1. Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA
2. Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA
3. National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK
4. School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3YE, UK
5. Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA

Correspondence to: Robert M. DeConto¹ Correspondence and requests for materials should be addressed to R.M.D. (Email: deconto@geo.umass.edu).

The long-standing view of Earth's Cenozoic glacial history calls for the first continental-scale glaciation of Antarctica in the earliest Oligocene epoch (33.6 million years ago¹), followed by the onset of northern-hemispheric glacial cycles in the late Pliocene epoch, about 31 million years later². The pivotal early Oligocene event is characterized by a rapid shift of 1.5 parts per thousand in deep-sea benthic oxygen-isotope values³ (Oi-1) within a few hundred thousand years⁴, reflecting a combination of terrestrial ice growth and deep-sea cooling. The apparent absence of contemporaneous cooling in deep-sea Mg/Ca records^{4, 5, 6}, however, has been argued to reflect the growth of more ice than can be accommodated on Antarctica; this, combined with new evidence of continental cooling⁷ and ice-rafted debris^{8, 9} in the Northern Hemisphere during this period, raises the possibility that Oi-1 represents a precursory bipolar glaciation. Here we test this hypothesis using an isotope-capable global climate/ice-sheet model that accommodates both the long-term decline of Cenozoic atmospheric CO₂ levels^{10, 11} and the effects of orbital forcing¹². We show that the CO₂ threshold below which glaciation occurs in the Northern Hemisphere (280 p.p.m.v.) is much lower than that for Antarctica (750 p.p.m.v.). Therefore, the growth of ice sheets in the Northern Hemisphere immediately following Antarctic glaciation would have required rapid CO₂ drawdown within the Oi-1 timeframe, to levels lower than those estimated by geochemical proxies^{10, 11} and carbon-cycle models^{13, 14}. Instead of bipolar glaciation, we find that Oi-1 is best explained by Antarctic glaciation alone, combined with deep-sea cooling of up to 4 °C and Antarctic ice that is less isotopically depleted (-30 to -35) than previously suggested^{15, 16}. Proxy CO₂ estimates remain above our model's northern-hemispheric glaciation threshold of 280 p.p.m.v. until 25 Myr ago, but have been near or below that level ever since^{10, 11}. This implies that episodic northern-hemispheric ice sheets have been possible some 20 million years earlier than currently assumed (although still much later than Oi-1) and could explain some of the variability in Miocene sea-level records^{17, 18}.

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