Preprints
https://doi.org/10.5194/egusphere-2024-3522
https://doi.org/10.5194/egusphere-2024-3522
22 Nov 2024
 | 22 Nov 2024
Status: this preprint is open for discussion.

Investigating Carbon and Nitrogen Conservation in Reported CMIP6 Earth System Model Data

Gang Tang, Zebedee Nicholls, Chris Jones, Thomas Gasser, Alexander Norton, Tilo Ziehn, Alejandro Romero-Prieto, and Malte Meinshausen

Abstract. Reliable, robust, and consistent data are essential foundations for analysis of carbon cycle feedbacks. Here, we consider the data from multiple Earth System Models (ESMs) participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6). We identify a mass conservation issue in the reported carbon and nitrogen data, with a few exceptions for specific models and reporting levels. The accumulated mass imbalance in the reported data can amount to hundreds of gigatons of carbon or nitrogen by the end of the simulated period, largely exceeding the total carbon/nitrogen pool size changes over the same period. Nitrogen mass imbalance is evident across all reported organic and inorganic pools, with mineral nitrogen exhibiting the most significant cumulative mass imbalance. Due to a lack of details in the reported data, we cannot uniquely identify the cause of this imbalance. However, we postulate that the carbon cycle imbalance in the reported data primarily stems from missing fluxes in the reported data and the inconsistency between the reported data and the definitions provided by the C4MIP protocol (e.g., land use and fire emissions), rather than from an underlying mass conservation issue in the models themselves. Our findings suggest that future CMIP reporting protocols should consider incorporating mass conservation into their data validation processes so that such issues are caught before users have to deal with them, rather than forcing all users to handle this issue in their own way. In addition, attention from model groups to the detailed diagnostic request and definitions, along with their own quality control will also help to avoid such issues in future. Given that CMIP6 data is no longer being reported, we recommend that data users that rely on a closed carbon/nitrogen cycle address potential flux imbalances by using the workarounds provided in this study.

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Gang Tang, Zebedee Nicholls, Chris Jones, Thomas Gasser, Alexander Norton, Tilo Ziehn, Alejandro Romero-Prieto, and Malte Meinshausen

Status: open (until 17 Jan 2025)

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Gang Tang, Zebedee Nicholls, Chris Jones, Thomas Gasser, Alexander Norton, Tilo Ziehn, Alejandro Romero-Prieto, and Malte Meinshausen

Data sets

Code and Data for "Investigating Carbon and Nitrogen Conservation in Reported CMIP6 Earth System Model Data" Gang Tang et al. https://doi.org/10.5281/zenodo.14060169

Gang Tang, Zebedee Nicholls, Chris Jones, Thomas Gasser, Alexander Norton, Tilo Ziehn, Alejandro Romero-Prieto, and Malte Meinshausen

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Short summary
We analyzed carbon and nitrogen mass conservation in data from CMIP6 Earth System Models. Our findings reveal significant discrepancies between flux and pool size data, particularly in nitrogen, where cumulative imbalances can reach hundreds of gigatons. These imbalances appear primarily due to missing or inconsistently reported fluxes – especially for land use and fire emissions. To enhance data quality, we recommend that future climate data protocols address this issue at the reporting stage.