Toward reconciliation of Late Ordovician (∼440 Ma) glaciation with very high CO2 levels

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Abstract
Although Phanerozoic glaciations usually coincided with times of estimated low atmospheric CO2, the Late Ordovician (440 Ma) glaciation is a significant exception. CO2 levels during that time may have been as much as 10 times greater than present. In an earlier paper we suggested that the unique geographic configuration of Gondwanaland may explain such a response, as the edge of the supercontinent was essentially tangent to the south pole, and the moderating effect of the nearby ocean may have suppressed the magnitude of summer warming on the landmass, thereby allowing glacial inception. One limitation to the earlier study was that it used a linear energy balance model (EBM). In this paper we further test the above hypothesis in a suite of experiments with a nonlinear EBM that allows for snow‐albedo feedback. We also utilize updated estimates for CO2 levels, decreases in solar luminosity, and variations in orbital forcing. Baseline experiments with no changes in luminosity or CO2 resulted in an ice‐covered area of 6.3 ×106 km2, 53% of the estimated area covered by the Ordovician ice sheet. Additional experiments for different combinations of orbital forcing, 7X/13X CO2, and −3.5%/−5.0% luminosity yielded 0–35% of the estimated ice area in the Late Ordovician. A crude estimate of possible topographic influences increased these numbers to7–47% of total estimated ice area. Additional factors related to ice sheet growth should increase these values somewhat. These results provide additional support for the high CO2/glaciation explanation, with the caveat that even the partial success occurs only when parameters are at the extreme end of their permissible range. The estimated duration of Ordovician glaciation is also consistent with the migration of Gondwanaland across the south pole, with a centrally located pole yielding ice‐free conditions in the summer. Thus identical levels of external forcing yield either glaciated or ice‐free conditions, with the solution dependent on location of the landmass. Although more work is required on this topic, our experiments suggest that there may be a relatively parsimonious explanation for this perplexing paleoclimate paradox.The results lend further support to the proposition that paleogeography significantly modifies the role of CO2 in the long‐term evolution of climate.