  It has recently been proposed that Earth-like planets in the outer regions of
the habitable zone experience unstable climates, repeatedly cycling between
glaciated and deglaciated climatic states (Menou 2015). While this result has
been confirmed and also extended to explain early Mars climate records
(Haqq-Misra et al. 2016; Batalha et al. 2016), all existing work relies on
highly idealized low-dimensional climate models. Here, we confirm that the
phenomenology of climate cycles remains in 3D Earth climate models with
considerably more degrees of freedom. To circumvent the computational barrier
of integrating climate on Gyr timescales, we design a hybrid 0D-3D integrator
which uses a general circulation model (GCM) as a short relaxation step along a
long evolutionary climate sequence. We find that GCM climate cycles are
qualitatively consistent with reported low-dimensional results. This
establishes on a firmer ground the notion that outer habitable zone planets may
be preferentially found in transiently glaciated states.
