Welcome aboard @bjlittle :boat:

The PP-netCDF compatibility problem seems unlikely to crop up, but the netCDF-netCDF (time series) problem does worry me.

As a temporary stance, requiring the inputs to concat to have the same var_name is OK by me. But I can't shake the feeling that leaving var_name in Cube.metadata is going to cause us problems later. I have similar nagging doubts about CoordDefn too.

I think of var_name as just a convenient way to remember what was in a netCDF file, and a way to influence what gets written to a CF-netCDF file. I still don't think CF gives it any semantic value. So including it in metadata comparisons seems a step beyond CF.

My "pragmatic" stance was influenced by the perceived complexity of trying to allow var_name to pass through cube merge when it's not part of the coordinate identity. Perhaps "pragmatic" would be better labelled "lazy" in this case?

In summary, bjlittle has tipped me back to being uneasy about the intrusion of var_name into identity/comparison.