Simulations

My other major worry relates to the quantity of simulations requested by the full experimental design. Whilst, I understand the justification of most of them from a rigorous scientific standpoint, I wonder if they will be tackled by sufficient groups to allow a model intercomparison. I also feel that most of the justification is for the Pliocene factoralisation experiments, and insufficient discussion is made of the CMIP DECK.

It is never mentioned that E\(^{280}\) (the preindustrial control run) must be performed as part of the DECK for CMIP6. As an outsider, I may read this paper and be really afraid of joining PlioMIP2 because of the quantity of simulations required. I suggest it is worth emphasizing that you can join (and be an important member of PlioMIP2) just be performing a single run (Eoi\(^{400}\)). I don’t know how many groups have signed up, but I’d expect most of them to only tackle that core simulation.

I was rather confused by the quantity of simulations requested for the Pliocene for Future side of Figure 2. There doesn’t seem to be much joined up thinking between this element and the wider picture of CMIP6. The purpose of the Tier 2 experiments E\(^{560}\) and Eoi\(^{560}\) appears to be to allow the Charney climate sensitivity to be calculated (Section 3.1.1). The DECK already involves a simulation specifically to calculate this metric - an abrupt quadrupling of CO\(_2\) - using a technique devised by \cite{Gregory_2004}. This approach only needs 150 years of computation rather than the minimum of 500 years proposed here. The DECK also involves standard transient forcing simulation where CO\(_2\) concentrations increase by 1% per year. Current approaches infer knowledge from past climates to constrain future projections involve either subsetting (i.e. discounting bad models) or using emergent constraints. Both of these approaches work based on just the CORE simulation and the simulations performed in the DECK. I therefore question whether any of the Pliocene for Future runs can be justified from that perspective.

The question of state dependence of climate sensitivity is an rather interesting one. The most efficient way to tackle it would be perform the Abrupt 4xCO2 experiment feature in the DECK, but using a Pliocene base state (perhaps denoted as Eio\(^{400}_{4xCO_2}\)). This would effectively replace Eio\(^{560}\), whilst E\(^{560}\) gains little over and above the E\(^{280}_{4xCO_2}\) simulation already in the DECK.

I don’t know whether there is any benefit to replicating the 1% simulation from a Pliocene base state, as I’m not sure people have previously investigated whether the ocean heat uptake efficiency is state dependent. It’s an interesting experiment to consider for some explorative science, but you’d probably need to do it in one model first to justify its inclusion.

There is an alternative potential justification for both E\(^{400}\) and E\(^{560}\), which is that they version of a stabilisation scenario. I didn’t immediately spot any runs in ScenarioMIP looking at determining what our target CO\(_2\) should be on the longer term. Nonetheless, I’d hestitate to include them in PlioMIP2 with that solely justification and feel they would sit better elsewhere. It would appear that you need E\(^{400}\) for the factorisation however. If you include that run, please talk to some folks involved in ScenarioMIP and try to get it used from both perspectives.

Despite all these negative comments about the scenario choices, you’re surely along the correct lines. You may want to instead have Eoi\(^{350}\) and Eoi\(^{450}\) as a Tier 1, that spans both the past and future elements, as I feel the CO\(_2\) uncertainty is pretty important to examine for both. This would mean that the Pliocene for Future section consists solely of Eio\(^{400}_{4xCO_2}\).

My recommended groupings would look like: