Effect of the Ordovician paleogeography on the (in)stability of the climate

Abstract

Abstract. The Ordovician Period (485-443 Ma) is characterized by abundant evidence for continental-sized ice sheets. Modeling studies published so far require a sharp CO 2 drawdown to initiate this glaciation. They mostly used nondynamic slab mixed-layer ocean models. Here, we use a general circulation model with coupled components for ocean, atmosphere, and sea ice to examine the response of Ordovician climate to changes in CO 2 and paleogeography. We conduct experiments for a wide range of CO 2 (from 16 to 2 times the preindustrial atmospheric CO 2 level (PAL)) and for two continental configurations (at 470 and at 450 Ma) mimicking the Middle and the Late Ordovician conditions. We find that the temperature-CO 2 relationship is highly non-linear when ocean dynamics are taken into account. Two climatic modes are simulated as radiative forcing decreases. For high CO 2 concentrations (≥ 12 PAL at 470 Ma and ≥ 8 PAL at 450 Ma), a relative hot climate with no sea ice characterizes the warm mode. When CO 2 is decreased to 8 PAL and 6 PAL at 470 and 450 Ma, a tipping point is crossed and climate abruptly enters a runaway icehouse leading to a cold mode marked by the extension of the sea ice cover down to the midlatitudes. At 450 Ma, the transition from the warm to the cold mode is reached for a decrease in atmospheric CO 2 from 8 to 6 PAL and induces a ∼ 9 • C global cooling. We show that the tipping point is due to the existence of a 95 % oceanic Northern Hemisphere, which in turn induces a minimum in oceanic heat transport located around 40 • N. The latter allows sea ice to stabilize at these latitudes, explaining the potential existence of the warm and of the cold climatic modes. This major climatic instability potentially brings a new explanation to the sudden Late Ordovician Hirnantian glacial pulse that does not require any large CO 2 drawdown.

Climate Sensitivity on Geological Timescales Controlled by Nonlinear Feedbacks and Ocean Circulation

“…However, the details of the coastlines in the paleogeographies are uncertain, in particular for greenhouse climates (Sømme et al, 2009). Furthermore, the nonlinearity found in our simulations supports previous studies that found that past climate sensitivity was a function of baseline CO 2 (e.g., Pohl et al, 2014, for the Ordovician) and suggests that further nonlinear behavior would occur if we carried out simulations at ×1 or ×8 CO 2 .…”

Earliest land plants created modern levels of atmospheric oxygen

“…The Late Ordovician (445 Ma, Hirnantian stage) setup of the model is fully described elsewhere (32). The model is driven by existing Late Ordovician climate simulations (31), conducted at a range of different atmospheric CO 2 and O 2 concentrations. Initially, we assume atmospheric O 2 = 0.6 PAL (∼14 vol.%) at 445 Ma, which is consistent with COPSE model simulations (Fig.…”

Possible patterns of marine primary productivity during the Great Ordovician Biodiversification Event

“…Because the location and depth of Ordovician ocean ridges are not well constrained, they are not included in the model. We use a flat-bottom ocean, the depth of which is set to present-day mean seafloor depth, that is À4000 m (Pohl et al 2014). The flat bottom is not expected to constitute a major bias.…”

“…These more reliable global palaeogeographical reconstructions have allowed the formulation of simple conceptual and more complex numerical models for ancient atmospheric and ocean circulation patterns, or to hypothetically locate upwelling zones (e.g. Wilde 1991;Christiansen & Stouge 1999;Herrmann et al 2004;Pohl et al 2014;Servais et al 2014). More recently, new constraints on the palaeobiogeography of marine living communities were provided by the publication of maps showing much more precisely the ocean surface circulation modelled at various atmospheric CO 2 levels during the Early, Middle and Late Ordovician (Pohl et al 2016b).…”