  Measurements of gravitational lensing of the cosmic microwave background
(CMB) hold the promise of yielding unique insights into cosmology at high
redshift. Uncertainties due to baryonic effects associated with galaxy
formation and evolution, including gas cooling, star formation, and feedback
from active galactic nuclei (AGN) and supernovae, have typically been neglected
when forecasting the sensitivity of future CMB surveys. In this paper, we
determine the impact of these effects using four suites of hydrodynamical
simulations which incorporate various prescriptions for baryonic processes,
namely OWLS, BAHAMAS, Horizon, and IllustrisTNG. Our analysis shows
characteristic power suppressions of several percent in CMB lensing due to
baryonic effects, compared to dark-matter only simulations, at experimentally
observable angular scales. We investigate the associated bias in the inferred
neutrino mass for experiments like the upcoming Simons Observatory and CMB-S4.
Depending on the experimental precision and the strength of the baryonic
feedback within the simulations, biases in the neutrino mass sum show
significant dispersion, ranging from very small to an over-estimation by
1.1$\sigma$. We conclude that baryonic effects will likely be non-negligible
for a detection of neutrino mass using CMB lensing.
