A couple of days back I showed one contemporary idea of how the world looked at the last glacial maximum, 20,000 years ago:
But after managing, using a simple simulation model, to advance an ice sheet all the way to latitude 26º (and beyond), I’m now considering a world that looks more like this:
In fact perhaps the oceans above 26º should be covered with ice as well. But I don’t yet know how to model convective heat flow in water.
In such a world pretty well all plant and animal life would be constrained to the region between the tropic of Cancer and the tropic of Capricorn. And this region would have been much cooler than it currently is. And with much of the Earth’s oceans covered in ice, and so with reduced scope for evaporation, it would have been a drier world than today.
The ice would not of course have extended to exactly 26º as I’ve shown it. In some places it would have not extended so far, and in other places farther. And high mountainous areas such as the Andes in South America would have been glaciated. So this map will need to be edited.
But more or less any geologist or glaciologist looking at this map would probably say that such a world is impossible, and that anyway it was never actually like that.
But according to my simulation models, such a world actually is possible. And it’s one in which the ice extends rapidly from about latitude 75º in a kind of domino effect, with the air in one cold latitude spilling into lower latitudes, lowering air temperatures below water freezing point, with rain turning to snow, latitude by latitude, as the ice marched south.
And if geologists don’t believe that the world was ever covered in so much ice, it would seem that it is because they have found no evidence of the presence of ice at latitudes lower than 51º N (in the UK). And this evidence is in the form of moraines and drumlins and scour marks on the ground over which the ice sheets moved. And these ice sheets were believed to be several kilometres deep in places. But is absence of evidence elsewhere ever evidence of its absence?
If it is entirely plausible that a sliding ice sheet a couple of km deep would create moraines and drumlins and other features on the ground beneath it, is it likely that ice sheets that were only tens or hundreds of metres deep would have the same effect? Surely it must be the pressure exerted on the ground beneath the ice sheets that must cause most of these effects? And this pressure is proportional to the depth of the ice. So it seems plausible that there is some threshold depth of ice below which it has little or no effect on the ground beneath it.
And do all ice sheets move? It seems entirely plausible that ice sheets should move on convex sloping ground: the base of these ice sheets are very often likely to be at 0ºC, with liquid water flowing under them, and so liable to slide in one mass. But what of ice sheets that stand on flat ground, or lie in concave hollows, or lie over undulating terrain? These might hardly move at all. And so it seems entirely plausible that while ice sheets moved in some places, they did not move in others. In Britain, the terrain grows more mountainous the further north one travels. In Scotland and Wales there are mountains, in northern England high hills, and in many places in southern England there are flat plains (e.g. Norfolk, Suffolk, Wiltshire, Somerset). It is currently believed that there were ice sheets covering much of Scotland and Wales and northern England, but none in southern England, because there is no evidence in the form of moraines and drumlins. But could this not have been because there were ice sheets over much of southern England, but they were largely motionless? And the same applies to the extensive plains in much of Europe, and to the steppes of Russia, and in many other places.
Let’s do some back-of-envelope calculations of the likely depths of ice sheets that extended from the poles to the tropics. About 71% of the Earth is covered with water, and sea levels have risen 120 m since the last glacial maximum 20,000 years ago. We also know that there are about another 66 m of ice remaining in the ice sheets of Antarctica and Greenland and elsewhere. So that’s a total of 186 m of water/ice. And this would cover the whole of the Earth to a depth of 132 m of ice.
But during an ice age it’s unlikely that the ice would be of equal depth everywhere. It would be more likely to be thicker at the poles than the tropics, because its much colder at the poles than the tropics, and the ice will have existed longer at the poles. So let’s now distribute the 132 m of ice so that it has a depth of twice this amount – 264 m – at the poles, and a depth of 0 m at the equator, with proportionate depths in areas in between. If the Earth’s surface is divided into 8 equal latitudinal zones, with 0 m of ice in the equatorial zone, and 264 m of ice in the polar zone, then the depths of ice in the 8 equatorial zones will be:
Latitude 0.00 7.10 14.5 22.0 30.0 38.7 48.6 61.0 90.0
Ice depth m. 0.0 37.7 75.4 113 151 189 226 264
So between the latitudes of 22º-30º (Luxor, Egypt 26º) there will be 113 m of ice, and between the latitudes of 30º-39º (Babylon 32º) there will be 151 m of ice, between the latitudes of 39º-49º (Rome 42º) there will be 189 m of ice, and so on. Using 264 x sin(lat) would probably give the exact values at every latitude.
This ice is not very thick. But that’s because it’s spread over much of the surface of the Earth rather than in kilometre-thick sheets in far northern latitudes. And it doesn’t matter how deep the ice is. All that matters is what its albedo is – how much light it reflects. It doesn’t matter whether it’s 1 km thick or 1 cm thick. Its thickness will only determine how rapidly it melts.
And when this not-very-thick ice melts, it will probably leave hardly a trace of its former existence, except perhaps in the beds of streams and rivers that once flowed under it.
Tomorrow I’ll consider what happens when the ice melts, and what most likely causes the ice to melt.