The Science of Discworld Revised Edition

the most recent snowball. About one and a half million years ago, round about the time that humans began to become the dominant species on Earth, the planet got very cold. The old name for this period was the Ice Age. We don’t call it that any more because it wasn’t
one
Age: we talk of ‘glacial-interglacial cycles’. Is there a connection? Did the cold climate drive the naked ape to evolve enough intelligence to kill other animals and use their fur to keep warm? To discover and use fire?
    This used to be a popular theory. It’s possible. Probably not, though: there are too many holes in the logic. But a much earlier, and much more severe, Ice Age very nearly put a stop to the whole of that ‘life’ nonsense. And, ironically, its failure to do so may have unleashed the full diversity of life as we now know it.
    Thanks to the pioneering insights of Louis Agassiz, Victorian scientists knew that the Earth had once been a lot colder than it is now, because they could see the evidence all around them, in the form of the shapes of valleys. In many parts of the world today you can find glaciers – huge ‘rivers’ of ice, which flow, very slowly, under the pressure of new ice forming further uphill. Glaciers carry large quantities of rock, and they gouge and grind their way along, forming valleys whose cross-section is shaped like a smooth U. All over Europe, indeed over much of the world, there are identical valleys – but no sign of ice for hundreds or thousands of miles. The Victorian geologists pieced together a picture that was a bit worrying in some ways, but reassuring overall. About 1.6 million years back, at the start of the Pleistocene era, the Earth suddenly became colder. The ice caps at the poles advanced, thanks to a rapid build-up of snow, and gouged out those U-shaped valleys. Then the ice retreated again. Four times in all, it was thought, the ice had advanced and retreated, with much of Europe being buried under a layer of ice several miles thick.
    Still, there was no need to worry, the geologists said. We seemed to be safe and snug in the middle of a warm period, with no prospect of being buried under miles of ice for quite some time …
    The picture is no longer so comfortable. Indeed, some people think that the greatest threat to humanity is not global warming, but an incipient ice age. How ironic, and how undeserved, if our pollution of the planet cancels out a natural disaster!
    As usual, the main reason we now know a lot more is that new kinds of observation became possible, propped up by new theories to explain what it is that they measure and why we can be reasonably sure that they do. These new methods range from clever methods for dating old rocks to studies of the proportions of different isotopes in cores drilled from ancient ice, backed up by ocean-drilling to study the layers of sediment deposited on the sea floor. Warm seas sustain different living creatures, whose death deposits different sediment, so there is a link from sediments to climate.
    All of these methods reinforce each other, and lead to very much the same picture. Every so often the surface of the Earth begins to cool, becoming 10°–15°C colder near the poles and 5°C colder elsewhere. Then it suddenly warms up, possibly becoming 5°C warmer than the current norm. In between big fluctuations, there are smaller ones: ‘mini ice ages’. The typical gap between a decent-sized ice age and the next is around 75,000 years, often less – nothing like the comfortable 400,000 years of ‘interglacial’ expected by the Victorians. The most worrying finding of all is that periods of high temperatures – that is, like we get now – seldom lasted more than 20,000 years.
    The last major glaciation ended 18,000 years ago.
    Wrap up well, folks.
    What caused the ice ages? It turns out that the Earth isn’t quite as nice a planet as we like to think, and its orbit round the sun isn’t quite as stable and repetitive as we usually assume. The currently accepted theory was devised in 1920 by a Serbian called Milutin Milankovitch. In broad terms, the Earth goes round the sun in an ellipse, almost a circle, but there are three features of the Earth’s motion that change. One is the amount through which the Earth’s axis tilts – about 23° at the moment, but varying slightly in a cycle that lasts roughly 41,000 years. Another is a change in the position of Earth’s closest approach to the sun, which varies in a 20,000-year