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Ice Sheet for PMIP3/CMIP5 simulations

This page synthesises the ongoing work for the definition of a new ice-sheet reconstruction to be used in LGM simulations for PMIP3/CMIP5.

Use the discussion panel at the end of the page for comments

Introduction

Several PMIP participants are in favour of using a new ice-sheet reconstruction that better matches the geomorphologic and glaciological constraints than ICE-5G used in PMIP2. We plan thus to have - a reference ice-sheet for the core simulations - alternatives for groups interested to run sensitivity experiment in order to discuss the sensitivity of the model results to the uncertainties in boundary conditions.

We are now considering the possibility to use a new ice-sheet reconstruction proposed by Lev Tarasov and/or Dick Peltier (see text below). The final decision on the ice-sheet will be taken in October. It will depend on what is available at that time, quality checking and climate model constraints. We will involve several PMIP participants in the process, and you are all invited to comment on the proposition in the discussion panel (at the bottom of this page). It is still possible that we have to find an alternative and less rigorous option by the end of September, which is the deadline to finalize the boundary condition for CMIP5.

From what is ready today, you can find below

  • a description of what is going on for the new reconstruction using both ICE-5G constraints and a glaciological model (the Lev-MOCA proposition discussed for PMIP3 deglaciation sub-project)
  • a new reconstruction using a new version of Dick's model (ICE-6G)

Please have a look at these files and make all the comments that can help us to finalise the CMIP5 boundary conditions rapidly.

Details on the proposed LGM and deglacial ice-reconstructions

Comparing the different reconstructions

Plots are corrected. Please download again (fuyuki, Nov 13 2009). The latest file names are gnh5.pdf and gaa5.pdf

The following plots were generated using the latest versions of the reconstructions available (inserted by Fuyuki Nov 13 2009).

  • Combined Surface Altitude and Fraction of Grid Cell Covered with Glacier
    • for ANU ice model (2009) incl. British area.
    • for ICE-6G v1.02
    • for GLAC-1 nn454 model for North America, and ne8234 for Eurasia
    • for GLAC-1 nn9014 model for North America, and ne8234 for Eurasia

The following plots have been prepared by Ayako Abe-Ouchi and Saito Fuyuki (put online, 11/13/09 @ 13h03)):


The following plots were generated using the latest versions of the reconstructions available to the persons who made the plots and may be more up-to-date than the plots available in the sections below. Please try to use the most recent plots when commenting the reconstructions!

The following 2 files combine some of the information available in the files below. If you are short on time, make sure you look at least at these 2 files:
PMIP3_ice-sheet_comparison_NH.pdf
PMIP3_ice-sheet_comparison_SH.pdf

GLAC-1

  • Surface Altitude, using nn454 model ensemble for North America, GrbB model for Greenland, and ne8234 for Eurasia

GLAC1_nn454_ne8234.pdf (generated by Jean-Yves Peterschmitt, put online, 10/29/09 @ 15h30)

ICE-6G v1.02

  • Surface Altitude and Fraction of Grid Cell Covered with Glacier
    • for ICE-6G v1.02
    • and for GLAC-1 nn442 model for North America, GLAC-1 nn8191 for Eurasia, and ICE-6G v1.02 for Antarctica

ICE-6G_compare_v1.02_GLAC-1.pdf (generated by Jean-Yves Peterschmitt, put online, 10/29/09 @ 16h40)

Figure 01 ⇒ [Var = orog] ice6g_v1.02 @ 21.0k
Figure 02 (polar) ⇒ [Var = orog] ice6g_v1.02 @ 21.0k
Figure 03 ⇒ [Var = orog] ice6g_v1.02 @ 21.0k - ice6g_v1.02 @ 00.0k
Figure 04 (polar) ⇒ [Var = orog] ice6g_v1.02 @ 21.0k - ice6g_v1.02 @ 00.0k
Figure 05 ⇒ [Var = orog] ice6g_v1.02 @ 21.0k - ice6g_v1.0 @ 21.0k
Figure 06 (polar) ⇒ [Var = orog] ice6g_v1.02 @ 21.0k - ice6g_v1.0 @ 21.0k
Figure 07 ⇒ [Var = sftgif] ice6g_v1.02 @ 21.0k
Figure 08 (polar) ⇒ [Var = sftgif] ice6g_v1.02 @ 21.0k
Figure 09 ⇒ [Var = sftgif] ice6g_v1.02 @ 21.0k - ice6g_v1.02 @ 00.0k
Figure 10 (polar) ⇒ [Var = sftgif] ice6g_v1.02 @ 21.0k - ice6g_v1.02 @ 00.0k
Figure 11 ⇒ [Var = sftgif] ice6g_v1.02 @ 21.0k - ice6g_v1.0 @ 21.0k
Figure 12 (polar) ⇒ [Var = sftgif] ice6g_v1.02 @ 21.0k - ice6g_v1.0 @ 21.0k
Figure 13 ⇒ [Var = orog] ice6g_v1.02_nn442_8191b @ 21.0k
Figure 14 (polar) ⇒ [Var = orog] ice6g_v1.02_nn442_8191b @ 21.0k
Figure 15 ⇒ [Var = orog] ice6g_v1.02_nn442_8191b @ 21.0k - ice6g_v1.02_nn442_8191b @ 00.0k
Figure 16 (polar) ⇒ [Var = orog] ice6g_v1.02_nn442_8191b @ 21.0k - ice6g_v1.02_nn442_8191b @ 00.0k
Figure 17 ⇒ [Var = orog] ice6g_v1.02_nn442_8191b @ 21.0k - ice6g_v1.0 @ 21.0k
Figure 18 (polar) ⇒ [Var = orog] ice6g_v1.02_nn442_8191b @ 21.0k - ice6g_v1.0 @ 21.0k
Figure 19 ⇒ [Var = sftgif] ice6g_v1.02_nn442_8191b @ 21.0k
Figure 20 (polar) ⇒ [Var = sftgif] ice6g_v1.02_nn442_8191b @ 21.0k
Figure 21 ⇒ [Var = sftgif] ice6g_v1.02_nn442_8191b @ 21.0k - ice6g_v1.02_nn442_8191b @ 00.0k
Figure 22 (polar) ⇒ [Var = sftgif] ice6g_v1.02_nn442_8191b @ 21.0k - ice6g_v1.02_nn442_8191b @ 00.0k
Figure 23 ⇒ [Var = sftgif] ice6g_v1.02_nn442_8191b @ 21.0k - ice6g_v1.0 @ 21.0k
Figure 24 (polar) ⇒ [Var = sftgif] ice6g_v1.02_nn442_8191b @ 21.0k - ice6g_v1.0 @ 21.0k

ANU ice model

Plots are modified. Please download again (fuyuki, Nov 13 2009)

  • Combined Surface Altitude and Fraction of Grid Cell Covered with Glacier
    • for ANU ice model (2009)
    • for ICE-6G v1.02
    • for GLAC-1 nn454 model for North America, and ne8234 for Eurasia
    • for GLAC-1 nn9014 model for North America, and ne8234 for Eurasia

The following plots have been prepared by Ayako Abe-Ouchi and Saito Fuyuki (put online, 11/13/09 @ 13h03)):

Lev-MOCA

Please check the Comparison section above for the latest plots

Northern Hemisphere (GLAC-1)

The Greenland model is from Tarasov and Peltier (2002 and 2003), a glaciological model with hand-tuned climate adjustments to enforce fit to Relative Sea-Level (RSL) records and the GRIP borehole temperature record. It was also validated against observed rated of present day uplift for 3 sites and against GPS measurements for horizontal ice surface velocity. A variant of it is the Greenland component of ICE-5G and ICE-6G.

The North American and Eurasian reconstructions are objective Bayesian calibrations of the MUN/UofT glacial systems model. The latter incorporates a 3D thermo-mechanically coupled (shallow) ice-sheet model, with permafrost resolving bed-thermal model, asynchronously coupled down-slope surface drainage/lake depth solver, and various other components such as a thermodynamic lake ice, sub-glacial till-deformation, bouyancy and temperature dependant calving law, ice-shelf represention, …, some of which are described in Tarasov and Peltier, QSR 2004, and Nature 2005, and a more complete description is currently being written up). The visco-elastic bedrock response uses either the VM2 (as used in ICE-5G) or VM5a (used in ICE-6G) earth rheologies. Relative Sea Level is computed using a gravitationally self-consistent formalism similar to that of Peltier, except for an eustatic approximation for dealing with changing ocean masks and the lack of accounting for rotational effects (which are mostly significant for far-field RSL records, ie records that do not locally constrain ice load history).

Climate forcing involves an interpolation between present day observed climatologies and the set of highest resolution LGM fields from PMIP I and II data sets. The interpolation is weighted according to a glaciological inversion of the GRIP record for regional temperatures over the last glacial cycle.

The calibration involves approximately 30 (currently 36 for North Am, 29 for Eurasia) ensemble parameters to capture uncertainties in deglacial climate and ice dynamics. The majority of these parameters are used for the climate forcing, including weighting the inter-model (ie between PMIP models) EOFs for LGM monthly precip and temperature, regional desert elevation effects, and LGM atmospheric lapse rate. Other ensemble parameters adjust calving response, effective viscosity of subglacial till, strength of margin forcing, and flow parameters for ice-shelves. Model runs are forced to stay within uncertainties of the independently derived ice margin chronologies (Dyke, 2004 for North Am, Gyllencreutz et al, in preparation for Eurasia).

Model runs cover a full glacial cycle. North America and Eurasia are calibrated separately. Calibration targets include a large set of RSL observations, geologically-inferred deglacial ice-margin chronologies, and geodetic constraints. For the case of North America, the calibrated ensemble is further scored with respect to strand-lines (paleo lake level indicators) and Marine Limit (maximum level of marine inundation) observations. A key point is that model runs are penalized in proportion to the amount of margin forcing required. So the calibration is directed towards a climate forcing that is consistent with the margin chronology.

The model was calibrated using the ICE4G ice load reconstruction for Antarctica and the VM2 earth rheology because the ICE6G Antarctic chronology and VM5a earth model along with a much expanded geodetic dataset were provided by Dick Peltier only in early September, which left too little time to recalibrate the models. There is the added issue that the ICE6G Antarctic chronology lacks error bars. The expanded geodetic data-set for North America included significant revisions to the previous geodetic constraints. This along with the significant reduction in LGM ice volume in ICE6G Antarctic as compared to ICE4 and 5G rendered a significant misfit with the far-field Barbados RSL record. With the limited time, a somewhat blind and largely random 2000 member ensemble was generated along with a rerun of the best 300 previously calibrated parameter sets and some 200 attempts at hand-tuning. nn450 is the weighted distribution of 7 model runs that passed certain hard threshold constraints. nn9021 is the best (though “best” depends to a certain extent on the weighting between various constraints) single run from the previous calibration and nn445 is the weighted ensemble mean for that previous calibration.

The Eurasian calibration did converge, and aside from issues with the Norwegian fjords (the latter are also a problem for ICE6G), the calibration was generally successful. nn8234 is one of the best runs with the largest 26ka RSL contribution to the Barbados record. A single run was chosen to ensure consistency between drainage fields and the surface topography. The mean distribution for the calibration can be made available upon request.

In summary, the GLAC-1 submission provides a set of glaciological models that are derived from a plausible climate forcing based on PMIP1 and PMIP2 results for LGM and that fit independently derived ice margin chronologies. This provides strong constraints throughout deglaciation. For Eurasia, the smaller set of constraints (among other more speculative reasons) resulted in a successful calibration that reasonably well covered the available constraint set. North America has a much larger and much more diverse set of constraints (and I suspect a much more complicated ice/climate interaction history), so the calibration has never been able to fully satisfy the whole set of constraints (strandlines are for instance a challenge to fit given their high sensitivity to drainage choke point elevations).

Unfortunately, these glaciological models in combination with the ICE-6G chronology for Antarctica (and Patagonia) and Dick Peltier's VM5a earth rheology have at best a weak fit to the the LGM segment of the Barbados record. There is a significant tradeoff between Barbados fit and fit to other constraints. What is unclear at this stage is the extent to which this is due to deficiencies in the glaciological models, to problems with the ICE-6G Antarctic ice chronology, or possibly with inferred uncertainties in the Barbados record and with the VM5a earth rheology.

One possibility for resolving Barbados, is to take the 1.5 sigma upper limit of the previously calibrated ensemble for North America which almost reaches the inferred Barbados record for 26 to 21 ka. Dick Peltier and Rosemarie Drummond will cross-check this dataset. The problem with using ensemble bounds is that this is no longer a glaciologically self-consistent model and RSL fits have also deteriorated.

References
Data
Note that the North Am model is calibrated with a stub Greenland by Ellesmere. When a final model is chosen, the North American and Greenland models can be easily melded.

Also, please note, that none of the above have yet been published, so these files are provided for discussion and consideration. Any fields and data not selected as a PMIP boundary condition are not to be used for research without prior consent.
  • Get in touch with Lev Tarasov or Jean-Yves Peterschmitt if you want to download the following NetCDF files:
    • NetCDF of 21 to 10ka surface elevation, thickness, basal velocities for the nn9021 North Am model and nn8234 Eurasian model: NAnn9021.nc, EAnn8234.nc
    • NetCDF of 21 to 10ka for nn445 ensemble means and 1.5 sigma range: NAnn445.nc
  • Other data available upon request from Lev Tarasov
Plots
  • RSL plots for North Am and Eurasia
    • North Am: (with nn8234 Eurasia)
      rslGLAC1RSLcompAc.pdf nn9021 (as calibrated, ICE4G Antarctica, VM2),nn9021 with ICE6G Antarctica/VM5a, nn7280, nn445

      rslGLAC1RSLcompBc.pdf nn9021, nn445, nn445+1sigma, nn445-1sigma
    • Eurasia:
      rslGLAC1RSLcompEc.pdf nn8234 (with nn9021), nn8234(with nn7280), nn8191(with nn9021)
    • Barbados: GLAC1RSLcompBbarbc.pdf (note will slightly over predict barbados RSL drop around LGM due to lack of rotational effects on RSL calculation)

Ice volume

The following table has been supplied by Lev Tarasov. It compares the ice amounts in the various regions in terms of eustatic sea level impact inferred by assuming the ocean area to remain fixed to the modern area (ice amount relative to now in m of ocean assuming ocean area = 360768576 km2).

GLAC-1
21ka
North America
(nn454)
76.6
Eurasia
(nn8234)
14.0

Southern hemisphere

Glaciological constraints have shown that the ICE-5G reconstruction was too high in East Antarctica. Several tests are now made using a revised version of ICE-5G (VM2=ICE-6G) to correct this aspect while keeping the global on sea level and other geomorphological constraints. Dick Peltier’s group is working now on smoothing the refined model and taking the extension of the loading of the shelf out to a distance from the present coast consistent with a specific bathymetry contour. The final version should be ready by September.

ICE-6G

Please check the Comparison section above for the latest plots

Using new calibrations and a revised version of his model, Dick Peltier proposes a revised version of the ice-sheet that should better match the different paleo data. The following figures are based on data supplied by Dick Peltier and Rosemarie Drummond (Sept 4th 2009 version). They are provided for evaluation and test, and do not correspond to the “official” boundary condition to be used for LGM simulations in CMIP5.

References

Note: get in touch with Jean-Yves Peterschmitt in you need a copy of the following papers.

  • Peltier, W.R., 2009, Closure of the budget of global sea level rise over the GRACE Era: The importance and magnitudes of the required corrections for global glacial isostatic adjustment. Quat. Sci. Rev. 28, 1658-1674.
  • Donald F. Argus and W. R. Peltier, 2009. Constraining models of postglacial rebound using space geodesy: A detailed assessment of model ICE-5G (VM2) and its relatives, Geophysical Journal International, submitted.
  • W.R. Peltier and R. Drummond, 2008. Rheological stratification of the lithosphere: A direct inference basd upon the geodetically observed pattern of the glacial isostatic adjustment of the North American continent. geophys. res. Lett., vol. 35, L16314, doi:10.1029/2008GL034586.
  • W. R Peltier, D. F. Argus, R. Drummond, R Gyllencreutz, J. Mangerud, J-I Swensen, and O. S. Lohne, Space Geodesy Constrains Ice-Age Terminal Deglaciation, Nature, submitted

Data

You should download the ice-ng_compare_polar.pdf file if you want to look at detailed graphics comparing the different versions of ICE-nG (56 Mb, 77 pages)

Notes:

  • Get in touch with Jean-Yves Peterschmitt if you want to download the netcdf files used to generate the figures below
  • Click on a figure to get the full size version
  • You can compare the figures with the ICE-5G versions available on the PMIP2 web site

 0 K 6 K 21 K 26 K

Ice volume

The following table has been supplied by Rosemarie Drummond. It compares the ice amounts in the various regions in terms of eustatic sea level impact inferred by assuming the ocean area to remain fixed to the modern area (ice amount relative to now in m of ocean assuming ocean area = 360768576 km2).

ICE-4G
final (1)
ICE-5G
v1.2 (2)
ICE-6G
v1.0
21k 26k 21k 26k 21k 26k
North America
(incl Innuit area)
64.24 54.92 81.47 83.71 79.82 88.14
Greenland & Iceland 6.38 5.43 2.49 2.45 2.36 2.34
Fennoscandia 10.39 8.91 11.19 11.79 10.22 12.31
Barents / Kara Seas 14.05 12.26 8.43 9.29 7.26 9.10
UK 0.42 0.35 1.48 1.65 0.58 0.82
Patagonia 0.55 0.47 0.55 0.55 0.83 0.87
West Antarctica 9.74 8.33 9.68 9.68 11.79 11.79
East Antarctica 8.35 7.12 8.36 8.36 1.45 1.44
Total 114.12 97.79 123.63 127.48 114.31 126.81
ice-shelves (3) 2.80 3.00
  • (1) The version of the ICE-4G (VM2) model for which data are provided is not exactly the version that was employed in the original PMIP project but rather one from which a significant amount of Antarctic ice was simply eliminated (version employed for illustrative purposes in Peltier's paper for the Mt Hood volume of QSR).
  • (2) The version of ICE-5G used for PMIP2/MOTIF was v1.1
  • (3) The calculation only cares about grounded ice, so where it detects that the ice-model has ice that is below the predicted sea-level, it deems the ice to be melted and added to the ocean. The ice may be melted or be part of an ice-shelf eg the Ronne, Ross ice-shelves. This is a feature of the most recent version of the solver.
  • The large change between ICE-4G (VM2) and ICE-5G (VM2) is due to the introduction of Art Dyke's margin chronology which had the ice being eliminated from North America earlier than in the original chronology obtained from John Andrews. This meant that more ice had to be removed in order to have the same impact on RSL during the ice-free Holocene interval.
  • The large differences between ICE-5G (VM2) and ICE-6G (VM5a) are a combined result of the introduction of two further constraints:
    • the analysis from Valerie Masson demonstrating that the elevation of East Antarctic ice was not significantly different than modern
    • the results of the analyses based upon the application of the space geodetic constraints described in the Argus and Peltier paper.

ANU Ice Model

Please check the Comparison section above for the latest plots

The ANU Ice Model description and data have been supplied by Kurt Lambeck

Model description

The ANU ice sheets are based on the inversion of geological sea level and shoreline data supplemented by observational evidence of ice margin locations and, in a few instances, by limiting ice thickness estimates. These models have evolved over a period of years in an iterative fashion.

Broadly, the first iterations are based on the analyses of far-field data where the sea-level signal is predominantly a measure of the changes in total ice volume (the ice-volume equivalent sea level or esl) with the principal isostatic component often being the water-load term and a function of the rate at which water is added into or removed from the oceans. Simple models are initially used for the ice sheets. The separation of mantle rheology from the esl function is achieved by using the spatial variability of the far-field sea-level signals (Nakada and Lambeck, 1990 #127 see http://rses.anu.edu.au/people/lambeck_k/index.php?p=pubs for references). The resulting ice function is then redistributed between the ice sheets by using simple scaling relations in the first place and the process is iterated to ensure some convergence (Lambeck, Yokoyama and Purcell, 2002 # 228).

In parallel inversions are attempted for the individual ice sheets using data from within and close to the ice margins. These observations are most sensitive to the ice models and mantle rheology. For the northern hemisphere these analyses are carried out separately for Scandinavia (Lambeck, Smither, and Johnston, 1998 #187, Lambeck et al., in press), Barents-Kara (Lambeck, 1995, 1996 #166, 170), Greenland (Fleming and Lambeck, 2004 #238), British Isles (Lambeck, 1993; 1995 #164, 156) and North America (as yet unpublished). In all cases new compilations of the field data have been made. These separate solutions allow for lateral variability in mantle viscosity. Some interactions between the ice sheets occur and the solutions are therefore iterated.

The Antarctic field data is insufficient for a similar analysis for the southern hemisphere and we use the difference between the global esl and the northern hemisphere esl to estimate the volume changes for Antarctica eslant (allowing for mountain deglaciation in both hemispheres, Lambeck and Purcell, 2005 #247). The ice in Antarctica is then distributed according to the LGM ice margins proposed by Anderson et al. (2002) and on the assumption that the ice profiles followed the quasi-parabolic function proposed by Paterson. The retreat history is determined by the eslant function. These models are not meant to be accurate reflections of the Antarctic ice history but as a convenient way of disposing of the ice volume that cannot be attributed to the northern hemisphere, in a way that will not impact in a major way on the far-field and northern hemisphere analyses.

With the new ice models the far-field analysis is repeated and the individual ice sheet analyses are also repeated. Several such iterations have now been carried out but the successive results have not yet been published. The LGM results provided here represent the most recent (2009) solution. The full solutions for some of the ice sheets extend back to MIS-6 (Lambeck et al., 2006 # 252).

The rebound inversions result in the changes in ice thickness compared to the present day ice volumes. Thus the LGM ice thickness is obtained by adding the present-day ice thickness. The LGM ice elevation, with respect to sea level at the LGM is obtained by subtracting the sea-level change (geoid change beneath the ice sheet) from the palaeo ice thickness.

The esl function as used in the ANU solutions is defined as all land ice and grounded ice on the shelves and the ocean margin at the LGM is defined by the ice grounding line (Lambeck et al., 2003 #233).

References

  • Lambeck & Johnston, 1998: The viscosity of the mantle: evidence from analyses of glacial rebound phenomena. “The Earth's Mantle” (ed. I. Jackson). Cambridge University Press, Cambridge. pp 461-502
  • Lambeck, Purcell, Johnston, Nakada & Yokoyama, 2003: Water-load definition in the glacio-hydro-isostatic sea-level equation. Quaternary Science Reviews, vol. 22, pp 309-318
  • Lambeck, & Chappell, 2001: Sea level change through the last glacial cycle. Science, 292, 679-686.
  • Lambeck, Yokoyama & Purcell, 2002: Into and out of the Last Glacial Maximum: sea-level change during Oxygen Isotope Stages 3 & 2. Quaternary Science Reviews, vol. 21, pp 343-360

Ice volume

The following table has been supplied by Ayako Abe-Ouchi. It compares the ice amounts in the various regions in terms of eustatic sea level impact inferred by assuming the ocean area to remain fixed to the modern area (ice amount relative to now in m of ocean assuming ocean area = 360768576 km2).

ANU ICE-6G
v1.0
21k 21k
ANT (1) 29.0 13.2
NA (2) 82.5 79.8
EUR (3) 18.2 18.1
Tot (4) 129.7 111.1
  • (1) ANT = East plus West Antarctica
  • (2) NA = Laurentide ice sheet
  • (3) EUR = rest of the ice sheets except for Greenland and Patagonia
  • (4) Total is excluding Greenland and Patagonia change

Plots

Plots are modified. Please download again (fuyuki, Nov 13 2009)

The following plots have been prepared by Ayako Abe-Ouchi and Saito Fuyuki:

The plots show the combined Surface Altitude and Fraction of Grid Cell Covered with Glacier, and use data from:

  • for ANU ice model (2009)
  • for ICE-6G v1.02
  • for GLAC-1 nn454 model for North America, and ne8234 for Eurasia
  • for GLAC-1 nn9014 model for North America, and ne8234 for Eurasia

Conclusion

In September we should be able to compare the results of the revised version of ICE-6G with those of the Lev-Dick collaboration and take a decision based on comparisons with data and on the advice of the community





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Discussion

Catherine Ritz, 2009/09/22 15:39

Hi,

Here are a few comments on the Antarctic reconstruction.ice6g for glacial (21 k)

The first look to figure 2 and 4 gives a rather strange impression because of the split between East and West Antarctica. This split has surely geophysical basis (not the same plate) but it is not realistic from the glaciological point of view (ice would flow to smoothe the step

I think most glaciologists will agree on a slighlty thinner (100 m ) Antarctic Plateau at LGM, From the color scale in figure 4(21-0), it seems almost ok

In the Ross sector of West Antarctica, rather small changes (especially compared with older reconstructions). It is consistent with the fact that Siple dome was still a dome. My own modelling agrees as well.

In the Weddel sector. The most striking feature is the huge increase in the Ellsworth ranges and where is presently the Rosnes ice shelf. The reconstruction displays a 4 km high dome in that place and I must say that from the ice sheet modelling point of view I am a bit skeptical.

Constrains from Berkner (Rob please correct is needed)

- Berkner Island was an isolated dome based on Raymond bump, underlying sand. It seems to be the case in the ice6g reconstruction - change in surface elevation at Berkner:

  • from isotope 400 m to 2000 m, best estimation 550 m
  • from total gas content 200-900 m
  • to keep a cold base requires a change smaller than 1500 m

Such a configuration means that this ice dome was likely surrounded by two active ice streams. In the reconstruction it seems a bit thick on the Filchner side to be an ice stream (but one could have resolution effets). At the present Rosnes ice shelf location, it seems to me that there is a depression and it seems too thick for an ice stream.

The thickness at the grounding line is larger than the one inside and is globally too high for all this region (1000 m high at the grounding line means ~ 10 km thick ice !)

Some other constrains : change in Shackleton range < 340 m change in Ellsworth Mountains > 1000 m,

I remember Rob saying that there are doubts on this point because the trimlines could be much older than LGM ? (as in many other places in Antarctica) I do not know whether there are new published data on this point but it is the key for the existence on a “Ellsworth dome ”

In conclusion and keeping in mind that I had only a look at the pictures, not the numerical data.

Compared to previous reconstructions

- East Antarctica more in agreement with glaciological model and ice core data. - West Antarctica : Ross sea, ok - West Antarctica : Weddel sea a bit too thick (especially at the grounding line) and I doubt from the existence of the huge dome first because I do not see which glaciological process could lead to it ; second because, with just a dome nearby the depression between this dome and Berker Island would be quickly filled and Berker Island would not be a local dome as we think it was.

Jerome Chappelaz (LGGE, Grenoble) , 2009/09/22 15:50

At this point I can only say that geomorphological information at Frontier Mountain (~50 km away from Talos Dome) indicate at maximum an increase of ice sheet elevation by ~200 m during the Last Glacial Maximum compared with today.

Bo Vinther, 2009/09/23 14:03

Our newest results working with Dominique Raynaud (total gas) and David Fisher (Canadian ice cores) are the following:

1) The Camp Century site (NW Greenland) was some 600m higher than present in the early Holocene. This suggest that the Canadian high arctic and Greenland was connected by an ice ridge and that the Greenland ice sheet (GIS) extended far out into Melville bay.

2) The DYE-3 site (SE Greenland) was 400m higher than present in the early Holocene. This suggest a wide southern GIS.

3) The GRIP and NGRIP sites (central Greenland) were 150m higher than present in the early Holocene. This suggest a a very wide GIS that could have extended out onto the entire continental shelf.

So to sum up, the GIS was both thicker and wider all over in the early Holocene and it was connected to the Innuitian ice sheet that covered the Canadian high arctic.

David Fisher, 562 Booth St Ottawa Ont. Canada K1A 0E8, 2009/09/23 16:32

wrt thickness history of Greenland since 12000 BP all future modelling efforts now must measure themselves against the very recent paper about that topic by Bo Vinther et al. in Nature (Sept 17 2009). Most of the 3D models for Greenland assumed the GRIP/GISP2 O18 records could be used for the temperature history. This has proven to be wrong and the Vinther et al. paper shows what the temperature history should be and indeed what the thickness history was.

Anders Carlson, UW-Madison, Dept. of Geoscience & CCR, 2009/09/23 21:40

While its good to see that the Keewatin Dome of the LIS is now thinner in 6G than 5G, the LIS is still really thick if its maximum height above sea level is in excess of 4 km. Also, I don't think there is good evidence to have the 21 ka global sea level ~12 m higher than the 26 ka sea level as the margin chronologies of all ice sheets do not show any significant retreat until 19 to 20 ka (see Clark et al., 2009, Science), at least not enough to justify 9 m of sea level rise from the Laurentide and 3 m from the Scandinavian/Barents/Kara. In addition, relative sea level records during this interval mainly consist of M. annularis (20 m growth habitat) (Peltier and Fairbanks, 2006) (one New Guinea coral with a 5 m growth habitat is in conflict with these data; Cutler et al., 2003), which doesn't allow determination of a sea level change between 26 and 21 ka. The first discernible rise comes between 20 and 19 ka. So, what is the basis for thinning of the Cordilleran Ice Sheet, the Keewatin Dome and the new dome over Hudson Bay?

In this vein, the LGM flow patterns of the LIS would not place a fourth dome over Hudson Bay, rather they indicate a dome over Keewatin, a dome over Baffin/Foxe Basin and a dome over Labrador/Quebec with a saddle running across southern Hudson Bay (i.e., the Dyke and Prest, 1987 pattern, or Prest et al., 1968).

Another odd feature is the very thick ice of the Barents Ice Sheet margin in the eastern Barents Sea at 26 ka, what is this based on?

Lev Tarasov, Lev Tarasov, 2009/09/24 03:10

The Barents/Kara component of ICE6G with a large ice-cube in the middle of a pancake makes no glaciological sense. Dick and I both face the challenge of trying to find places to stick all the ice apparently indicated by the Barbados record. Dick's geophysical hand-tuned Lego block approach lends itself to sticking ice in places where there are not geophysical observations to constrain the ice load (ie the Barents/Kara ice cube). I don't have this option with a calibrated glaciological model. Furthermore, the max/bestguess/min deglacial ice margin chronology for Eurasia from our Bergen colleagues (Gyllencreutz, Mangerud, Oystein, Svendsen has about 20% less ice area at 26ka than at 21 ka, yet ICE6G has 23% more ice volume at 26ka than at 21ka

Wrt Anders' comments for North Am, contrary to Eurasia, I think some 26ka to 21ka thinning for North Am is plausible with Heinrich event 2. The M. annularis data actually does provide a challenging constraint if one takes (which Dick and I do) the 20 m growth habit of as the error bar. Unless Dick's VM2/5a earth rheologies are way out, it's a major challenge to get enough ice on land in a glaciological model to fit the Barbados record given the ice margin, RSL, and geodetic constraints.

More fundamentally, this community needs to decide on what criteria will be used for choosing the deglaciation models. At this stage, there are significant trade-offs between:

-fit to RSL data (I'll leave Dick to deal with an associated earth rheology issues)

-fit to geodetic data (””)

-glaciological self-consistency

-fit to the best available margin chronologies

-derivable from plausible climatological forcing

-fit to paleo lake level indicators (strandlines)

-inclusion of error bars

-inclusion of self-consistent meltwater drainage pointer field

Peter Clark, Geosciences, Oregon State University, 2009/09/24 05:38

I like Lev's description of these as “Lego ice sheets” - they are certainly not glaciological phenemona, and their use in climate models with increasingly high resolution leaves something to be desired. As far as I understand, the geomorphic “trimlines” in Antarctica are very ambiguous with respect to constraining LGM ice limits (cosmogenic nuclides suggest that they likely represent transition from warm-based ice below to cold-based ice above). What is the basis for reducing E. Antarctica down to 1.45 m excess ice? Why can't E. Antarctica have substantially more ice? Regarding Lev's comments, I don't know of any evidence for thinning of ice from 26-21 ka (any sea level contribution from H2 is unknown), so what is the basis for thinning in the model? If Barbados, see below. For clarification, I'm not sure what Lev means by the 20 m error on the Barbados M. annularis corals providing a “challenging constraint” - as in it's a challenge to use these data with this error to constrain ice volume? What are the RSL and geodetic constraints on 21-26 ka ice? With respect to modeling sea level with Dick's VM2/5a earth rheologies, I refer the reader to Jerry Mitrovica's model results of LGM sea level and Barbados using a different Earth rheology that are presented in Clark et al. (2009, Science - see more complete discussion and results in the accompanying Supporting Online Material).

Lev Tarasov, Lev Tarasov, 2009/09/24 13:58

Response to Peter's comments:

1) it should be noted that all current geophysical models for deglaciation are Lego block or effective variants thereof.

2) From conversations with other modellers along with the published litterature, it appears that glaciological models tend to also produce 10-18 m contributions for Antarctica. I have a student attacking Antarctica now, and will have a clearer sense of the extent to which Antarctica contributions are constrained in a few months.

3) There is no evidence that I've seen to refute a substantial H2 contribution. Dick has tuned his model against the RSL and Barbados records, so within the uncertainties that all modellers face (earth rheology,…), I'm not clear on what basis his 26 to 21 ka sealevel rise could be refuted.

4) Given the modelling I'm doing (large ensemble glaciological model with climate forcing derived from PMIP I and II results) and the set of constraints that I'm using (Art Dyke's North American RSL database, Dick's Eurasian RSL database, Barbados and Sunda Shelf records, Art Dyke's margin chronology for North Am., the Bergen group (Gyllencreutz et al) margin chronology for Eurasia), strandline records for North Am., present day rates of observed uplift for North Am.) along with ICE6G for the Antarctic component, it's very difficult to fit the Annularis records (ie within 20 m uncertainty from the growth range), and when fit, it's at the cost of poorer fits to other constraints.

5) It would sure help the community to resolve this earth rheology question, but this will require getting past some bad blood and politics so that the various rheological models can be properly compared. The geophysicists need to create their on MIP with the clear metrics for rheological intercomparison. They also need to build some community bedrock response and RSL post-processing modules following current initiatives in the climate and glaciological fields.

6) Wrt the science paper that Peter mentions, I see no methodological difference from what Dick does (time-varying shoreline geometry, rotational feedbacks) except that it appears on my quick read of the supplement to not use near-field RSL constraints and as such is much less constrained (Peter please correct me/clarify if I'm wrong here).

Gifford Miller, INSTAAR, Univ Colorado, 2009/09/24 07:18

I have trouble not seeing a LIS dome over Foxe Basin, and instead a strong dome over Hudson Bay for Ice 6g at both 26 and 21 ka. The large scale geomorphic flow patterns on Baffin Is suggest the dome is over Foxe Basin most of the time.

Lev Tarasov, Lev Tarasov, 2009/09/24 14:08

This is something I've pondered with my glaciological models, and there is a caveat here. Geomorphic flow patterns generally require warm-based conditions with sliding or till-deformation. If the core is cold-based during LGM, there will be no signature of flow except at the margins and at high elevation points. Once Hudson Strait surges, flow patterns can then be generated that are more consistent with the geomorphic record.

Really pinning down H2 contributions would sure help resolve such issues.

Peter P. Smolka, University Muenster(*), D-48149 Muenster, Germany, 2009/09/24 10:37

Refering to the ice_ng_compare_polar.pdf: Kuhle (Prof. Kuhle, Goettingen) provided for the LGM and Last Glacial Period as he calls it since more than a decade quite a lot of hard data (ice scoures, ice-levels etc etc) from the Tibet Plateau (also presented on the IGC in Oslo 2008).

Although it is physical geography (geomorphology, mapping) these are data, comparable to ODP/IODP cores. The evidence as reported and document I regard as normal credible geological data.

Several figures from above show on the relevant maps “fraction of grid-cell covered with ice” no ice on the Tibet Plateau.

If I understood the maps correctly (reading quite fast) I recommend to include above-mentioned data.

I have an updated version of a newer manuscript by Prof. Kuhle here (intended for a special issue). Thus you might either refer directly to him or to me for this.

If, reading quite fast, I misunderstood something, e.g. respective maps with ice on the Tibet Plateau appear below Fig. 33 of above please apologize this comment.

The _data_ I think should be included in the reconstructions.

The interpretations regarding the impact of the ice on the Tibet-Plateau are a “different story” - independent of the data themselves. I referred to the data.

Robert Mulvaney, British Antarctic Survey, Cambridge, CB3 0ET, UK, 2009/09/24 15:21

Commenting on the points made by Catherine about the Weddell Sea region. The evidence from a bedrock ice core from Berkner Island seems to imply that the Weddell Sea ice sheet was not as thick as the model suggests.

It looks as though there was no (or little) basal melting during the LGM which a quick re-run of the model requires ice less that 1600m thicker than today (if thicker than that, even with a surface temperature 20 degrees colder and an accumulation rate half that of today, geothermal heat flux leads to basal melting).

Data from analysis of the core are also limiting on the thickness, with the enclosed air suggesting <900m, and the isotope derived temperature profile around 600m (though could be interpreted as up to ~1500m). Sub-ice sand is morphologically consistent with a stable ice sheet, rather than one that is subsumed into the larger WAIS and subject to ice flow near the base.

Mike Bentley at Durham will have the data on the exposure ages and trimlines in the Shackleton and Ellesworth ranges and be better placed to give a definitive response.

Massimo Frezzotti, ENEA Via Anguillarese 301, 00123 Roma , Italy, 2009/09/24 15:32

Hi

I have some comment about Ross sea area:

Geomorphological evidence points out that the former profile of outlet glaciers along TAM and Ross Sea coast afford surface elevation changes of about 1100 m at Hatherton Gl. (80 S, 157 ° 30’E), 600 m at Mackay Gl. (77° S 162°E), and 360 m Reeves Gl (74°45’ S 162° E).

Whereas the glacier surface at margin between TAM and East Antarctic Ice Sheet (EAIS) remained virtually unchanged in the western Ross Sea area.

While glacier surface at margin between TAM and EAIS showed a modest thickening (100–200 m) of glacier drainnig directly to Southern Ocean (Rennick Glacier 160°E from 70 to 73°S). The extensive presence of meteorites in the Frontier Mountain blue-ice-field (73°S 160°E) area and their terrestrial ages, attributed to the presence of blue-ice meteorite traps during full glacial times, suggest that a moderate change in elevation occurred in agreement with modest thickening (100–200 m) of Rennick Gl.

Isotopic composition of Talos Dome Ice core indicates that Talos Dome was not overrun by East Antarctic ice during the LGM.

The elevation changes at Talos Dome are due to a combination of glacial dynamic effects during LGM and subsequent climate effects (increase of accumulation) during deglaciation. The former is related to an advance of the grounded ice sheet up to the continental margin during the sea level drop occurring at LGM. The latter is related to a doubling of accumulation rate linked to atmospheric warming and sea-ice reduction during deglaciation. Analogously, the thinning of the dome started after the retreat of the grounding line from LGM position in Ross Sea occurring from 14.3 to 12 ky BP.

On the base of C14 date Terra Nova Bay area was free from Ross Ice Sheet up to around 25 kyr BP, this suggest that the main thickening of glaciers along the Ross west Coast (Mackay and Reeves) occur between 25 kyr and 16 kyr and a significant difference should be occur between Ice6G26 and Ice6G21. I think that is also important to see an Ice6G15 before the start of retreat of Ross Ice Sheet.

The drastic change of morphology between present and LGM at the boundary between TAM and EAIS is not compatible with knowledge and appear as an artefact of model constrain.

Anders Carlson, UW-Madison, Dept. of Geoscience & CCR, 2009/09/24 17:18

In agreement with Peter and Giff, I think it is quite important to have the ice domes, particularly for the LIS, at the best estimate of height and location as this clearly impacts atmospheric circulation (see Otto-Bliesner et al., 2006 for ICE-5G vs. 4G and the warming over Alaska from the high Keewatin Dome). The impact will only grow as models become higher resolution and add isotopes to the hydrologic cycle, which requires then the ice ablation to interact with the ocean and its circulation. For example, at the last PMIP meeting, Allegra showed ModelE-R simulations using 5G and a lower LIS from Licciardi et al. (1998). AMOC was reduced in the Licciardi experiment because of the high overturning of the thinner ice sheet, while AMOC was enhanced in the 5G run.

Dr. Mahesh Gaur, Associate Professor - Geography, Remote Sensing & GIS; Government PG College, Pali. INDIA (11/948 Chopasni Housing Board, Jodhpur - 342008. INDIA), 2009/10/03 16:19

Dear fellow scientists and researchers,

I am working as Associate Professor and have more than 17 years of thematic and research for societal needs. I belong to the Thar Desert of India and it has always fascinated me since my childhood days. I have studied about its origin and other geological upheavals during my masters and doctoral study. Still I am engaged.

I would appreciate receiving support (in the form of un/published research materials; financial and others) from all of you in framing a research proposal to study (and even try to reconstruct) its glacial and holocene age. I am keen to associate (and even undergo training) with any research organization / Institution / university Department to undertake this study. I would feel obliged I am offered training (with financial support) for carrying out this study.

A quick response of assistance will be highly solicited. I am looking forward to hear favorably from world scientific community in my endeavor.

best regards,

Mahesh Gaur

Mark Siddall, 2009/10/09 16:32

PALSEA meeting WHOI September 21-25, 2009, consensual statement on PMIP ice sheet boundary conditions:

1. Alternative ice sheet boundary conditions must be considered as a means to consider the sensitivity of the PMIP simulations to ice sheet boundary conditions.

2. We recommend as an ice sheet boundary condition the existing Dyke (2004) database of ice sheet extent covering the last 20 kyr of ice-sheet retreat. This is a reasonable alternative reconstruction of the Laurentide for this purpose. A parabolic ice sheet can be fitted to this reconstruction in a similar fashion to Carlson et al (2008).

3. We recommend, as an ice sheet boundary condition, alternative, independent isostatic rebound models such as that of Kurt Lambeck (e.g. Lambeck et al 2002). These boundary conditions will be made available to PMIP.

4. We note that an underlying inconsistency in isostatic models is with the relative sea level databases input to them. These are not the same between models and almost certainly contain relative sea level data which has been dated with inconsistent and inaccurate techniques. To address this problem in the longer term PALSEA will construct an open access, quality controlled, self consistent, state-of the art database of relative sea level for use in isostatic models.

Carlson, A.E., A.N. LeGrande, D.W. Oppo, R.E. Came, G.A. Schmidt, F.S. Anslow, J.M. Licciardi, and E.A. Obbink, (2008), Rapid early Holocene deglaciation of the Laurentide ice sheet. Nature Geosci., 1, 620-624.

Dyke, A. S. in Quaternary Glaciations — Extent and Chronology Part II Vol. 2b (eds Ehlers, J. & Gibbard, P. L.) 373–424 (Elsevier, Amsterdam, 2004).

Lambeck, K., Y. Yokoyama, and A. Purcell (2002), Into and out of the Last Glacial Maximum: sea-level change during Oxygen Isotope Stages 3 and 2, Quat.Sci.Rev., 21, 343-360

signed (workshop participants):

Morten Andersen, University of Bristol Andrew Ashton, Woods Hole Oceanographic Institution Edouard Bard, CEREGE, College de France Hai Cheng, University of Minnesota Dan Condon, British Geological Survey H. Allen Curran, Smith College Pierre Deschamps, CEREGE Andrea Dutton, Australian National University Norbert Frank, LSCE Edward Gasson, University of Bristol Paul Hearty, University of North Carolina Gideon Henderson, Oxford University Mick O'Leary, Manchester Metropolitan University David Richards, University of Bristol Laura Robinson, Woods Hole Oceanographic Institution Peter Rowe, University of East Anglia Denis Scholz, University of Bristol Mark Siddall, University of Bristol Alex Thomas, Oxford University William G. Thompson, Woods Hole Oceanographic Institution

Dick Peltier, Dept. Physics, University of Toronto, 2009/10/29 17:18

As I was saying when the nasty wiki cut me off! I had already entered text discussing the new data that has been employed to adjust the north american element of the model from ICE-5G to ICE-6G. There has been no recognition in the previous comments of this important new information which is entirely of a space geodetic origin. Please read the Argus and peltier paper submitted to GJI and the Peltier et al paper submitted to nature. Simply put these data require that LGM ice was thinner to the west and to the south of HB but thicker over Labrador and northern Quebec and along the northern border between Alberta and British Columbia.

Some consternation has been expressed over the ridge of LGM ice that is evident in ICE-6G within HB itself. This is clearly a transient feature that is posited to exist in the model prior to the time of Heinrich event 1 which leads to a rapid draw down of ice from the Bay itself. Because i is lost quickly it makes no contribution to the time dependent gravitational field signal which the GRACE satellites are now recording to be present over the north american land mass. Inspection of the prediction of the GRACE signal by ICE-6G documented in the Peltier et al (2009, submitted) publication demonstrates that this model beautifully reconciles this observation whereas the prediction by the previous ICE-5G model, with its overly thick ice to the west of Hudson Bay, provides a very poor fit to this important observation. Since the GRACE data has not been employed to construct ICE-6G this is an important confirmation of the quality of the model.

Dick Peltier, Dept of Physics, University of Toronto, 2009/10/29 17:44

Concerning the Antarctic component of ICE-6G we have employed the following types of information to constrain this component of the reconstruction (Much of this information was unavailable at the time ICE-5G was produced). First we have now exlicitly employed the ice-core derived constraint provided by the work of Valerie Masson-Delmotte et al This work very strongly suggests that there was very little if any loss of ice from the central dome of the east Antarctic ice sheet. What loss of ice that did occur from east Antarctica must therefor have been associated with a diminution of its lateral extent. At LGM East Antarctic ice extended outwards to the shelf break. During deglaciation it retreated inwards in the same way as the Greenland ice sheet adjusted to the marked change in climatic conditions in the northern hemisphere (more to follow).

In West Antarctica the situation is known to have been much more dynamic. In constraining the geographical regions from which ice was lost from this region during deglaciation use has been made of the original geomorphological constraints provided by the work of George Denton and colleagues. So far as I am aware this work remians unchallenged. Comparing this aspect of the reconstruction with the IJ05 reconstruction of Ivins and James there is very little difference between the models in this respect. In constraining the amount of ice that was lost from these geographical regions, two sources of information have been employed. First all available relative sea level data have used to constrain the mass loss in regions from which such data is available. These data are not voluminous, however, and furthermore are available only from coastal regions! The additional constraint that has been employed to approximate the amount of mass loss is that provided by the most recent glaciological reconstructions by Ritz et al and Philippon et al, analyses which appear to restrict the total mass loss to be equivalent to approximately 11 m of eustatic sea level rise. The timing of this mass loss is also critical, of course. In ICE-6G this aspect of the deglaciation history of Antarctica has been fixed by the work of Eugene Domack and colleagues whose work in the context of the US Antarctic programme has allowed them to identify the timing of the pull-back of grounded ice from the shelf-break at a very large number of sites surrounding both East and West Antarctica to the time of meltwater pulse 1b in the extended Barbados sea level curve of Peltier and Fairbanks (2006). In ICE-6G it is assumed that the fraction of the total Antarctic ice that was lost during the glacial-interglacial transition was such as to explain the observed amplitude of meltwater pulse 1b in the Barbados record.

Dick Peltier, Dept of Physics, University of Toronto, 2009/10/29 22:06

I thought it might be helpful, prior to the discussion tomorrow morning (?-depending upon where you are)if I were to comment upon the two complementary methodologies that are being employed by Lev and I. Although they are closely linked, these two methodologies each have strengths and weaknesses. I' ll refer to them below as follows: (1) The geophysics based methodology. This is the methodology employed to develop the ICE-NG (VMX) sequence of models.

  The only glaciological inputs to this methodology consist of time dependent margin positions from LGM to present, wherever 
  this information is available. The only exception to this in the past has been for Greenland in the ICE-5G (VM2) 
  reconstruction for which the model developed in the two papers by Tarasov and Peltier (referenced by Lev above) has been
  introduced.

(2) The trained ice sheet methodology employed in the several papers by Tarasov and Peltier now available from our eb sites.

  In this method the same geophysical data employed in method (1) are used to train a fairly standard shallow-ice 
  representation of the dynamics of a continental scale ice sheet for either North America and seperately for Eurasia. The
  same methodology may of course be applied to Antarctica. The data required to apply the method in this region, however, is
  still rather sparse.

The breakthrough that has occurred in the past year or so that has enabled to refinement of the ICE-5G (VM2) model to produce the the ICE-6G (VM5a) model consists of the space geodetic information presented for the first time in the paper by Argus and Peltier (2009, submitted; available as a preprint from the PMIP3 eb site. The new model has been developed so as to satisfy these newly available constraints. The new model is described in some detail n the Peltier et al (2009,preprint) paper also available for download frmo the PMIP3 web site.Since these results have only recently been finalized they have yet to be proprly integrated into the reconstructions that have been performed using method (2) above. Through the application of method (1), however, they have been shown to have an extremely important impact upon the inferred topography of the LIS that is expected to have a significant impact upon the atmospheric circulation of the northern hemisphere at LGM.

Method (1) of course does have an important weakness. This is that no attempt is made to ensure that the ice sheets delivered by the analysis are glaciologically rationale. This is why Lev and I have been working to develop method (2). The weakness of method (2)is that it requires knowledge of the climate forcing that is responsible for driving the accumulation of mass on the continents that evolves so as to produce the LIS and other ice sheets. This is of course unknown and so must be varied so as to produce a “best possible” fit to the available constraints. This is a highly under constrained inverse problem and has to be regularized by the aplpication of a number of assumptions. A crucial constraint, however, is on the total mass of ice that is removed from the continents across the glacial-interglacial transition. We employ far field relative sea level data to provide the necessary constraints, Barbados being the most significant data set available for this purpose.

The circumstance in which we find ourselves at present is that method (2) seems unable to deliver models that both satisfy the constraints on regional ice thickness provided by the rsl and space geodetic data from the ice-covered regions AND the far field constraint on total mass of all ice sheets provided by the record from Barbados. My expectation is that once the new space geodetic constraints are fully integrated into the training procedure employed in method (2) this unfortunate circumstance will be alleviated. Until this has been carefully checked, however, we cannot rule out the possibility that the simple shallow ice approximation to the dynamics we have been employing might in fact be the cause of the problem. Of course the net mass constraint is a zeroth order constraint that any accaeptable model must satisfy.

Lev Tarasov, 2009/10/29 23:08

A few points:

1) Some clarifications wrt Dick's description of the glaciological calibration. “Training” is an erroneous description. The order 30 ensemble parameters for the glaciological model were calibrated using MCMC methods against the RSL data that Dick used for tuning ICE-6G. As detailed above, it was also calibrated against some surface drainage constraints (eg drainage into the Gulf of Mexico during MWP1-a), some geodetic data, and to maximize consistency with the climate forcing while maintaining ice margins with the error bars of the independent margin chronologies. Ensemble results were further scored wrt marine limit and strandline observations. Though the model uses shallow ice physics, ice streams and ice-shelves are implemented with the potential for very high (up to 100km/year) velocities.

2) The glaciological model was calibrated with the old ICE-4G Antarctic chronology. The ICE6-G chronology was not made available in time for recalibration, so an attempt was made at some random parameter searches and previous best fits with some attempts a further hand-refinement of the model parameters. With ICE-4G, the calibration had difficulty fitting the far-field Barbados record. The ICE6-G chronology has made the mis-fit to Barbados much stronger. It should be noted though, that it would be much easier to fit the Barbados record, if all non-RSL constraints were ignored.

3) What is still unclear to me is to what extent the Barbados misfits may be due to problems with the ICE-6G Antarctic ice load chronology. As Dick mentions above, his Antarctic model has little constraint. And no geophysical (lego-block) model is going to have any significant constraint aside from independent margin chronologies beyond a few relaxation time constants of the local geophysical data. In other words, at LGM, geophysical reconstructions have no real ice thickness constraint, only ice extent (if independently available) and a global far-field volumetric constraint. Geophysical reconstructions can also ignore any glaciological sensibility in order to fit geophysical constraints (eg the deep incision across the Hudson Bay/Keewatin ice dome in ICE-6G).

While I'm not disputing the Antarctic contribution to mwp-1b, I have yet to be presented with evidence that Antarctica could not have significantly contributed to mwp-1a as well thereby also allowing more ice volume to better enable a fit to Barbados with GLAC-1. The WAIS shelf retreat inference is as I understand based on marine cores. But continuation of perennial sea-ice would still inhibit sedimentation and hide shelf retreat. If anybody could provide a clear refutation of this possibility, that would be much appreciated.

Anders Carlson, UW-Madison, Dept. of Geoscience & CCR, 2009/10/29 23:03

Assessing the three presented ice topographies for the LGM, it looks like the main difference between the 6G and MOCA reconstructions for the Eurasian Ice Sheet is the thickness of the ice and extent. Based on the cosmogenic dates of Goehring et al. (2008, QSR) and Linge et al. (2006, QSR), ice was at least up to 1400 m asl in SE Norway near the coast and 1800 m at the Norway-Sweden border, which would indicate that 6G and ANU are more in line with the data on ice sheet height than MOCA (Moca is <1000 m near the coast). The ANU model does not, however, have a British ice sheet, which there clearly was.

With regards to the North American Ice Sheet, I would say the ANU and MOCA models look similar, but with MOCA model having a more pronounced dome structure, similar to the flow patterns of Dyke and Prest (1987) (Keewatin, Labrador, Foxe Domes with the trough between the Keewatin and Lab Domes). Nevertheless, I think MOCA does a better job of resolving the Foxe Dome than ANU. It would be nice to know what the ice volume is in MOCA. As I mentioned before, 6G does not produce the dome pattern one would expect from the well mapped flow patterns of the LGM and early deglacial. Similarly, it looks like the Laurentide is flowing over the Canadian Rockies in 6G to the Pacific Ocean, which did not occur. The other two models produce a discrete Cordilleran Ice Sheet. Both 6G and the ANU models produce lower southern LIS lobes than the MOCA model, in agreement with reconstructions (Clark, 1992, GSA Bull; Hooyer and Iverson, 2002, J. Glac.).

Lev Tarasov, 2009/10/30 01:02

GLAC-1 nn454 has 76.6 m eustatic equivalent over North America at 21ka, nn8234 has 14.0 m eustatic over Fennoscandia and Northern Russia at 21ka.

Lev Tarasov, 2009/10/30 21:17

I check the papers that Anders cited above, did the local comparison to the models and actually the GLAC-1 nn8234 model better fits the three clear elevation constraints in those works:

hd=contemporaneous elevation, H=ice thickness, and obs are PD(present day) elevation from Quaternary Science Reviews 27 (2008) 320-336, Goehring et al.

and note due to isostastic adjustment, hd=hd(LGM)< hd(PD) which actually corresponds to “obs”

site obs ::::: H(8234) hd(8234) hd(6G)

Skala 1440 :::: 480.8, 1196. 1120.

Blåhø <1800,>1620 : 1364. 2088. 2274.

Elgahogna >1460 ::: 1154. 1627. 2706. ”

(FYI, ferret commands: set mode interp Skala (61.81N, 6.91E; 1848 m a.s.l.) list hd[x=6.9505,y=61.8666]

Blahø is located in central southern Norway (61.81N,

9.21E; 1617 m a.s.l.) north of Ottavatnet (Figs. 1). list hd[x=9.21,y=61.81]

Elgahogna (62.11N, 12.01E; >1460 m a.s.l.) list hd[x=12.01,y=62.1]

Anders Carlson, UW-Madison, Dept. of Geoscience & CCR, 2009/11/02 16:34

With respect to Lev's numbers above, I don't think there is much difference between the MOCA and 6G models as they both fit the constraints at Blaho and Elgahogna, but are a bit low at Skala. I'd say a little thicker ice by at least 200 m at Skala would be a better fit. At that site, ice height is greater than or equal to 1440 m, more likely greater than 1440 m, and ice thickness at least 1300 m. One boulder age of ~14 ka provides the height constraint. The deglacial age of the sample suggests that it was deposited during deglaciation and thinning down to 1440 m, rather than demarcating the LGM ice surface. Furthermore, with the well documented preservation of “old” landscapes under cold-based ice at Blaho and Elgahogna by Goehring et al. (2008), the presence of a trim line at Skala cannot be used as an indicator of former LGM ice thickness.

With this data removing what I saw as an inconsistency between 6G and MOCA, I'd say MOCA is a better model. It puts the highest dome along the Norway-Sweden border, which is where I think it would be. 6G has it further west, which I think wouldn't be the case as the precip source would be from the West and much of the precip for building the dome would rain out moving eastward, leaving a drier interior part of the ice sheet. The MOCA pattern is also closer to the reconstructed flow lines of Boulton et al. (2001, QSR) (similar reconstruction to the Dyke and Prest, 1987 work).

Peter Clark, Geosciences, Oregon State Univ., 2009/10/29 23:38

There is reasonably good evidence that significant deglaciation of the Antarctic ice sheet began between 15-14 ka.

Constraints from trimlines:

The significance of the vertical zonation of a landscape according to differences in rock-surface weathering (so-called weathering zones) to ice-sheet history in Scandinavia and the eastern Canadian Arctic has been debated for over a century, with the upper limit of any given weathering zone (trimline) interpreted as either a former ice surface (Boyer and Pheasant, 1974; Ives, 1978; Nesje and Dahl, 1990) or an englacial change in basal thermal regime from non-erosive cold-based ice above to erosive warm-based ice below the limit (Sugden and Watts, 1977). Terrestrial cosmogenic nuclide (TCN) ages from upper weathered zones on Baffin Island (Briner et al., 2005; Miller et al., 2002) and Fennoscandia (Goehring et al., 2008) have now conclusively demonstrated that the limit marks an englacial thermal boundary, whereby the upper, more weathered surface was covered by cold-based, largely non-erosive ice at the LGM. Recent work from Antarctica has similarly used TCN ages to demonstrate that highly weathered rock surfaces were preserved beneath cold-based ice during the last glaciation (Di Nicola et al., 2009; Stone et al., 2003; Strasky et al., 2009; Sugden et al., 2005). Trimlines are thus not a reliable constraint on former ice thickness.

Constraints from the shallow-marine record.

Most constraints on deglaciation from marginal shelf areas are based on AMS radiocarbon of the bulk acid insoluble organic (AIO) carbon fraction. AIO 14C ages from surface sediments in the western and central Ross Sea, however, are substantially older than the reservoir age, with a range from core to core of 2000 to >6000 14C yr B.P. (Andrews et al., 1999; Licht et al., 1999). Corresponding age corrections for each core site are conventionally established by subtracting the age of the AIO at the modern sediment-water interface (Andrews et al., 1999; Domack et al., 1999), which assumes that both the reservoir age and the amount of reworked organic matter has remained the same at each core site. This seems highly unlikely. The marine record of deglaciation from marginal shelf areas is established by using a characteristic stratigraphic succession reflecting grounding line retreat as represented by the transition from subglacial facies to sub-ice shelf facies to open-marine facies (Anderson et al., 2002). AIO 14C ages obtained from the diatomaceous mud or ooze that comprises the open-marine facies are the most reliable (Licht et al., 1996; Andrews et al., 1999; Domack et al., 1999), and thus closely constrain the timing of the transition from sub-ice shelf to open marine conditions, but provide only limiting minimum ages for grounding line retreat. In any event, Mackintosh, Domack et al. recent presentation at the ACE-Grenada meeting conclude the following: “Our reconstruction demonstrates that deglaciation of deep-shelf troughs and lowering of the ice sheet surface occurred in two phases, from 14-12 and 12-7 ka before present (BP). Our consideration of possible mechanisms for the observed retreat of the marine ice margin of Mac.Robertson Land favours rapid rates of eustatic sea level rise associated with Meltwater Pulse 1a (MWP-1a) at ~14 ka BP and warming of the marginal oceans and atmosphere to near-modern levels ~2 ka later.” (http://www.apcongress.es/misc/webantartic/docs/resumen/V11-9.40.pdf) It certainly seems reasonable, given dating uncertainties, to argue that initial ice retreat was associated with a contribution of Antarctica to MWP-1A.

Constraints from ice cores:

The isotopic (delD and del18O) records from two ice cores bordering the Ross Sea (Siple and Taylor Domes) show an abrupt warming at ~14.5 ka, which may reflect ice-surface lowering or some regional climate change induced by ice-surface lowering. If attributed solely to a change in ice-surface elevation, the 3-4oC warming at Siple Dome (Steig et al., 2000) would indicate 500-650 m of ice-surface lowering assuming a free atmospheric lapse rate of 6oC/1000 m. This magnitude of lowering is supported by ice-sheet modeling, which suggests thinning of Siple Dome ice by 350 m between 14 and 15 ka (Price et al., 2007).

Constraints from Antarctic relative sea level: Bassett et al. (2007) approached the question of an early and rapid deglaciation of Antarctica associated with MWP-1A by using an earth model to compute the near-field Antarctic sea-level history for two scenarios, one in which MWP-1A was produced entirely from the northern hemisphere, and one in which there was a dominant contribution from Antarctica. The former scenario would most closely correspond to the mid-Holocene deglaciation and retreat of the grounding line in the Ross Sea suggested from the RSL history (Baroni and Hall, 2004; Hall et al., 2004), whereas the latter scenario would test the hypothesis of whether deglaciation associated with MWP-1A can be accommodated by the RSL history. The best RSL records from Antarctica come from the Ross Sea region. For both scenarios, there is a sea-level rise at the Ross Sea sites associated with the northern hemisphere melting until this region deglaciates, after which there is a sea-level fall resulting from the isostatic uplift. The combined effect of these two processes is a sea-level highstand, which occurs earlier in the Antarctic source model and thus results in a more gradual sea-level fall during the mid-to-late Holocene than is the case for the northern hemisphere source model. Bassett et al. (2007) concluded that the more gradual sea-level fall predicted by the Antarctic source model is more consistent with the data than the larger and steeper fall predicted by the northern hemisphere source model.

Constraints from the Ross Sea Glacial Geological Record:

As summarized in Clark et al. (2009, Science), most of our best-constrained information on the chronology of the Antarctic ice sheet comes from the extensive radiocarbon dating from the Ross Sea sector of the WAIS, particularly that from Taylor Valley, by Denton and colleagues.  (1) The youngest 14C age (14.7-15.1 cal ka; AA-20667) constraining the time of the local last glacial maximum (LLGM) ice margin at the Hjorth Hill moraine provides a limiting maximum age for retreat from the moraine. (2) Two 14C ages on algae that grew in a glacial lake that formed following ice-margin retreat from the Hjorth Hill moraine provide limiting minimum ages for this retreat (14.8-15.2 cal ka; AA-13576, and 14.9-15.2 cal ka; AA-17342). (3) The glacial lake in Taylor Valley permanently dropped below ~80 m after 13.9-14.7 cal ka (AA-17314) (14C age on youngest high-elevation delta) and before 13.9-14.1 cal ka (QL-1707) (14C age on oldest low-level delta after permanent drop).  (4) Lacustrine deposits occur in a stream exposure cut into the moraine deposited by the LLGM ice margin on the floor of Taylor Valley, with a 14C age requiring deglaciation from the moraine before 14.3-14.8 cal ka (AA-17333). (5) Glaciolacustrine sediments containing dropstones occur along the distal side of the valley-floor threshold moraine, indicating that the ice margin was at the moraine.  The youngest 14C age from this unit (13.6-14.7 cal ka; QL-1794) provides a limiting maximum age for retreat from the moraine. Given these dating constraints, we suggested an age for the termination of the LLGM (13.9-15.2 cal ka)in this region, and assigned the error based on the youngest limiting maximum 14C age for ice-margin retreat (13.9-14.7 cal ka (AA-17314) and the oldest limiting minimum 14C age for ice-margin retreat (14.9-15.2 cal ka; AA-17342).  This only identifies when ice began to retreat, but does not constrain any subsequent rate of retreat.
Dick Peltier, Dept of Physics, University of toronto, 2009/10/30 01:41

Peter's comments above concerning the (re)interpretation of Eugene Domack's data are interesting. I've sent a copy of his text above to Gene in order to have him (Eugene) respond directly.Peter's interpretation above would appear to contradict Gene's own as recently expressed to me.

From a physical perspective, however, I would find it unusual if there were not some reaction around Antarctica to the dramatic warming of the northern hemisphere that was caused by the re-invigoration of the NADW formation that in turn caused the Bolling-Allerod warming and the release of meltwater that was MWP1A. This re-invigoration of northern hemisphere deep water formation occurred as a consequence of the cessation of the freshwater forcing associated with Heinrich event 1 (H1). The evidence of a strong contribution to MWP1A from the northern hemisphere is clearly evident in records from the GOM as is well known. The northern hemisphere warms because the NADW process resumes. This causes the main reaction of the northern hemisphere ice sheets which deliver the majority if not all of MWP1A. I would expect that the grounded ice surrounding Antarctica should have reacted to some degree to this. The question is to what degree. I do not believe that we have any evidence that would allow us to quantitatively measure this. I can say , however, that all of the relative sea level records available from Antarctic sites are well fit by the ICE-6G (VM5a) model, including those of Hall et al from the Scott coast referred to above. These data fits have been sent previously to the PMIP3 group. Clearly this model for Antarctica is based upon the radical assumption that the only mass loss from east Antarctica occurred from the exposed shelf that would have existed at the time of the low stand of the sea. However the discrepency between the eustatic equivalent ice amounts melted from Antarctica in the ANU model are ~29 m rather than the ~13 m in ICE-6G, ie more than double. I believe this model has been constructed on the assumption that essentially ALL of MWP1A was delivered by Antarctica. Since the proponents of this model have not seen fit to provide us with the time series of the meltwater production by their model I'm in no position to know whether this interpretation is correct. So , again, please provide us all with the detailed ice thickness history for all ice sheets for this model so that they may intercompared directly and fully.

What I would find helpful insofar as the ANU model is concerned is to have Kurt forward the complete thickness history of all of the ice sheets in this model to me in Toronto so that I can run them through the complete suite of analyses that will allow me to compare the predictions of this model to the space geodetic constraints that are now available, especially the GRACE data which are crucial in enabling us to clearly see what wee the dominant geographical regions from which unloading of ice occurred. In order for the model we fasten upon to be a truly community model there needs to be full disclosure of the thickness isopacks so that detailed inter-comparisons of the models can be peformed.

Lev Tarasov, 2009/10/30 03:15

Wrt MWP1-a, I would expect the sea-level rise to have had a significant impact on grounding line migration and thereby on grounded ice volume, especially in the WAIS. The question is what is “significant impact”, which my group will hopefully better constrain over the next few months.

Anders Carlson, UW-Madison, Dept. of Geoscience & CCR, 2009/10/30 02:08

In quick response to one of Gene's comments provided by Dick, the evidence for a large MWP-1A contribution from the LIS in the form of benthic and planktonic d18O, particularly in the Gulf of Mexico, is not supported by the existing data as shown in Carlson (2009, QSR). Rather all of these records (Aharon, 2003, Paleoc, 2006, EPSL; Keigwin et al., 2005, Paleoc; Poore et al., 1999, Geology; Hall and Chan, 2004, Paleoc) record just a small 1A LIS contribution. When considering a reasonable ice-melt d18O end member (-25 per mil for conservative, noting that -35 per mil reduces the calculations below), the maximum contribution the LIS provided to MWP-1A through the Mississippi River is 2.7 m of sea level rise. Aharon (2006, EPSL) assumed a -25 per mil end member as well (and -35 per mil), but also assumed a constant P-E-ice melt relationship where only 12% of the water down the Mississippi River was sourced from LIS melt. This led to the use of a total river end member d18O value of -9.8 to -11 per mil (-25 and -35 per mil melt end members, respectively), and thus his high discharge calculations. In reality, he should have then taken the 0.28-0.33 total Sv calculated and adjusted for the 88% contribution from P-E he modeled, which gives a southern LIS sea level rise contribution (P-E does not raise sea level) of only 1.4-1.7 m. Similarly, the northern outlet record indicates a maximum contribution of 0.5 m and the eastern outlet record a maximum contribution of 2.1 m. Thus the total LIS MWP-1A contribution is less than 5.3 m; not a significant contribution.

The records from other Northern Hemisphere ice sheets also do not show a large MWP-1A contribution, contrary to the Gene's assertion. The southern margin of the Scandinavian Ice Sheet retreated less than 100 km during MWP-1A (Rinterknecht et al., 2006, Science). Similarly, planktonic and benthic d18O records from the Norwegian Sea suggest a reduction in freshwater discharge during MWP-1A relative to earlier meltwater discharge from the Scandinavian Ice Sheet (Lehman et al., 1991, Nature; Karpuz and Jansen, 1992, Paleoc; Clark et al., 1996, Paleoc). The much smaller Cordilleran Ice Sheet also retreated only a relatively small distance during MWP-1A when compared to later retreat (Dyke, 2004). The major retreat and melt-back of the Barents-Kara Ice Sheet occurred ~18–17 ka, well before MWP-1A leaving significantly less ice available at ~14.6 ka to contribute to MWP-1A (Jones and Keigwin,1988, Nature; Koç and Jansen,1994, Geology; Svendsen et al., 2004).

Richard Gyllencreutz, Jan Mangerud, John Inge Svendsen, Oystein Lohne, Dept. of Earth Science, University of Bergen, 2009/10/30 11:59

This is a joint comment from the Bergen group (wemade the ice margin constraints for MOCA and ICE-6G):

1) ICE-6G: The apparent ice bridge between British-Irish ice sheet (BIIS) and the Scandinvavian ice sheet (SIS) in ICE-6G at 21 k must be a result from coarse resolution, which can be seen in the MOCA model, where there is just a narrow strait separating the two. There is firm evidence that BIIS-SIS were not confluent after 25 k.

2)ANU: BIIS is entirely missing, which must have an important impact on climate modelling based on this model. There is no ice on easternmost Iceland. Western Barents Sea ice sheet has a marked bight in Bjornoyrenna, which shouldn't be there. The ice in Russia between 45-55 deg E, 70 deg N is too large and goes slightly beyond the maximum Weichselian limit from QUEEN.

Richard Gyllencreutz, Jan Mangerud, John Inge Svendsen, Oystein Lohne

pmip3/design/21k/icesheet/index.txt · Last modified: 2009/12/23 14:25 by jypeter