  Dansgaard-Oeschger events are a prominent mode of variability in the records
of the last glacial cycle. Various prototype models have been proposed to
explain these rapid climate fluctuations, and no agreement has emerged on which
may be the more correct for describing the paleoclimatic signal. In this work,
we assess the bimodality of the system reconstructing the topology of the
multi--dimensional attractor over which the climate system evolves. We use
high-resolution ice core isotope data to investigate the statistical properties
of the climate fluctuations in the period before the onset of the abrupt
change. We show that Dansgaard-Oeschger events have weak early warning signals
if the ensemble of events is considered. We find that the statistics are
consistent with the switches between two different climate equilibrium states
in response to a changing external forcing (e.g. solar, ice sheets...), either
forcing directly the transition or pacing it through stochastic resonance.
These findings are most consistent with a model that associates
Dansgaard-Oeschger with changing boundary conditions, and with the presence of
a bifurcation point.
