  The Earth is well-known to be, in the current astronomical configuration, in
a regime where two asymptotic states can be realised. The warm state we live in
is in competition with the ice-covered snowball state. The bistability exists
as a result of the positive ice-albedo feedback. In a previous investigation
performed on a intermediate complexity climate model we have identified the
unstable climate states (Melancholia states) separating the co-existing
climates, and studied their dynamical and geometrical properties. The
Melancholia states are ice-covered up to the mid-latitudes and attract
trajectories initialised on the basins boundary. In this paper, we study how
stochastically perturbing the parameter controlling the intensity of the
incoming solar radiation impacts the stability of the climate. We detect
transitions between the warm and the snowball state and analyse in detail the
properties of the noise-induced escapes from the corresponding basins of
attraction. We determine the most probable paths for the transitions and find
evidence that the Melancholia states act as gateways, similarly to saddle
points in an energy landscape.
