Results from coupled ice-sheet and atmospheric general circulation models show that the waxing and waning of ice sheets during the Late Ordovician were very sensitive to changes in atmospheric pCO2 and orbital forcing at the obliquity time scale (30–40 k.y.). Without orbital forcing, ice sheets can grow with pCO2 level as high as 10 times preindustrial atmospheric level (PAL). However, with orbital forcing, ice sheets can grow only with pCO2 levels of 8 times PAL or lower. These results indicate that the threshold of pCO2 for the initiation of glaciation is on the lower end of previously published estimates of 8–20 times PAL. The ice-sheet model results further indicate that during exceptionally long periods of low summer insolation and low pCO2 levels (8–10 times PAL), large ice sheets could have formed that were able to sustain permanent glaciation under subsequently higher pCO2 values. This finding suggests that in order to end the Late Ordovician glaciation with a rise in pCO2, atmospheric pCO2 must have risen to at least 12 times PAL. Ice sheets therefore introduce nonlinearities and hysteresis effects to the Ordovician climate system. These nonlinearities might have also played a role in the initiation and termination of other glaciations in Earth history.