State of the planet· The columns of time

Ice-free to temperate

635 Ma → 2.6 Ma

For six hundred and thirty million years, from 635 to 2.6 million years ago, the Earth had no permanent polar ice sheet, and the whole history of animals before the present ice ages took place in that world.

Regime of six hundred and thirty million years without a permanent polar ice sheet, in which animals appear and the whole of their history prior to the present glaciations takes place.

What the evidence shows

From the end of the last snowball Earth to the beginning of the Quaternary glaciations, the planet retains no permanent polar ice sheet. The interval is not uniform. Two major glaciations fall within it: the end-Ordovician, around 445 million years, cold enough to lower tropical sea temperatures by about five degrees and contemporaneous with the first of the great mass extinctions; and the Late Palaeozoic, which maintained ice on the southern continents intermittently for some hundred million years, with a maximum in the Carboniferous at high latitudes. Neither produced the two-ice-sheet configuration of the present, and the rule across the whole interval remains a warm planet with poles free of permanent ice. The band represents that rule and not each of its exceptions.

Bounds and conventions

Designating six hundred and thirty million years with a single term is the strongest simplification in the series, and the two glaciations spanned were not minor: examination at the scale of the Ordovician or the Carboniferous reveals a colder world than the band indicates. What the band asserts is narrower than its name suggests: the absence of a permanent polar ice sheet, which did hold throughout.