So What Is in an Earth System Model?

Use of numerical models is at the heart of climate science. Models underpin process understanding; our ability to explain observed past changes and subsequently making predictions/projections of the coming decades to help guide climate policy. Roland Séférian and colleagues (2019, https://doi.org/10.1029/2019MS001791) have written an excellent paper documenting CNRM‐CERFACS's latest complex model, "CNRM‐ESM2‐1," which will be used for CMIP6 simulations and will produce data to be analyzed by the research community for many years to come. Here I explain the importance of Seferian et al. and why it showcases a new degree of comprehensiveness needed for model documentation activity. Plain Language Summary: Just like weather forecasting, climate science relies heavily on simulating the Earth's weather patterns using complicated numerical models on very powerful supercomputers. These models often represent tens or hundreds of thousands of lines of computer code and encompass as much understanding of how the real world operates as we can include into them. Their complexity means that it is very important to document what is in them and how they behave. In this commentary I explain why Roland Séférian and colleagues (2019, https://doi.org/10.1029/2019MS001791) do a particularly good job of explaining and documenting the developments in their new model: "CNRM‐ESM2‐1," Key Points: This commentary describes Seferian et al. documentation of CNRM's new Earth System ModelSeferian et al represents an example of model documentation and traceability for others to follow [ABSTRACT FROM AUTHOR]