  North Atlantic climate during glacial times was characterized by
large-amplitude switchings, the Dansgaard-Oeschger (DO) events, with an
apparent tendency to recur preferably in multiples of about 1470 years. Recent
work interpreted these intervals as resulting from a subharmonic response of a
highly nonlinear system to quasi-periodic solar forcing plus noise. This
hypothesis was challenged as inconsistent with the observed variability in the
phase relation between proxies of solar activity and Greenland climate. Here we
reject the claim of inconsistency by showing that this phase variability is a
robust, generic feature of the nonlinear dynamics of DO events, as described by
a model. This variability is expected from the fact that the events are
threshold crossing events, resulting from a cooperative process between the
periodic forcing and the noise. This process produces a fluctuating phase
relation with the periodic forcing, consistent with proxies of solar activity
and Greenland climate.
