Model-dependent latitudinal temperature gradient drives Late Ordovician climate stability

Among the five great extinction events of the Phanerozoic, the Late Ordovician stands out as it is
concomitant with a massive glacial event under high atmospheric pCO2. This apparent climate
paradox was addressed in numerous climate modeling studies. In particular, [1] showed that under
the specific palaeogeographical conditions of the Hirnantian (445 Ma), with an ocean-dominated
Northern Hemisphere, the climate system may undergo a “tipping point” where a small pCO2
variation leads to either glacial or ice-free warm equilibrium state.
Those results were obtained with the intermediate complexity Fast Ocean Atmosphere Model
(FOAM). We have conducted new simulations using the state-of-the-art coupled IPSL-CM5A2-LR
Earth System Model [2], spanning a wide range of pCO2 for the Hirnantian. We find that the climate
tipping point is entirely absent, and that the equilibrium climate sensitivity is strikingly linear in this
set of simulations.
We conducted a detailed model intercomparison and we have identified major differences between
the models in the representation of the radiative transfer, cloud cycle and oceanic eddy dynamics
which contribute to the qualitatively different model behaviors, enhanced under high atmospheric
pCO2 content. Specifically, the FOAM tipping point corresponds to an abrupt transition from a sharp
Northern latitudinal temperature gradient at low pCO2 (cold state) to a flattened gradient with warm
polar latitudes (ice-free warm state). In contrast, the IPSL-CM5A2 temperature gradient is relatively
constant across pCO2, with year-long sea ice confined in the Northern latitudes even under 15X
preindustrial pCO2 level (4200 ppm).
We propose a physical mechanism to link the warm FOAM flattened latitudinal temperature gradient
to the dramatic sea-ice albedo feedback sensitivity via the increased stratification of the superficial
ocean. Since this mechanism is independent of the physical parameterizations and relative
complexity of the models, and comparing our results with other scarce published climate simulations
of the Hirnantian [3,4], we propose that the latitudinal temperature gradient, seen as a model-
dependent emerging feature, may be the main driver of the previously unveiled sea-ice albedo
climate tipping point.
References:
[1] Pohl et al. (2014), Climate of the Past, 10, 6
[2] Sepulchre et al. (2020), Geoscientific Model Development, 13,7
[3] Pohl et al. (2017), Paleoceanography, 32, 4
[4] Valdes et al. (2021), Climate of the Past, 17, 4