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Earth’s climate underwent a major transition from the warmth of the late Pliocene, when global surface temperatures were ~2-3°C higher than today, to extensive Northern Hemisphere glaciation (NHG) at ~2.73 Ma. We show that North Pacific deep waters were significantly colder (4°C) and likely fresher than North Atlantic deep water prior to the intensification of NHG. At ~2.73 Ma, the Atlantic-Pacific temperature gradient was reduced to <1°C suggesting the initiation of stronger heat transfer from the North Atlantic to the deep Pacific. We posit that increased glaciation of Antarctica, deduced from the 21 ± 10 m sea-level fall from 3.15-2.75 Ma, and the development of a strong polar halocline, fundamentally altered deep ocean circulation, which enhanced inter-hemispheric heat and salt transport thereby contributing to the NHG.