  A measurement of the sum of neutrino masses is one of the main applications
of upcoming measurements of gravitational lensing of the cosmic microwave
background (CMB). This measurement can be confounded by modelling uncertainties
related to so-called "baryonic effects" on the clustering of matter, arising
from gas dynamics, star formation, and feedback from active galactic nuclei and
supernovae. In particular, a wrong assumption about the form of baryonic
effects on CMB lensing can bias a neutrino mass measurement by a significant
fraction of the statistical uncertainty. In this paper, we investigate three
methods for mitigating this bias: (1) restricting the use of small-scale CMB
lensing information when constraining neutrino mass; (2) using an external
tracer to remove the low-redshift contribution to a CMB lensing map; and (3)
marginalizing over a parametric model for baryonic effects on large-scale
structure. We test these methods using Fisher matrix forecasts for experiments
resembling the Simons Observatory and CMB-S4, using a variety of recent
hydrodynamical simulations to represent the range of possible baryonic effects,
and using cosmic shear measured by the Rubin Observatory's LSST as the tracer
in method (2). We find that a combination of (1) and (2), or (3) on its own,
will be effective in reducing the bias induced by baryonic effects on a
neutrino mass measurement to a negligible level, without a significant increase
in the associated statistical uncertainty.
