A controlled vocabulary is an agreed list of terms definitions used to provide a unique label to a concept. Controlled vocabularies are usually discipline related, their main aim is to facilitate sharing of data in the same community. For this reason is important that the commun ity participate in the development of the vocabulary and agrees to its adoption for them to be useful.
In some case vocabularies have been created in relation to a specific project, and then more widely adopted. As an example, since CMIP is an intercomparison project with modelling groups participating from across the world, it was essential to its success to define and use controlled vocabularies. CMIP6 controlled vocabularies cover many different aspects: experiments, variables, realms, models, sub-projects, frequency, resolution and grid labels. Their definition and labels for variables, frequency and realms are often adopted by others climate data producers.
Controlled vocabularies also provide keywords to use when publishing data. Keywords are a powerful instrument when used properly. They can greatly increase the discoverability of a dataset, which is why it is one of the few highly recommended attributes in the ACDD conventions. Unfortunately there is not yet an agreed controlled vocabulary to be used specifically for climate science. Lots of climate terms are however covered by the Global Change Master Directory Keywords, maintained by NASA