  A generic saddle-node bifurcation is proposed to modelize fast transitions of
finite amplitude arising in geophysical (and perhaps other) contexts, when they
result from the intrinsic dynamics of the system. The fast transition is
generically preceded by a precursor phase which is less rapid, that we
characterize. In this model, if an external source of noise exist, the
correlation length of the fluctuations increases before the transition, and its
spectrum tends to drift towards lower frequencies. This change in the
fluctuations could be a way of detecting catastrophic events before they
happen.
