Question on general CMIP inquiry of mass balance

[oyvindseland]

PhD Gang Tang from the University of Melbourne, Australia sent this e-mail to a number of CMIP6 model groups early December.
Neither myself or anyone else replied at the time. I do not know if it is still relevant, but I will reply to the author with a link to this issues
The topic seems to be on carbon cycle so I have suggested Mats and Joerg as assignees.

Dear Earth System Model Teams,

I hope this email finds you well. My name is Gang. I am a PhD researcher at The University of Melbourne and the Max Planck Institute for Biogeochemistry. I work on the coupled carbon-nitrogen cycle in the MAGICC reduced-complexity model. I am reaching out to inquire about mass conservation checks in the CMIP6 data submission process and to discuss potential solutions for identified issues.

The email is a bit long. In short, I lay out the issue I identified in our paper (link here). Then, I ask a few questions. The point of considering these questions now is to make life for CMIP7 easier through my role on the data request team. If you have any time, I would greatly appreciate your thoughts. The better we do this now, the fewer headaches there will be at the crunch time of submitting model simulation results.

The Mass Conservation Problem:
Over the past two years, I have been analyzing CMIP6 data, focusing on C4MIP variables, and have found carbon and nitrogen mass conservation issues in the reported data for many experiments and models. These issues arise for both the total land pool and the subpools such as vegetation, litter, and soil (details in our preprint).

Our analysis suggests that the problem primarily stems from data reporting inconsistencies—either incomplete reporting or flux definitions that differ from C4MIP guidelines. Land-use fluxes and fire emissions are likely contributors to the carbon conservation issue, but even accounting for these, some models remain unconserved. For nitrogen, the situation is more complex due to the fluxes interacting between organic and mineral nitrogen pools. These issues hinder data usability and could compromise analyses like carbon cycle feedback quantifications.

Challenges from MIPs and the CMIP Data Request:
As part of the CMIP7 Data Request team (land and land ice), I recognize that some problems may arise from the Data Request itself. For example, for the carbon fluxes, LUMIP and FireMIP request some variables similar to C4MIP (i.e., different variables share the similar/identical definition), which could cause confusion for modeling groups.

Currently, there is no mandatory requirement for modeling groups to submit all variables when participating in MIPs, which is understandable given model differences. However, this means a "black box" for data users - It is impossible to know if the variable is not modeled, not reported, or reported under other variables. Proper documentation of missing fluxes and their causes is the only way I can see to address this, but perhaps there are other ways.

Our Questions:
To address these issues, we kindly ask for your input on the following:

Question 1: Does your modeling group currently perform mass conservation checks for CMIP data submissions? If not, would you prefer to implement them internally, or would a general purpose validation tool for quality checks be preferable?
Question 2: Regarding the CMIP7 Data Request (e.g., v1.0beta), have you encountered challenges in reporting variables for different MIPs (e.g., duplicates, inconsistencies)? Would your team be willing to provide comments on challenging variables, such as whether they are unsupported by your model or need redefinition for clarification?
Question 3: For the current CMIP6 data conservation issues, do you have any insights into potential causes or solutions?  

The Plan for Moving Forward:
As the first step, this email aims to gauge interest from modeling groups in addressing these issues. If your group is interested, a response to question 1 (and, if possible, question 3) would be greatly appreciated.

Later, I will compile a list of CMIP7 Data Request land carbon-nitrogen variables categorized by MIPs. We will need your input on whether these variables are reportable or require adjustments for your specific model. The key point is to identify the incompatibility of model outputs with MIPs.

Finally, I plan to summarize these challenges and present them to the MIPs and CMIP7 Data Request team, with the aim of developing a harmonized data request from MIPs that is convenient for model teams and ultimately resolves the mass conservation issue.

We sincerely appreciate your time and efforts and look forward to your responses!

Best regards,
Gang
On behalf of the author team
Tang, G., Nicholls, Z., Jones, C., Gasser, T., Norton, A., Ziehn, T., Romero-Prieto, A., and Meinshausen, M.: Investigating Carbon and Nitrogen Conservation in Reported CMIP6 Earth System Model Data, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-3522, 2024.

[JorgSchwinger]

This is mostly about the land carbon cycle, so I have added Rosie and Kjetil

[kjetilaas]

This is mostly about the land carbon cycle, so I have added Rosie and Kjetil

Yes, I saw the same. I don't have the answers to the three questions now, but we should be able to respond to this early in the new year.