Toward reconciliation of Late Ordovician (∼440 Ma) glaciation with very high CO2 levels
Level 5 - mechanism / opinion, no new human data
Mechanistic numerical climate modeling (Level 5 by design analogy, non-clinical scholarship)
OpenAlex W2046810737 · doi:10.1029/91jd02449
What was done
The authors used a nonlinear energy balance model (EBM) with snow-albedo feedback to simulate Late Ordovician (~440 Ma) glacial inception under high atmospheric CO2 (7X to 13X present levels). The simulations evaluated the impact of Gondwanaland's geographic position tangent to the south pole, updated reductions in solar luminosity (-3.5% to -5.0%), orbital variations, and estimated topographic influences on permanent ice-covered area.
What was found
Baseline experiments with no changes in luminosity or CO2 produced an ice-covered area of 6.3 × 10^6 km^2 (53% of the estimated Ordovician ice sheet). Varying orbital forcing, CO2 (7X/13X), and luminosity (-3.5%/-5.0%) yielded 0% to 35% of the estimated ice area. Incorporating crude estimates of topographic influences raised this to 7% to 47% of total estimated ice area. Identical forcing produced ice-free conditions when the continent was centrally located over the pole.
Why it matters
The study shows how continental geometry and oceanic moderation can substantially alter climate sensitivity, providing a plausible physical mechanism for glaciation during periods of high atmospheric greenhouse gas concentrations.
Limits
The model achieved only partial ice coverage (at most 47% of geological estimates) and required forcing parameters at the extreme ends of their permissible ranges. The study relied on a simplified energy balance model with crude topographic parameterization rather than a coupled ocean-atmosphere general circulation model.
Cited by
- context There have been periods in Earth's history when atmospheric carbon dioxide levels were significantly higher than today while global temperatures were significantly colder.