The Coupled Model Intercomparison Project (CMIP) data has become one of the most relevant inputs for the IPCC Assessment Reports (ARs). The IPCC Data Distribution Centre (DDC) Partner DKRZ preserves long-term the CMIP input data underpinning the IPCC ARs since the Second Assessment Report (Stockhause and Lautenschlager, 2022). The main tasks of the DDC have shifted over time: from providing model data for the Working Groups (WGs), to preserving IPCC-related CMIP input data, to providing both services. Meanwhile, WCRP CMIP has continuously refined the experimental protocols, reflecting evolving data formats, standards and leveraging emerging new technologies to facilitate data access and analysis.
All users and service providers including IPCC authors and the DDC benefit from these WCRP CMIP developments. The IPCC FAIR Guidelines have enhanced the transparency of the IPCC AR6 process and its outcomes and made the DDC an active partner in the Sixth cycle: CMIP6 data citations in Appendix II of WGI AR6 give credit to the modeling centers, the Supplementary Chapter Materials document dataset usage, and cross-references between DDC data and AR6 enhance CMIP6 data usage discoverability. Finally, by making the IPCC-related CMIP6 data subset accessible through the Earth System Grid Federation (ESGF), the data hosted at the DDC becomes part of the CMIP6 archive.
Both DDC and CMIP face challenges from increasing data volumes, growing diversity and complexity of scientific experiments and data uses, a rapidly expanding downstream user base, and sustainability of their core services. While IPCC has established a DDC Fund, the CMIP infrastructure still relies on temporary project grants and institutional commitments. Both are increasingly integrating solutions to technical issues into their governance and procedures. Enhancing the established collaboration between IPCC and WCRP CMIP on aspects of data usability for IPCC authors and the visibility of CMIP data in IPCC outputs can streamline procedures for the benefit of IPCC and CMIP.