MODELING LATE ORDOVICIAN (~ 445 MA) CLIMATIC CHANGES WITH THE OCEAN-ATMOSPHERE MODEL FOAM AND THE ICE-SHEET MODEL GRISLI
Sensitivity tests with the ocean-atmosphere climate model FOAM are achieved for CO2 values ranging from 4480 to 560 ppm and with the Late Ordovician paleogeography. Contrary to what was previously shown based on simpler models, it is demonstrated that the temperature-CO2 relationship is highly non-linear when ocean dynamics is taken into account. For high CO2 values, climate is relatively warm, with no sea-ice in the Northern Hemisphere. When CO2 is slightly decreased (from 2240 ppm to 1680 ppm), global temperature falls by 9 K and sea-ice extends down to the mid-latitudes. That critical non-linearity potentially brings a new explanation to the Hirnantian glacial peak that does not require any sharp drop in CO2.
FOAM was then asynchronously coupled to the ice-sheet model GRISLI. By taking the feedback of the ice-sheet on global climate into account, continental-size ice-sheets are obtained for higher CO2 values than it was proposed so far. Our study supports the current vision of a long-lived glaciation with the persistence of South-Polar ice-sheets throughout most of the Ordovician.