In a world where lots of people think that the world is experiencing Global Warming, I’ve been thinking all year about ice and ice ages. For according to the Vostok and EPICA ice cores, it was only 12,000 years ago that the Earth emerged from an ice age that had lasted for 100,000 years. How did humans survive during such a prolonged ice age?
I used to work, 4o years ago, building heat flow models, and so I’ve constructed a computer simulation model of heat flow up a core of rock that extends from the centre of the Earth to the top of its atmosphere, 6371 km long, and 1 metre square at the Earth’s surface.
Heat flows conductively from the centre of the Earth to its surface, and radiatively from the surface of the Earth into the atmosphere and to outer space (and from the Sun to the Earth).
The first idea I had was that a layer of ice at the surface of the Earth would act as insulation, and warm the rock beneath. And according to my simulation model, this did indeed happen. The rock beneath the ice cooled at first, and then warmed up.
My second idea was that if the rock beneath the ice warmed up enough, it would start to melt the overlying ice. And according to my simulation model, this is exactly what happened when the subglacial ice temperature rose above 273ºK, the melting point of ice.
And my third idea was that if all the ice melted, the insulation would disappear, and the hot rock beneath the ice would cool down again. And in this manner there could be a cyclic series of ice ages as ice was melted away by hot rocks, and then deposited again on cold rocks. And according to my simulation model, this could indeed sometimes happen.
And my fourth idea was that if the ice under ice sheets was melting, then the ice sheets would be gradually descending, even if new ice was being added at the surface. And according to my simulation model, that’s exactly what the ice did. Typical figures were around 1 cm/year. And it would take about 100,000 years for a kilometre of ice to completely melt away.
And my fifth idea was that if the ice sheets were always melting at their base, there would be streams and lakes beneath the ice. And if the ice covered a large area of land, these streams would become tributaries to rivers flowing beneath the ice, much like those found today on the surface of the Earth. And these rivers would flow out of the edge of the ice sheet into adjoining unglaciated land or sea.
And my sixth idea was that, while the air temperature above the upper surface of the ice might fall as low as 173ºK (-100ºC), the temperature beneath the ice would be stable at 0ºC or slightly higher.
And my seventh idea was that many animals would inhabit the ice caverns beneath the ice, near where the rivers flowed out from under the ice. The ice caves would be inhabited by bats and rats and all sorts of other furry animals. And perhaps some fish would swim up the streams under the ice, to spawn there (e.g. salmon). And if there were animals living under the ice that only came out to feed in daylight, and in summer, then they would have attracted predators like wolves and cats and bears, who would follow them under the ice. And in places where the ice was both translucent and shallow, some plants (mosses?) might have found enough light to survive. And where the caves were large enough, men would have ventured as well, to hunt for sheep and goats and pigs that sheltered there.
And so, by easy steps, we have entered a subglacial underworld. For there once actually was a world that was covered with large areas of ice, 4 km thick in places. And the ice beneath these ice sheets would have been melting, and the meltwaters would have flowed in streams and rivers beneath them out into the adjoining land and sea beyond the perimeter of the ice. And where these streams flowed out, there would have been cave mouths into which animals would have entered, to find shelter in an icy world. There’s nothing irrational about this. And it’s not a mere imaginative thought experiment: it’s all backed up by a heat flow simulation model. And if people don’t believe mine, they can always build their own.
But how extensive were these ice sheets during the last ice age? At present it is believed that they covered much of what is now Canada and parts of the USA. And in Europe they covered all of Scandinavia and areas of north-western Russia. Here’s one map of the world 20,000 years ago:
And much of the evidence for the presence of these ice sheets is derived from their effect on the terrain beneath them. For most of these ice sheets were moving, and where they moved they scoured the ground beneath them, and formed moraines and drumlins made up of transported rocks.
Tomorrow I’ll produce my own suggestions about how far the ice sheets extended at the last glacial maximum.