Sometimes the world seems rational. Ice ages are thought to start when the Isthmus of Panama arose 2-4 million years ago either from volcanic action or the lifting of the Caribbean plate by subduction of the Cocos and South American plates. The would-be Isthmus consisted of a chain of volcanoes along a tectonic plate boundary. When they went active or the Panama plate was pushed upward, the Isthmus formed, connecting North and South America and cutting the warmer currents from Pacific to Atlantic.
Core sediments support this timing beautifully. Pre-Isthmus temperatures show a stability higher than our present levels, then slipped below and into the current ice-age oscillations.
The basic point is that this started the ice ages, which now cycle every 100K years, and they will continue until some major geological change alters it. 
Further, the temperatures of this interglacial trend toward an end of this interglacial. We're cooling, not warming. The small, red segment to the right (click the graph to enlarge it) is the "hockey-stick" warming coming out of the Little Ice Age. Not impressive in the larger perspective.
