State of the planet· The columns of time
Quaternary ice ages
2.6 Ma → Today
The regime we are in, and the only one on the ribbon that oscillates by definition: ice sheets advance and retreat on cycles of tens of thousands of years, driven by the geometry of Earth's orbit.
What the evidence shows
Ice does not arrive and stay. What moves it is the slow change in the shape of Earth's orbit and the tilt of its axis, which redistributes sunlight across latitudes and seasons without changing how much of it arrives. The beat itself changes: about a million years ago, cycles of 41,000 years give way to cycles of roughly 100,000, longer and far deeper, so that the last few glaciations were the coldest of the whole series. We are in a warm phase of this regime, not outside it.
Bounds and conventions
The regime has no sharp beginning: northern ice sheets expand around 2.7 million years ago and the swings settle into their rhythm by about 2.5, so the base of the Quaternary at 2.58 million years is a convenient line through a gradual change rather than a moment. Why the beat lengthened a million years ago has no accepted answer, and the orbit alone does not supply one, since it did not change then.