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1. Exp 1 does not allow for any particular useful comparisons to observed data, and so does not provide a good evaluation of model responses. Specifically, using a modern ice sheet (GR and WAIS) is a waste of time. I strongly suggest that the group use any pre-calculated LIG ice sheet as a first cut, and allow (and encourage) different variations as and when they become available through either Exp 2 or other efforts. These can be seen as forcing variations, and not new experiments.
2. Having extra diagnostics just for this is not sensible. Instead, lobby and make sure that sufficient diagnostics are included within the basic CMIP6 specification. This is not the only experiment where people might want to run off-line ISMs.
3. Specifying coupling strategies is too prescriptive for exp. 2. Absent any good scientific rationale, groups should do what they can here.

To add on to Gavin's comments, the likely minimum in the Greenland ice sheet was ~120 ka so the CMIP exp. 1 and 2 really won't catch that interval. The PMIP approach would. It is likely that the GIS was ~similar to its present extent (at the GCM scale) around 128 ka and so modern topography could be okay.

I already made this comment offline to some of you, but Bette encouraged me to add this to the wiki. It adds to what Gavin has raised. For the CMIP 128 ka experiment, even with correct ice sheets, there is a problem comparing to the data. For sure, we can (for example using the data syntheses in Capron et al 2014) create a 128 ka timeslice. However, remember that 128 ka is very close to the time when things change rapidly: the fast methane jump in TII centres at 128.6 on AICC2012 (very close to the fast speleo jump in Cheng et al (2009)). The implication is that northern hemisphere temperature and tropical hydrology jumped rapidly somewhere around that point. The problem is that it remains impossible to synchronise records precisely enough at this period, so we would regard any time slice as being at best a 2ka wide window. That means we run the danger that, if we create a 128 ka slice, some records are from before the rapid jump and some are after, making a highly inhomogeneous data target: much more so than if the timeslice was firmly before or after 128 ka.

Of course this also highlights the question of why there was a rapid change in NH climate around this time, which many of us think is likely to be due to a bipolar seesaw response. That then raises the question of what AMOC will be doing in your model runs. Presumably the default would be as Holocene, in which case a data slice firmly after the rapid climate changes would be the more appropriate data target.

I'm afraid there isn't an easy answer to this. 128 ka will give the strongest astronomical/GHG forcing, but you'll definitely have a cleaner data target if you compare to something a little later (or earlier but then you may need to worry about doing something to force AMOC).

Eric's comment is a very poignant deal for proxy comparison. This whole LIG kinda brings up the idea that with a bit more forcing, things get 'weird'. I would suggest contacting Jeremy Hoffman Oregon State (jeremy.hoffman@geo.oregonstate.edu) who has a full paleoclimate database through this interval to say when is the best time to assess pure orbital effects (this would include all uncertainties).

Like Eric says, the 128 ka would be like using a 9 or 8 ka simulation for the Holocene, or not but that timing is undatable.

I think that experiment two is the pathway forward. Since, a good chunk of the polar amplification will come from the changes in topography, I'd really like to include these.

For experiment one, couldn't we just do more Holocene time slices… and maybe then single forcing Holocene instead? (Better proxy coverage.) Although, I feel like another mid-Holocene experiment will be very marginal in the amount of extra info we can get that we didn't already understand from the previous PMIPs.

 Eric -- what period of time during the Eemian do you find to be the most promising.

To do experiment two well, we need strong input from the ISMIP folks. I emailed Sophie Nowicki to see if she would chime in here.

One thing I remember from their meeting last summer is (as usual), everyone wants/needs higher, and higher resolutions from the GCMs. Have we thought about how we will do the down-scaling?

Not sure what you expect to get out of an accelerated setup such as experiment 2 (or are you also accelerating orbital forcing, GHGs,..)? Results would also depend on the spinup/initialization of the ice sheet model.

I second that climate downscaling to ISM scale needs attention.

For a devil's advocate” instead of focussing on transient response during a period with little paleo-data coverage, why not transient response during H1 or YD onset or 8.2 ka for CMIP AR6? Sure the Eemian has little ice, but 128 ka ice and ocean state will have a strong imprint from a poorly constrained penultimate deglaciation.

As Eric noted above, a first order comparison between model simulation/data around Greenland can now be done with data presented in Capron et al. (2014) but do note that their relevant time slices are centered on 130- and 125- ka, with little explicit information about the total uncertainty for time slop in their estimates. This suggests that comparison of a 126- or 127-ka model slice may be just as valid with respect to the data realm, if we suspect a modest uncertainty around +/- 2ka for the compilation. However, the Capron et al. tuning scheme uses the final TII methane jump (~128.6 ka) as one of its characteristic tie-points for developing the age models of N. Atlantic marine records (assigned a relative uncertainty of +/- of 0.5 ka), which would actually reduce the uncertainty of what N. Atl SSTs were doing at/around that time, as long as we agree that SST-based stratigraphies are well-founded. As Anders/Eric/Lev noted full age model uncertainty does need to be addressed by the data folks before truly meaningful comparisons can be made, but I feel that a comparison of the ~transient simulations (exp 2) to the data compilations would/could be useful and at least improve uncertainties for AR6, if the other issues (downscaling et al.) are sufficiently addressed.

In line with what Bette noted, PMIP3 Last Interglacial equilibrium and transient experiments show that it does not make too much of a difference whether one picks 128ka or 125ka, as long as you're after the early CO2 maximum. Other differences like fixed/dynamic vegetation and the prescribed changes in ice sheet configuration and freshwater fluxes are of larger impact. Perhaps an argument in favor of 125ka is that many more groups performed a 125ka simulation as part of PMIP3 (~10) while only ~3 groups did a 128ka experiment. As such, the proposed experiment 1 could be a continuation of the PMIP3 125ka experiment.

I would like to add another twist into the rapid transition that is occurring at ~128.5 ka as others have mentioned above. There is evidence to suggest that sea level rose extremely rapidly at this time, surpassing present eustatic sea level and possibly reaching as high as 6 m above present at this time (according the the Seychelles data) or at least a few meters above present (according to Western Australia data). Either way, this early sea-level rise was most likely sourced from Antarctica if Greenland mass loss occurred gradually and peaked much later (see Anders' comment above). This is to say that in my mind there is a distinct possibility that some sector of the AIS collapsed – coincident with the onset of the sea level highstand. If the models start at some ice sheet configuration similar to present, then they may miss this scenario entirely. Is there a possibility to, for example, set it up such that the WAIS has collapses at 128.5 ka or start with no WAIS, or something that captures this possibility? Surely this will have a significant impact on how things play out from there.

I apologise for starting what seems to have been a blizzard, and I stood back a bit because I think the modelling folks need to decide what the experiment is for before they can decide what it is: I see a few motivations in the messages here and you can't study them all with 2 experiments.

One practical issue is to make sure you are aware that there is a new, more comprehensive compilation of CO2 data now available. (Bereiter, B., S. Eggleston, J. Schmitt, C. Nehrbass-Ahles, T. F. Stocker, H. Fischer, S. Kipfstuhl, and J. Chappellaz (2014), Revision of the EPICA Dome C CO2 record from 800 to 600 kyr before present, Geophys. Res. Lett., 2014GL061957, doi: 10.1002/2014GL061957.) While the main part of the paper is a small correction to values older than 600 ka, they also provide a file that includes and compiles all measured data for the LIG. I don't think it changes much for you (I would read 276 at 128 ka) but you should be aware especially if anyone plans transients, as the new data are much less noisy than the old Vostok data.

When I read the 3 purposes near the top of this page (just above where it says CMIP6), it seems to me that purpose 1 and 3 require roughly modern sea level, ice sheets and maximum insolation, and that this would be the point of expt 1. So why not do as you planned with 128 ka insolation and GHG, and modern sea level - that will give the strongest forcing for intermodel comparisons. BUT do data comparisons with a slightly later time (like 125 ka) when you expect that AMOC and sea level are at least close to modern, with no data straddling regimes. I am sure this kind of asynchronous data-model comparison hurts, but I don't see much alternative unless you simply want to use the weaker insolation forcing at 125 ka anyway.

Eric

Regarding the snapshot LIG experimental design, I don't think there is any merit in prescribing ice sheets which are different to modern. This is because: (a) we don't know what the LIG ice sheets looked like - we have some idea of total sea level change, but within error bars of at least +- 2 metres - and we have very little idea of how this sea level change was distributed between Greenland and Antarctica. (b) the impact of a smaller Greenland ice sheet on global climate is very limited - there is a strong surface air-temperature effect locally, due to lapse-rate and albedo, but outside of the ice sheet itself, the signal is quite small (see e.g. Toniazzo et al, Lunt et al, Ridley et al). Granted - removing parts of West Antarctica may have a larger non-local effect, but i woudl argue that given the uncertainties this shoudl be a subject of sensitivity studies, rather than in the core simulation. © given (a) and (b), and that changing the ice sheets may be a barrier to participation for some groups, I would conclude that we should keep modern ice sheets.