At ~2.7 million years ago the warm equable climates of early and middle Pliocene time (used here to mean from ~5 to ~2.7 Ma) were replaced by recurring ice ages. Most attempts to explain the change appeal either to changes in CO2 in the atmosphere or reduced heat transport by the Atlantic Ocean. The sources of the strongest teleconnections in the current climate, however, lie in the tropics, and such connections occur by transport of heat and moisture by the atmosphere. The most prominent of these teleconnections link aberrations in sea surface temperatures in the equatorial Pacific, El Ni¿o--Southern Oscillation (ENSO) variations, with warm and dry or cool and wet anomalies in extratropical climates. We show that in most cases early and middle Pliocene climate both in equatorial regions and in the extratropics differ from present-day climates with the same spatial pattern as that associated with ENSO. For instance, not only was Canada warmer during early Pliocene time than at present, as it is during El Ni¿o, but the region surrounding the Gulf of Mexico appears to have been cooler and a bit wetter, as it commonly is during El Ni¿o. A virtually permanent El Ni¿o-like state appears to have characterized pre-Ice Age climates, suggesting that transport of heat by the atmosphere was the principal mechanism that maintained extratropical warmth. Accordingly, cooling and the growth of recurring ice sheets in the Northern Hemisphere resulted from the development of a strong Walker circulation and a weakening of the Hadley circulation.