Climate scientists have long debated whether climate shifts abruptly or gradually. It is clear that sometimes climate regimes changed very quickly, in as little as 10 years. A classic climate shift is the start of a typical “glacial” period, or “Ice Age.” Many glacial periods lasting from 40,000 to 100,000 years long have occurred repeatedly over the last 2 ¾ million years; they have been punctuated by warm climatic periods known as “interglacials” (We are now in an interglacial period.) But before 2 ¾ million years ago, an extended regime of warm climate (the Pliocene) had higher sea level, higher temperatures, and higher levels of CO₂ than today. Thus 2 ¾ million years ago, the Earth transitioned to a cooler regime with frequent glacial periods.

        Was that transition global and was it abrupt? Ana C. Ravelo and colleagues at the University of California in Santa Cruz assembled evidence from diverse parts of the globe. They reported in Nature¹ that the climate regimes underwent transitions at different times in the Tropics, the subtropics, and the temperate and polar regions where the Ice Ages were felt. The Tropics and subtropics went through two transitions at different times, unlike the Ice Age regions which experienced the major shift at the end of the Pliocene. Although particular regions sometimes went through a quick transition, the globe as a whole cooled gradually from the Pliocene warmth to the subsequent cool regime.