  We present models of CO(1-0) emission from Milky Way-mass galaxies at
redshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate the
molecular abundances by post-processing the simulations with an equilibrium
chemistry solver while accounting for the effects of local sources, and
determine the emergent CO(1-0) emission using a line radiative transfer code.
We find that the results depend strongly on the shielding length assumed, which
in our models sets the attenuation of the incident UV radiation field. At the
resolution of these simulations, commonly used choices for the shielding
length, such as the Jeans length, result in CO abundances that are too high at
a given H$_2$ abundance. We find that a model with a distribution of shielding
lengths, which has a median shielding length of $\sim 3$ pc in cold gas ($T <
300$ K) for both CO and H$_{2}$, is able to reproduce both the observed CO(1-0)
luminosity and inferred CO-to-H$_{2}$ conversion factor at a given star
formation rate compared with observations. We suggest that this short shielding
length can be thought of as a subgrid model which controls the amount of
radiation that penetrates giant molecular clouds.
