Abstract. It is widely accepted that orbital variations are
responsible for the generation of glacial cycles during the late
Pleistocene. However, the relative contributions of the orbital forcing
compared to CO2 variations and other feedback mechanisms causing the
waxing and waning of ice sheets have not been fully understood. Testing
theories of ice ages beyond statistical inferences, requires numerical
modeling experiments that capture key features of glacial transitions. Here,
we focus on the glacial buildup from Marine Isotope Stage (MIS) 7 to 6
covering the period from 240 to 170 ka (ka: thousand years before present). This
transition from interglacial to glacial conditions includes one of the
fastest Pleistocene glaciation–deglaciation events, which occurred during MIS 7e–7d–7c (236–218 ka). Using a newly developed three-dimensional coupled
atmosphere–ocean–vegetation–ice sheet model (LOVECLIP), we simulate the
transient evolution of Northern Hemisphere and Southern Hemisphere ice sheets during
the MIS 7–6 period in response to orbital and greenhouse gas forcing. For a
range of model parameters, the simulations capture the evolution of global
ice volume well within the range of reconstructions. Over the MIS 7–6
period, it is demonstrated that glacial inceptions are more sensitive to
orbital variations, whereas terminations from deep glacial conditions need
both orbital and greenhouse gas forcings to work in unison. For some
parameter values, the coupled model also exhibits a critical North American
ice sheet configuration, beyond which a stationary-wave–ice-sheet
topography feedback can trigger an unabated and unrealistic ice sheet
growth. The strong parameter sensitivity found in this study originates from
the fact that delicate mass imbalances, as well as errors, are integrated
during a transient simulation for thousands of years. This poses a general
challenge for transient coupled climate–ice sheet modeling, with such
coupled paleo-simulations providing opportunities to constrain such
parameters.