State of the planet· The columns of time
Ice-free to temperate
635 Ma → 2.6 Ma
Six hundred and thirty million years mostly without permanent ice caps. Animals appear at its opening, and everything they did before the ice ages happened inside it.
What the evidence shows
From the end of the last Snowball to the start of the present ice ages, the planet keeps no permanent polar cap. Those six hundred and thirty million years are not uniform. Two major glaciations fall within them: the Late Ordovician, around 445 million years ago, cold enough to drop tropical sea temperatures by about five degrees and coinciding with the first of the great mass extinctions; and the late Palaeozoic icehouse, which kept ice on the southern continents, on and off, for roughly a hundred million years, and peaked in the Carboniferous, at least in the high-latitude record. Neither built the two-cap world we live in, and between and around them the default is a warm planet with open poles. The ribbon draws that default rather than every excursion: the simplification every band on this timeline makes, and the reason each one carries a note saying so.
Bounds and conventions
Calling six hundred and thirty million years by one name is the largest simplification on the ribbon, and the two glaciations it passes over were not small: a reader zooming into the Ordovician or the Carboniferous will find a colder world than the band admits. What the band asserts is narrower than its name suggests: only that no permanent polar cap existed, and that much held throughout.