This post starts with a story, and then moves to a diagram. I was once a keen bushwalker, and puzzled over the gorge that the Molonglo River has made through the Cullarin Range near Canberra. The river runs across the Bungendore plain, cuts through a small mountain range, reappears a little lower in the Canberra Plain, and then joins with the Murrumbidgee River. How did that gorge happen?
The geologists I knew explained that it all started to occur 10 million years ago: the range began to appear as tectonic uplift, and the river maintained its course by eroding its base fast enough to match the rate of uplift. Was that just luck? How did the river manage it? I tried to imagine, and a geologist friend just laughed. ‘Don, you’ll go mad trying to think in human terms about geological time’, he said. So I gave up.
For those who find this sort of stuff interesting, another stream further north of the Molonglo gave up the unequal struggle, and in doing so quickly produced Lake George, currently about half full, and sometimes completely dry, because the stream (Taylor Creek) hasn’t much of a catchment.
Now to the diagram, which shows the last half-million years, and the temperature anomaly measured through ice-cores. Our time is on the right-hand side of the diagram. You can see that there have been five inter-glacial periods, and each has been quite short compared to the glacial periods. Ours has lasted longer than the last three, and is continuing.
In looking at the diagram I have that odd feeling that I had when wondering how the Molonglo River had cut through the gorge: geological time is vast, and beyond human comprehension. You can see that interglacial periods happen quickly, and they die quickly. How long will ours last? No one knows.
I took this diagram from an excellent summary of the sceptical perspective on global warming that I found on Watts Up With That, and you can read the whole essay here. The temperatures are of course from ice-core data. And while the ends of the glacial and of the inter-glacial periods occur, as I have said, very quickly, you need to remember that the smallest division on the horizontal axis is 25,000 years — speed here is relative!
What do we know about our inter-glacial, which we call the Holocene? Well, it’s about 10,000 years since it began, and it is in that time that humanity has prospered. The oldest human settlements of which we have knowledge, like Jericho, date from the very beginning of the Holocene, and they represent what we call ‘civilisation’, which has three core elements: writing of some kind, a city, and a ceremonial culture. In fact, the word ‘civilisation’ comes from the Latin word civitas, meaning city, or city-state.
The previous glacial period was very cold and dry, and on the whole humans were hunter-gatherers, moving about as food in one location was exhausted, or the first brush with the winter freeze arrived and they moved to a warmer climate. Interestingly, the ice-core data suggest that the previous inter-glacial, known as the Eemian, was on average a couple of degrees warmer than ours. And ours has got slowly cooler: the warmest time in our inter-glacial was a little after the beginning, the so-called ‘Holocene Optimum’, 8,000 years ago.
It seems that though homo sapiens, our lot, has been about for at least the last 200,000 years, it is only in our inter-glacial period that human civilisation has developed. At the least, there are no structural relics from the Eemian period, only flint tools. When you look at the diagram above, human civilisation exists only at the far right-hand corner. You could say, also, that in terms of communication and technologies of all kinds, it has really been in the last two hundred years, at the far edge of that corner.
And you begin to wonder — how long will this inter-glacial last? Several explanations are offered for what causes the beginning and the end of ice ages, and much of what we know comes from ice-core data. But these data show that warm periods have been unusual in the last half a million years, and there is no reason to expect that our period is any different to the earlier ones.
The recent pause in temperature, and the growing cold and increasing extent of sea-ice in the Antarctic, have caused some to warn us that the end is nigh — not Thermageddon but Refrigeration! And there are others still telling us that the next ice age is on hold indefinitely because we are pumping such a lot of carbon dioxide into the air. Given that the effect of CO2 is logarithmic I find that hard to accept.
For what it’s worth my own view is that, like me with the Molonglo Gorge, people forget about the difference between geological and human time. My optimistic spirit tells me that human ingenuity and technology will allow my descendants even to deal with the approach of the next ice age, whenever that is. But I don’t at all expect it next year.
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Excellent post, Don.
The Nepean River is an even better example than the Molonglo. the Nepean meanders in and out of the Blue Mountains.
Most enlightening. I can only suggest a de-freeze tax.