Obliquity forcing with 8–12 times preindustrial levels of atmospheric pCO2 during the Late Ordovician glaciation

Abstract

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 p CO 2 and orbital forcing at the obliquity time scale (30–40 k.y.). Without orbital forcing, ice sheets can grow with p CO 2 level as high as 10 times preindustrial atmospheric level (PAL). However, with orbital forcing, ice sheets can grow only with p CO 2 levels of 8 times PAL or lower. These results indicate that the threshold of p CO 2 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 p CO 2 levels (8–10 times PAL), large ice sheets could have formed that were able to sustain permanent glaciation under subsequently higher p CO 2 values. This finding suggests that in order to end the Late Ordovician glaciation with a rise in p CO 2 , atmospheric p CO 2 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.