Miocene Climatic Optimum

The Miocene Climatic Optimum was a warm interval from about 17 to 15 million years ago, the hottest time since the early Eocene. Global temperatures were several degrees warmer than today's, forests extended to high latitudes, and the ice sheet on Antarctica shrank and may have retreated in places. It followed a long cooling that began when Antarctica froze and was reversed for about two million years.

A likely driver was carbon dioxide released by the Columbia River Basalts, flood eruptions in what is now Oregon, Washington and Idaho between about 16.7 and 15.9 million years ago. They poured out roughly 210,000 cubic kilometres of lava, and some flows ran more than 500 kilometres to the Pacific coast. Changes in Earth's orbit and ocean circulation also contributed. The cause is debated, and some studies stress carbon dioxide while others stress ocean currents. After about 14 million years ago the climate cooled steeply in the Middle Miocene Climate Transition, and the East Antarctic ice sheet grew back to near its present size. The warm years suited the diverse apes then living in Europe and Africa, and the following cooling dried the continents and favoured grasslands.