Add a "scheme" for GCOS Essential Climate Variables

[6a6d74]

GCOS defines a set of Essential Climate Variables (ECV). These are all Quantity Kinds ...

It would be useful to

• ensure that /common/quantity-kind included all the GCOS ECVs
• add a SKOS Concept Scheme that includes only the set of ECV terms

Note that the ECV list is hierarchical:

• Atmospheric
  • Surface: Air temperature, Wind speed and direction, Water vapour, Pressure, Precipitation, Surface radiation budget.
  • Upper-air: Temperature, Wind speed and direction, Water vapour, Cloud properties, Earth radiation budget (including solar irradiance).
  • Composition: Carbon dioxide, Methane, and other long-lived greenhouse gases, Ozone and Aerosol, supported by their precursors.
• Oceanic
  • Surface: Sea-surface temperature, Sea-surface salinity, Sea level, Sea state, Sea ice, Surface current, Ocean colour, Carbon dioxide partial pressure, Ocean acidity, Phytoplankton.
  • Sub-surface: Temperature, Salinity, Current, Nutrients, Carbon dioxide partial pressure, Ocean acidity, Oxygen, Tracers.
• Terrestrial: River discharge, Water use, Groundwater, Lakes, Snow cover, Glaciers and ice caps, Ice sheets, Permafrost, Albedo, Land cover (including vegetation type), Fraction of absorbed photosynthetically active radiation (FAPAR), Leaf area index (LAI), Above-ground biomass, Soil carbon, Fire disturbance, Soil moisture.

Outstanding questions:

• how should the concepts relating to top and middle nodes in the hierarchy be captured; they're not quantity-kinds themselves but categories.
• where should the GCOS ECV scheme be located; should we create a new top-level register for GCOS? More though required.

[6a6d74]

Note: the top-level in the hierarchy is defined as "Domain"; perhaps this is a separate code list for GCOS?