
, 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.
& Ed Bueler
Click the image to go to the Earth Planet. Sci. Lett. page.
Spontaneous ice-front retreat caused by disintegration of adjacent ice shelf in Antarctica
investigators:
and A. Levermann
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 , 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.
& Ed Bueler
Click the image to go to the Journal of Glaciology page.
Resolution-dependent performance of grounding line motion in a
shallow model compared with a full-Stokes model according to the
MISMIP3d intercomparison
investigators:
, 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.
& Ed Bueler
Click the image to go to the EGU Outstanding Student Poster (OSP) Awards 2013 page.
Paleo-glaciations of the Shaluli Shan, southeastern Tibetan Plateau
investigators:
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.
& Constantine Khroulev
Click the image to go to the Palaeogeography, Palaeoclimatology, Palaeoecology page.
Mountain building and the initiation of the Greenland Ice Sheet
investigators:
, J. Bonow, P. Langen, P. Japsen, and C. Hvidberg
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.
& Constantine Khroulev
Click the image to go to the Quaternary Science Reviews page.
Glaciology and geological signature of the Last Glacial Maximum Antarctic ice sheet
investigators:
and 12 others
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.
& Ed Bueler
Click the image to go to the GRL page.
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
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.
& Ed Bueler
Click the image to go to the University of Maine news item.
Increasing the Scalability of PISM for High Resolution Ice Sheet Models
investigators:
and T. Morey
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.
& Constantine Khroulev
Click the image to go to the paper at The Cryosphere website
An iterative inverse method to estimate basal topography and initialize ice flow models
investigators:
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 Nordenski?ldbreen, 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.
& Ed Bueler
Click the image to go to the paper at the J. Geophys. Res. (Earth Surface) website
Insights into spatial sensitivities of ice mass response to environmental change from the SeaRISE ice sheet modeling project II: Greenland
investigators:
and others
This second paper explores Greenland climate scenarios and forcing experiments from the 31 member
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.
& Ed Bueler
Click the image to go to the paper at the J. Geophys. Res. (Earth Surface) website
Insights into spatial sensitivities of ice mass response to environmental change from the SeaRISE ice sheet modeling project I: Antarctica
investigators:
and others
Antarctic climate scenarios and forcing experiments from the 31 member
project are applied to six three-dimensional thermomechanical ice-sheet models, including a PISM model lead by .
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.
& Ed Bueler
Click the image to go to the paper at The Cryosphere.
Hindcasting to measure ice sheet model sensitivity
investigators:
A. Aschwanden, G. A?algeirsdóttir, 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
for simulations of the Greenland ice sheet.
The results are compared to observations in the
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.
& Ed Bueler
Click the image to go to the paper at J. Glaciol.
Grounding-line migration in plan-view marine ice-sheet models: results of the ice2sea MISMIP3d intercomparison
investigators:
F. Pattyn and others, including
and M. Huetten
These are the results of a comparison between plan-view marine ice-sheet models, .
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.
& Ed Bueler
Click the image to get the PDF (17.5 MB).
Changing basal conditions during the speed-up of Jakobshavn Isbrae, Greenland
investigators:
, Martin Truffer, and David Maxwell, University of Alaska Fairbanks
conference:
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.
& Andy Aschwanden
Click the image to get the PDF (1 MB).
Modelling the outlet glaciers terminating in Godthab fjord
investigators:
and Dorthe Dahl-Jensen, Centre for Ice and Climate, Copenhagen
conference:
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
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?
& Ed Bueler
Click the image to get the PDF (4 MB).
Are the simulated climatic and dynamic mass losses of the Greenland Ice Sheet decoupled during the next 100 years?
investigators:
Gu?finna A?algeirsdóttir and Andy Aschwanden
conference:
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.
& Andy Aschwanden
Click the image to go to the J Glaciol page.
Ice-sheet model sensitivities to environmental forcing and their use in p}