Earth's climate underwent a major transition from the warmth of the late Pliocene, when global surface temperatures were similar to 2 degrees to 3 degrees C higher than today, to extensive Northern Hemisphere glaciation (NHG) similar to 2.73 million years ago (Ma). We show that North Pacific deep waters were substantially colder (4 degrees C) and probably fresher than the North Atlantic Deep Water before the intensification of NHG. At similar to 2.73 Ma, the Atlantic-Pacific temperature gradient was reduced to < 1 degrees 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-meter sea-level fall from 3.15 to 2.75 Ma, and the development of a strong polar halocline fundamentally altered deep ocean circulation, which enhanced interhemispheric heat and salt transport, thereby contributing to NHG.