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Abstract. It is widely accepted that orbital variations areresponsible for the generation of glacial cycles during the latePleistocene. However, the relative contributions of the orbital forcingcompared to CO2 variations and other feedback mechanisms causing thewaxing and waning of ice sheets have not been fully understood. Testingtheories of ice ages beyond statistical inferences, requires numericalmodeling experiments that capture key features of glacial transitions. Here,we focus on the glacial buildup from Marine Isotope Stage (MIS) 7 to 6covering the period from 240 to 170 ka (ka: thousand years before present). Thistransition from interglacial to glacial conditions includes one of thefastest Pleistocene glaciation–deglaciation events, which occurred during MIS 7e–7d–7c (236–218 ka). Using a newly developed three-dimensional coupledatmosphere–ocean–vegetation–ice sheet model (LOVECLIP), we simulate thetransient evolution of Northern Hemisphere and Southern Hemisphere ice sheets duringthe MIS 7–6 period in response to orbital and greenhouse gas forcing. For arange of model parameters, the simulations capture the evolution of globalice volume well within the range of reconstructions. Over the MIS 7–6period, it is demonstrated that glacial inceptions are more sensitive toorbital variations, whereas terminations from deep glacial conditions needboth orbital and greenhouse gas forcings to work in unison. For someparameter values, the coupled model also exhibits a critical North Americanice sheet configuration, beyond which a stationary-wave–ice-sheettopography feedback can trigger an unabated and unrealistic ice sheetgrowth. The strong parameter sensitivity found in this study originates fromthe fact that delicate mass imbalances, as well as errors, are integratedduring a transient simulation for thousands of years. This poses a generalchallenge for transient coupled climate–ice sheet modeling, with suchcoupled paleo-simulations providing opportunities to constrain suchparameters.