High priority scenarios

[nwieters]

Hello,
in the last CMIP7 Forcings drop in session there was a timeline given for the scenario forcings. And there was a date mentioned for the high priority scenarios.
Which are the high priority scenarios? Are there any information about it on the CMIP7 webpage that I might have missed?
Thank you very much.

[eleanororourke]

You can find the slides from the drop in session at https://doi.org/10.5281/zenodo.17100872

The suggested first in running order scenarios are H and VLLO (name likely to be updated) but there have been a number of requests to also include M.

As the discussion is continuing we have not posted definitive information on the website yet.

[nwieters]

Thank you very much for your answer.

Is this for emission or concentration driven scenarios, or both?

[robinsmith-ncas]

That said, as a fully paid up member of Team UKESM (Team Colin?) I have a strong personal preference for as emission-driven as possible ;)

[znicholls]

Thanks for the comments @jesusff and @robinsmith-ncas. In terms of concentration-driven vs. emissions-driven spread, one helpful figure could be cross-chapter Box 7.1 Figure 1 from the latest IPCC report (https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-7/, screenshot below). It's not entirely ESM-based, but I would argue that it's our quantification of multiple lines and shows that yes, the uncertainty is wider when the carbon cycle is included, but it's definitely not the leading uncertainty (and, for CORDEX, can probably just be rolled into general climate uncertainty).

[znicholls]

Could you point to some info on these identifiers?

In the experiment ID, there will be different names. The exact form is still being decided I believe, but it will be something like vl and vlc (c suffix for concentration-driven) or vle (e suffix for emissions-driven) and vl or something so clear already from the filename/DRS which is which.

[eleanororourke]

@jschewe if you/ISIMIP have any thoughts about any preference for emission or concentration driven scenarios.

[jschewe]

For the Assessment Fast Track, we'd have a preference for concentration driven scenarios. Simply because emissions driven scenarios would likely introduce new complexities that we need to adapt to, and this might cost us precious time. One such complexity is that historical CO2 concentrations will differ between ESM simulations and observations (and between different ESMs). CO2 concentration is an input for some of the impact models. So far (with CMIP6) we have bias-corrected all relevant atmospheric variables, but not CO2, as this was prescribed to observations (or, for future, by the RCPs). If ESM's historical CO2 concentrations now diverge from observations, that could pose challenges to impact models. Plus impact models would need to implement different CO2 forcings between different ESMs which costs some time.
Another complexity is potential double-counting of carbon cycle processes between ESMs and offline land-surface/biome model simulations in ISIMIP. Similar inconsistencies exist already (ESMs account for land-atmosphere feedbacks but we apply their atmospheric outputs to different land surface models) but there could be more, or more serious, inconsistencies arising that impact modelers need to think about.
None of this is irresolvable, and there'll probably be some interesting science coming out of testing these things. So we don't oppose emissions driven runs in general. Just in the AFT, we'd be keen to avoid anything that could delay the process ever so slightly, given it's already a crunch.