  Starburst galaxies are undergoing intense periods of star formation.
Understanding the heating and cooling mechanisms in these galaxies can give us
insight to the driving mechanisms that fuel the starburst. Molecular emission
lines play a crucial role in the cooling of the excited gas. With SPIRE on the
Herschel Space Observatory we have observed the rich molecular spectrum towards
the central region of NGC 253. CO transitions from J=4-3 to 13-12 are observed
and together with low-J line fluxes from ground based observations, these lines
trace the excitation of CO. By studying the CO excitation ladder and comparing
the intensities to models, we investigate whether the gas is excited by UV
radiation, X-rays, cosmic rays, or turbulent heating. Comparing the $^{12}$CO
and $^{13}$CO observations to large velocity gradient models and PDR models we
find three main ISM phases. We estimate the density, temperature,and masses of
these ISM phases. By adding $^{13}$CO, HCN, and HNC line intensities, we are
able to constrain these degeneracies and determine the heating sources. The
first ISM phase responsible for the low-J CO lines is excited by PDRs, but the
second and third phases, responsible for the mid to high-J CO transitions,
require an additional heating source. We find three possible combinations of
models that can reproduce our observed molecular emission. Although we cannot
determine which of these are preferable, we can conclude that mechanical
heating is necessary to reproduce the observed molecular emission and cosmic
ray heating is a negligible heating source. We then estimate the mass of each
ISM phase; $6\times 10^7$ M$_\odot$ for phase 1 (low-J CO lines), $3\times
10^7$ M$_\odot$ for phase 2 (mid-J CO lines), and $9\times 10^6$ M$_\odot$ for
phase 3 (high-J CO lines) for a total system mass of $1\times10^{8}$ M$_\odot$.
