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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
Please have a look at these files and make all the comments that can help us to finalise the CMIP5 boundary conditions rapidly.
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 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.
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.
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.
Note: get in touch with Jean-Yves Peterschmitt in you need a copy of the following papers.
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).
| North America|
(incl Innuit area)
|Greenland & Iceland||6.38||5.43||2.49||2.45||2.36||2.34|
|Barents / Kara Seas||14.05||12.26||8.43||9.29||7.26||9.10|