  We calculate the polarization function of Dirac fermions in metallic armchair
graphene nanoribbons for an arbitrary temperature and doping. We find that at
finite temperatures due to the phase space redistribution among inter-band and
intra-band electronic transitions in the conduction and valence bands, the full
polarization function becomes independent of the temperature and the position
of the chemical potential. As a result, for a given width of nanoribbons there
exists a single plasmon mode, with the energy dispersion determined by the
graphene's fine structure constant. In Coulomb-coupled nanoribbons, this
plasmon splits into the basic in-phase and out-of-phase plasmon modes, with the
splitting energy determined additionally by the inter-ribbon spacing.
