If clouds of ice formed high above the poles during the night (as they sometimes do today) computer simulations indicate that temperatures would have stayed above freezing all year round in the Bering Strait land bridge and other putative high-latitude migration routes. Thus the mammals of the late Palaeocene might have survived their long, cold journeys to new lands.

'Polar stratospheric clouds', as these ice-clouds are properly called, absorb and trap some of the warmth absorbed by the ground during the day. They then re-radiate it, preventing it from escaping into space and keeping the poles warmer than they would be if cloudless.

Methane in the air can provide the water vapour to make the ice in these clouds, because methane molecules undergo reactions that form water and other substances.

And indeed, scientists believe there was a sudden injection of methane into the atmosphere around the time of the third great wave of trans-continental migrations, 55 million years ago. As a greenhouse gas, methane is thought to have caused the sudden rise in global temperatures associated with that time.

This methane may have been released from ice-like minerals called methane hydrates on the sea floor, where the low temperatures and high pressures trap methane molecules into cages formed from water molecules.

Methane hydrates are thought to harbour large amounts of methane in the deep sea today. If these deposits are disturbed, by seismic tremors, for example, they can decompose and release the methane in a great belch.

Bubbling into the atmosphere, a burst of methane would warm the planet via the greenhouse effect. Something like this is believed to have happened towards the end of the Palaeocene period.