The impact of paleogeography, pCO2, poleward ocean heat transport and sea level change on global cooling during the Late Ordovician
Abstract We performed sensitivity experiments with the global climate model GENESIS on two stages of the Upper Ordovician (Caradocian, ∼454 Ma; Ashgillian, ∼446 Ma) under a range of atmospheric p CO 2 values (8–18× PAL; Pre-industrial Atmospheric Level), high and low sea level, and two values of poleward ocean heat transport in order to determine the importance of these variables on global cooling. We then coupled a three-dimensional ice sheet model to the global climate model in order to investigate the necessary boundary conditions for ice sheet formation. All simulations with a high sea level and normal heat transport remain free of ice sheets, even with p CO 2 levels as low as 8× PAL. In the Caradocian simulations, ice sheets form in three scenarios: (1) with p CO 2 of 8× PAL and a low sea level and normal poleward ocean heat transport, (2) with p CO 2 of 8× PAL and a high sea level and reduced (50% of normal) poleward ocean heat transport, and (3) with p CO 2 of 15× PAL and a low sea level and reduced poleward ocean heat transport. In the Ashgillian simulations, ice sheets form in only two scenarios: (1) with p CO 2 of 8× PAL and a low sea level and normal poleward ocean heat transport, or (2) with p CO 2 of 8× PAL and a high sea level and reduced poleward ocean heat transport. The ice sheets in the Ashgillian experiments are larger and thicker than the ice sheets in the Caradocian simulations because the southward movement of Gondwana increased land area in the higher southern latitudes where ice sheets could grow. The threshold for glaciation under Ashgillian paleogeography is 8× PAL and either a low sea level (exposed shelves) or a reduced poleward ocean heat transport. While the paleogeographic evolution and a drop in p CO 2 during the Late Ordovician cooled the global climate, changes in additional factors were required to initiate ice sheet formation, such as a drop in sea level, a reduction in poleward ocean heat transport, or a combination of both.