  Observations of line ratios in the Milky Way's warm ionized medium (WIM)
suggest that photoionization is not the only heating mechanism present. For the
additional heating to explain the discrepancy it would have to have a weaker
dependence on the gas density than the cooling rate, $\Lambda n_e^2$.
\cite{reynolds99} suggested turbulent dissipation or magnetic field
reconnection as possible heating sources. We investigate here the viability of
MHD-wave mediated cosmic ray heating as a supplemental heating source. This
heating rate depends on the gas density only through its linear dependence on
the Alfv\'en speed, which goes as $n_e^{-1/2}$. We show that, scaled to
appropriate values of cosmic ray energy density, cosmic ray heating can be
significant. Furthermore, this heating is stable to perturbations. These
results should also apply to warm ionized gas in other galaxies.
