MODELING LATE ORDOVICIAN (~ 445 MA) CLIMATIC CHANGES WITH THE OCEAN-ATMOSPHERE MODEL FOAM AND THE ICE-SHEET MODEL GRISLI

The Late Ordovician Hirnantian is characterized by the onset of large ice-sheets on the supercontinent Gondwana. The mechanisms permitting to enter glacial conditions at that time are still debated but the best hypotheses consider today a CO₂ drop. Several studies used climate modeling to determine the CO₂ threshold required to trigger glaciation. The last one dates now from more than 10 years ago. Here, we propose to investigate the response of the Ordovician climate to a CO₂ decrease with an Earth system model accounting for the ice-sheet feedbacks.

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.