Description
The space-time cube is a netCDF (.nc) file, and the CF conventions define metadata for netCDF files. These conventions provide a definitive description of the data in each variable and their spatial and temporal properties. Following CF conventions ensures that the space-time cube can be shared, processed, and read by other netCDF tools and software.
The coordinate system of the features is not included in these conventions. If you intend to perform all analysis in ArcGIS AllSource, the warning can be ignored. However, the netCDF file may produce errors when used in other softwares.
Solution
To use the netCDF file in other softwares, use the Project tool to project the features to a coordinate system that is part of the CF conventions and re-create the space-time cube. For more information and to review the CF conventions guidelines, see CF Conventions and Metadata at cfconventions.org.