[CF-metadata] Proposed standard names for biological model outputs

[Jonathan Gregory]

Dear Michael

These look very good to me, and thanks for your clear definitions. I have one
question, about silica (=SiO2). Do you mean silicon? 

In the discussions about Christiane's aerosol names we appear to have agreed
that we need only say X_expressed_as_Y when X and Y are different; that is
the rule you have used, so yours and hers are consistent.

Best wishes

Jonathan