CDO

Hello,
I am working with the TRENDY output and struggling to calculate the global average for the CLASS-CTEM output. Since it's GPP, I wanted to get the global sum. The command cdo gridarea results in

cdo gridarea CLASS-CTEM_S2_gpp_2000.nc test.nc
cdo gridarea (Abort): grid_cell_area: Unsupported grid type: generic

the header

netcdf CLASS-CTEM_S2_gpp_2000 {
dimensions:
lon = 128 ;
lat = 64 ;
time = UNLIMITED ; // (12 currently)
variables:
double time(time) ;
time:standard_name = "time" ;
time:units = "days since 1861-1-30 00:00:00" ;
time:calendar = "proleptic_gregorian" ;
time:axis = "T" ;
double gpp(time, lat, lon) ;
gpp:_FillValue = -99999. ;
gpp:missing_value = -99999. ;

// global attributes:
:CDI = "Climate Data Interface version 1.8.0 (http://mpimet.mpg.de/cdi)" ;
:Conventions = "CF-1.6" ;
:history = "Wed Jun 05 15:52:14 2019: cdo selyear,2000 CLASS-CTEM_S2_gpp.nc CLASS-CTEM_S2_gpp_2000.nc" ;
:units = "kgC/m^2/s" ;
:long_name = "Gross_primary_production" ;
:title = "CLASS-CTEM output generated for 2016 TRENDY analysis of GCP" ;
:contact = "Joe Melton, joe.melton@canada.ca" ;
:code_id = "57efbfab328a5e54d96ab50ca024498477d36a97" ;
:CDO = "Climate Data Operators version 1.8.0 (http://mpimet.mpg.de/cdo)" ;

and grid description

1. gridID 1 #
   gridtype = generic
   gridsize = 8192
   xsize = 128
   ysize = 64

look a bit funny. Just as a check I tried fldmean, it works but with a warning that I suspect tells me it hasn't calculated the average in the correct way (I'm looking at gpp)

cdo fldmean CLASS-CTEM_S2_gpp_2000.nc test.nc
cdo fldmean (Warning): Grid cell bounds not available, using constant grid cell area weights for variable gpp!
cdo fldmean: Processed 98304 values from 1 variable over 12 timesteps ( 0.11s )

Is there anyway to fix this with CDO? I randomly selected a year and attached it.

Thanks a lot,
Lina