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Please use the Discussion section below to specifically comment on the choice of ice sheet reconstructions for the core experiment.

For the core experiment, there is a choice of two global ice sheet reconstructions:

• ICE-6G_C, provided by Dick Peltier, Rosmarie Drummond and co-authors
• GLAC-1D, provided by Lev Tarasov and co-authors.

Please use one of these reconstructions for your Last Glacial Maximum (LGM) experiment (e.g. equilibrium type spinup at 21 ka), and continue to use the same reconstruction through the transient last deglaciation Core simulation.

Those groups that are able may wish to carry out two simulations; one with each ice sheet reconstruction.

This page contains a lot of information; use the 'Table of Contents' (top right) to navigate through!

• Argus, D. F., Peltier, W. R., Drummond, R. & Moore, A. W. The Antarctica component of postglacial rebound model ICE-6G_C (VM5a) based on GPS positioning, exposure age dating of ice thicknesses, and relative sea level histories. Geophys. J. Int. ggu140 (2014).
• Peltier, W. R., Argus, D. F. & Drummond, R. Space geodesy constrains ice age terminal deglaciation: The global ICE-6G_C (VM5a) model. J. Geophys. Res. Solid Earth (2015).
  
  

Information provided by Dick Peltier, October 2014:

[The data] contain the paleobathymetry of the oceans over oceanographic regions as well as the paleotopography of the continents.

The ice mask in this reconstruction is fractional. For the purpose of the animations (below), we have used > 80 % ice cover per grid cell. The timestep is 500 years. Horizontal resolution is 1 degree, but it will also be available at 10 arcminute resolution. ^{[1-2] 1)}

The information in this section was provided directly by Dick Peltier et al., October 2014:

Time dependent ice-equivalent contribution to eustatic sea level rise [relative to present day] from each of the primary geographical regions from which grounded ice loss occurred during the [last] deglaciation process. ²⁾

Although ice meltwater fluxes should not be prescribed for the Core simulation, groups may wish to run alternative simulations with meltwater fluxes to the oceans.

At this stage it is proposed that ICE-6G_C meltwater routing can be calculated from ice thickness at each timestep. It will not be explicitly provided by the working group.
See the alternative full transient simulations page for more info. [not live yet]

Dick Peltier has suggested that ICE-6G_C topographies could be provided as smoothed fields (October 2014):

…we could…provide these [ICE-6G_C] topographies in the form of the smooth fields obtained by projecting them onto the set of spherical harmonics employed in a 1 degree by 1 degree model in the CMIP5 class. The results you obtain when you do this are illustrated in [Peltier and Vettoretti (2014)]^[3]. Would you rather have these fields in the smoothed form actually seen by such a climate model? This might be a good idea since some groups may be employing grid point models and these groups would probably appreciate being given a smooth topography field to start with.

Provided by Dick Peltier, October 2014:

Paleo-topography data sets contain information on BOTH the topography of the continents with respect to sea level at specific times in the past as well as paleo-bathymetry of the oceans at the same sequence of times. It is not only the former field that is important but also the latter. Although we could produce a separate data set for an “altitude anomaly” for the continents we would also have to produce a “bathymetric anomaly” for the oceans . The latter is as important as the former because modern climate models , eg the NCAR CESM model, have begun to include a diapycnal diffusivity field that is linked to mixing associated with the dissipation of the internal tide produced by the flow of the barotropic tide over ocean bottom topography. In order to re-tune such mixing parameterizations for past times, it is necessary to produce new models of the barotropic tide and this requires the different bathymetry of the oceans for these past times. It is very important in my opinion that the paleo-bathymetry be made available to the community as well as the paleo-topography.

Paleobathymetry is required by the groups interested in properly running coupled atmosphere ocean models of climate state as the bathymetry of the oceans is highly variable through the glaciation deglaciation process. Although some groups may want to simplify the analysis by not including the paleobathymetry of the oceans in the ocean component of their coupled model, it is important in my opinion that the data are available that will make it possible to do the analysis properly if they wish to do so.

AKA Tarasov or Tarasov et al. reconstruction.

• Tarasov, L. & Peltier, W. R. Greenland glacial history and local geodynamic consequences. Geophys. J. Int. 150, 198–229 (2002).
• Tarasov, L., Dyke, A. S., Neal, R. M. & Peltier, W. R. A data-calibrated distribution of deglacial chronologies for the North American ice complex from glaciological modeling. Earth Planet. Sci. Lett. 315–316, 30–40 (2012).
• Briggs, R. D., Pollard, D. & Tarasov, L. A data-constrained large ensemble analysis of Antarctic evolution since the Eemian. Quat. Sci. Rev. 103, 91–115 (2014).
• Tarasov et al. Eurasian ice sheet evolution (in prep.).
  
  

The ice mask in this reconstruction is based on 100 % ice or no ice. The timestep is currently 1000 years, but could be provided at 500 years, if desired. Horizontal resolution is 1 degree. ^[4-7]4)

Ice volume of the constituent ice sheets through time.^{[4-7] 5)}

Although ice meltwater fluxes should not be prescribed for the Core simulation, groups may wish to run alternative simulations with meltwater fluxes to the oceans.

A timeseries of global meltwater routing (river mouth discharge) that is consistent with GLAC-1D's ice sheet reconstruction will be provided.

See the alternative full transient simulations page for more info. [not live yet]

Provided by Lev Tarasov, October 2014:

The data set has surface elevation (ice if present, otherwise ground) relative to contemporaneous sealevel, so the land/seal mask is the 0 elevation contour. It also has an ice mask.

…[It does not currently include] a floating ice mask. I can easily add that later if someone is modelling sub ice shelf circulation…

The Eurasian (EA) and North American (NA) components are from Bayesian calibrations of a glaciological model. The Antarctic (ANT) component is from the recently published scored ensemble of 3344 model runs. The Greenland (GR) component is my old hand-tuned GrB model. The constraint data sets for these models includes RSL (all), marine limits (NA), present-day vertical velocities of the solid earth (NA and EA), geologically inferred deglacial margin chronologies (NA and EA), strandline proxies for pro-glacial lake levels (NA), ice core vertical temperature profiles (GR), cosmo data for constraining past ice thickness (ANT), and present-day ice configuration (ANT and GR).

Details on the constraint data sets for the NA, GR, and ANT components are in the above refs. The Eurasian component is in the process of completion and uses the geologically inferred DATED deglacial ice margin chronology which includes max/min uncertainty isochrones for each timeslice. Each of these glaciological models employed fully-coupled visco-elastic isostatic adjustment of the solid earth.

The ANT model uses the dynamical core of the Pennstate model that includes shallow-shelf ice physics (the other 3 components [have] just the shallow ice approximation).

These 4 components have been combined under GIA post-processing for a near-gravitationally self consistent solution (the approximation is explained in my 2004 QSR paper^[8] and has been tested against complete GIA solutions). The global combined solution includes ICE5-G^[9] components for Patagonia and Iceland for topography (but they were not transfered to the ice mask). There is likely a 10-15 m eustatic equivalent shortfall of LGM ice, the “missing ice” issue that has long challenged GIA-based deglacial reconstructions (uncertainty associated with proxies, tidal changes, and earth viscosity structure).

Please think about the following points and add any comments on these or any other aspects of the experiment design to the discussion section below: [Topics will be added here as they are raised below or by email.]

• No specific points raised yet.
  
  

1. Argus, D. F., Peltier, W. R., Drummond, R. & Moore, A. W. The Antarctica component of postglacial rebound model ICE-6G_C (VM5a) based on GPS positioning, exposure age dating of ice thicknesses, and relative sea level histories. Geophys. J. Int. ggu140 (2014).
2. Peltier, W. R., Argus, D. F. & Drummond, R. Space geodesy constrains ice age terminal deglaciation: The global ICE-6G_C (VM5a) model. J. Geophys. Res. Solid Earth 2014JB011176 (2015).
3. Peltier, W. R. & Vettoretti, G. Dansgaard-Oeschger oscillations predicted in a comprehensive model of glacial climate: A ‘kicked’ salt oscillator in the Atlantic. Geophys. Res. Lett. 41, 2014GL061413 (2014).
4. Tarasov, L. & Peltier, W. R. Greenland glacial history and local geodynamic consequences. Geophys. J. Int. 150, 198–229 (2002).
5. Tarasov, L., Dyke, A. S., Neal, R. M. & Peltier, W. R. A data-calibrated distribution of deglacial chronologies for the North American ice complex from glaciological modeling. Earth Planet. Sci. Lett. 315–316, 30–40 (2012)
6. Briggs, R. D., Pollard, D. & Tarasov, L. A data-constrained large ensemble analysis of Antarctic evolution since the Eemian. Quat. Sci. Rev. 103, 91–115 (2014).
7. Tarasov et al. Eurasian ice sheet evolution (in prep.).
8. Tarasov, L. & Peltier, W. R. A geophysically constrained large ensemble analysis of the deglacial history of the North American ice-sheet complex. Quat. Sci. Rev. 23, 359–388 (2004).
9. Peltier, W. R. Global glacial isostasy and the surface of the Ice-Age Earth: The ICE-5G (VM2) model and GRACE. Annu. Rev. Earth Planet. Sci. 32, 111–149 (2004).
   
   

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