Time varying orography

[martinjuckes]

The "orography_type" enumeration has options "present_day" and "modified". The phrasing implies that these are considered as fixed during a simulation, but some CMIP6 models will have evolving ice sheets and hence evolving orography. I suggest changing this list to "fixed_present_day", "fixed_modified" and "evolving". From the perspective of the data request it will be important to know which (configured) models have evolving ice sheets.