import numpy as np
import matplotlib.pyplot as plt
import pandas as pd
import netCDF4 as nc

#-- open net-cdf and read in variables
data = nc.Dataset('test.nc')

time = nc.num2date(data.variables['Time'][:],
                   data.variables['Time'].units)

df = pd.DataFrame(data.variables['mgpp'][:,0,0], columns=['mgpp'])

df['dates'] = time
df = df.set_index('dates')
print(df.head())

mgpp
dates
1901-01-01  0.040735
1901-02-01  0.041172
1901-03-01  0.053889
1901-04-01  0.066906

df_cp = df[df.index.year == 2001]