As a former historian, I see the emergence of the world from the last ice age as the starter’s gun for the advance of humanity (see here), and I’ve read a lot about the ice ages and the pattern that the recent ones have. From time to time you get people telling us that the ‘interglacial’ we are in is about to end, while others have suggested that this one will last for 50,000 years, which would mean that I wouldn’t be around to see the next glacial come in. Still others suggest that the more CO2 we pump out the better, because it will delay the arrival of the next ice age.
No one is quite sure why we have had, in the geologically recent period, the Pleistocene, a pattern of glacials and inter-glacials, with the glacials lasting much longer. As to the second point, it takes a lot of energy to melt ice, but it doesn’t take so long for ice to form if there is no heat. As to the pattern, the Milankovitch cycles and the theory contained in them seems to be the conventional wisdom as to why ice ages occur, but they don’t explain everything, in particular why they started when they did.
In the last half million years or so there have been a number of ice ages, which you can see on the following chart.
You can see that the interglacial we are in (at the right-hand end of the chart) seems to be slightly different to the others. The other four have had higher temperatures, but the last three peaked and declined rapidly; ours seems to have held up longer. I’ve wondered a bit how different these patterns are, and someone has done the work for me. It appears in the next chart, which relies on the EPICA ice core data.
Here the quick declines from the high temperatures of the earlier inter-glacials are more obvious. Ours has already lasted longer than two earlier ones, and is about four thousand years shorter than the last interglacial, which began 130,000 years ago. Why? What has caused the difference?
The guy that did this nice work you can encounter at <http://oz4caster.wordpress.com>. His name is Bryan, he is a Texan with forty years in meteorology, and he has normalised [the beginning of each interglacial] to the year where the estimated global temperature first reached the level of our modern “normal” climate. The approximate year where each interglacial episode first reached the modern “normal” temperature is shown in the legend. There is no doubt that our interglacial, the Holocene, is very different to the others.
A few months ago I wrote a piece on ‘the Ruddiman hypothesis’, the essence of which was that humans have been altering the climate for a long time, through burning, clearing and maintaining domesticated animals in large numbers — not just in developing heavy industry from the the late 18th century. Well, you can see why Ruddiman might have thought so given Bryan’s chart above. But as with so much else on climate science, it’s a hypothesis that can’t really be tested in any way.
Look at the declines again. In every case but ours the drop from the interglacial peak temperature is more or less symmetrical. The peak lasts a little while — a couple of thousand years — and then down the temperature goes. The Holocene, however, is chugging along some 12,000 years after temperature reached its peak. It is most like the interglacial 400,000 years ago, which lasted perhaps 28,000 years.
So, how much time do we have left before the ice returns? Bryan argues that we have no better data than those you see above, and that there is no reason to expect that the Holocene will last indefinitely. Equally, he sees no sign of its imminent decline. It’s worth noting that the difference between today’s ‘modern normal’ temperature and that of the glacial standard is about 5.5 degrees C. Much of the present northern hemisphere would be only sparsely populated at average temperatures so cold.
And a final thought. Bryan’s graph truncates the long ice ages, and you might get the impression that the interglacials and the glacials are of much the same length. It ain’t so. Very roughly, the glacial lasts for about a hundred thousand years, and the average of the interglacials so far is about 16,000 years. The ice ages build up a colossal amount of ice — it was two kilometres thick over the site of modern Chicago, for example — and the more ice the higher the albedo, meaning that more sunlight is reflected away, so the planet remains cold.
The colder it is the less life there is. Our modern civilisation is simply unworkable in an ice age, so the arrival of one — and the arrival can occur within a century or so — is a threat of great moment, much greater, in my opinion, than that of a warmer world, where we know that life flourishes. Much too little attention has been given, by those crying doom, to the possibility of the return of serious cold.
I recognise that the recent icy snap in North America is simply weather, as are early hot temperatures here, but it ought to be a reminder that human, animal and plant life all welcome summer, and go rather dormant in winter. A hundred-thousand-year winter really would be the end of civilisation as we know it.