It is widely accepted that an ice-covered Antarctica first occurred ∼34 million years ago, but the glacial history of the northern hemisphere is less clear. This paper investigates the possibilities of when the continental-scale glaciation of the north occurred with a global climate/ice-sheet model that takes into account the long-term decline of atmospheric CO2 levels during this period. The CO2 threshold for glaciation in the north seems to be much lower, and so will have been crossed much later than ∼34 million years ago, suggesting that episodic northern-hemispheric ice sheets have been possible only for the past ∼25 million years.
- Robert M. DeConto
- David Pollard
- Mark Pagani
