State of the planet· The columns of time

Quaternary ice ages

2.6 Ma → Today

For the last 2.6 million years, the current regime, ice sheets have advanced and retreated on cycles of tens of thousands of years set by the geometry of the Earth's orbit.

Current regime, and the only one in the series defined by an oscillation: ice sheets advance and retreat on cycles of several tens of thousands of years governed by the geometry of the Earth's orbit.

What the evidence shows

Ice does not settle permanently. What moves it is the slow variation in the shape of the Earth's orbit and in the tilt of its axis, which redistributes solar energy between latitudes and seasons without altering the total received. The pacing itself changes: about a million years ago, cycles of 41,000 years give way to cycles of about 100,000, longer and of greater amplitude, so that the last glaciations are the coldest of the series. The present interval is a warm phase of this regime and not an exit from it.

Bounds and conventions

The regime has no sharp beginning: northern hemisphere ice sheets expand around 2.7 million years and the oscillations settle into their rhythm around 2.5 million, so that the base of the Quaternary at 2.58 million years is a convenient line drawn across a gradual change. The slowing of the pacing a million years ago has no accepted explanation, and the orbit supplies none, not having changed at that date.