Thresholds for Cenozoic bipolar glaciation

Northern Hemisphere glaciation: all in the timing It is widely accepted that an ice-covered Antarctica, like that of today, first occurred around 34 million years ago. The glacial history of the Northern Hemisphere is, however, less clear. The prevailing view is that continental-scale glaciation arrived in the north about 3 million years ago, although some argue that widespread glaciation might also have occurred at about the time of the first continental-scale glaciation of Antarctica. DeConto et al. investigated these possibilities with a global climate/ice-sheet model that takes into account the long-term decline of atmospheric carbon dioxide levels during this period. Their findings agree with an earlier study that identified a CO2 threshold for the onset of Antarctic glaciation that was plausibly crossed about 34 million years ago. But the carbon dioxide threshold for glaciation in the north appears to be much lower, and so will have been crossed much later, suggesting that episodic Northern Hemispheric ice sheets have been possible only for the past 25 million years or so: still much earlier than commonly assumed, although much later than some have suggested.