Let's look at the dimensions, variables, and attributes from the netCDF file co2.nc:
ncdump -h co2.nc
netcdf co2 {
dimensions:
T = 456 ;
variables:
float T(T) ;
T:units = "months since 1960-01-01" ;
float co2(T) ;
co2:long_name = "CO2 concentration by volume" ;
co2:units = "1.0e-6" ;
co2:_FillValue = -99.99f ;
// global attributes:
:references = "Keeling_etal1996 Keeling_etal1989 Keeling_etal1995" ;
co2, representing averaged concentrations.T, representing a time axis.units, associated with the co2 variable.references, which
applies to the whole file.