The Science of Discworld Revised Edition

by Terry Pratchett

  Maybe in the past there were vast civilizations of which we now know nothing, all traces having vanished – except for their effect on the global temperature. Maybe the Earth seethed with vast herds of cattle, buffalo, elephants busily excreting methane. But most scientists think that climate change results from variations in five different factors: the sun’s output of radiant heat, the Earth’s orbit, the composition of the atmosphere, the amount of dust produced by volcanoes, and levels of land and oceans resulting from movement of the Earth’s crust. We can’t yet put together a really coherent picture in which the measurements match the theory as closely as we’d like, but one thing that is becoming clear is that the Earth’s climate has more than one ‘equilibrium’ state. It stays in or near one such state for a while, then switches comparatively rapidly to another, and so on.

  The original idea was that one state was a warm climate, like the one we have now, and the other was a cold ‘ice age’ one. In 1998 Didier Paillard refined this idea to a three-state model: interglacial (warm), mild glacial (coldish), and glacial (very cold). A drop in heat received from the sun below some critical threshold, caused by those astronomical cycles, triggers a switch from warm to coldish. When the resulting ice builds up sufficiently, it reflects so much of the sun’s heat that this triggers another switch from coldish to very cold. But when the sun’s heat finally builds up again to another threshold value, thanks once more to the three astronomical cycles, then the climate switches back to warm. This model fits observations deduced from the amount of oxygen-18 (a radioactive isotope of oxygen) in geological deposits.

  Finally, some drama. About 700 million years ago there was an ice age so severe that it very nearly killed off all of the surface life on Earth. This ‘big freeze’ lasted for between 10 and 20 million years, the ice reached the equator, and it seems that the seas froze to a depth of half a mile (1 km) or more. According to the ‘Snowball Earth’ theory, proposed by Paul Hoffman and Daniel Schrag in 1998, ice covered the entire Earth at this time. However, if ice really covered the whole Earth, it should have done more damage than the fossil record indicates.

  And that’s not the only problem. One key piece of evidence for the Big Freeze is a layer of sedimentary rock that formed just after the glaciers melted and left huge quantities of debris. This layer contains less carbon-13, in proportion to ordinary carbon-12, than normal. Marine photosynthesis converts carbon-12 into carbon dioxide more readily than it does carbon-13, so an excess of carbon-13 is left behind in seawater and in the layers of sediment in it that later turn to rock. So a low ratio of carbon-13 to carbon-12 indicates low biological activity.

  The scientist’s task is to find ways to try to disprove things that seem to make sense. In 2001, Martin Kennedy and Nicholas Christie-Blick measured this ratio for sediments that formed during the alleged Big Freeze. If the world was miles deep in ice, the ratio ought to be low. But in fact it was high – in Africa, Australia, and North America. This suggests that the global ecosystem was going strong at that time.

  Computer models of the climate system show that the oceans strongly resist freezing over completely, too.

  Like many attractive scientific theories, Snowball Earth is not at all clear-cut, and further research will be needed to find out who is right. Maybe Earth wasn’t a really solid snowball after all. Or maybe, as Schrag responded, there were patches of open water big enough to change the carbon chemistry of the ocean as it absorbed atmospheric carbon dioxide. Maybe the Earth’s axis tilted a lot more than astronomers are willing to concede, and the poles lost their ice while equatorial regions gained it. Or perhaps continental drift was more rapid at that time than we think, and we’ve mapped out the extent of the ice incorrectly. Whatever the details, though, it was a spectacularly icy world.

  Although the big freeze came close to wiping out all surface life, it may indirectly have created a lot of today’s biodiversity. The big shift from single-celled creatures to multi-celled ones also happened 800 million years ago. It is plausible that the big freeze cleared away a lot of the single-celled lifeforms and opened up new possibilities for multi-celled life, culminating in the Cambrian Explosion 540 million years ago. Mass extinctions are typically succeeded by sudden bursts of diversity, in which life reverts from being a ‘professional’ at the evolutionary game to being an ‘amateur’. It then takes a while for the less able amateurs to be eliminated – and until they are, all sorts of strange strategies for making a living can temporarily thrive. The succession of icy periods that followed the big freeze could only have assisted this process.

  However, it may have been the other way round. The invention of the anus by triploblasts may have changed the ecology of the seas. Faeces would have dropped to the sea-bed, where bacteria could specialize in breaking them down. Other organisms could then become filter feeders, living on those bacteria, perhaps sending their larvae up into the plankton for dispersal, as modern filter-feeders do. Several new ways of life depended on this primeval composting system. And it’s possible that the successful return of phosphorus and nitrogen into the marine cycles led to an explosion of algae, which reduced atmospheric carbon dioxide, cut back on the greenhouse effect, and triggered the big freeze.

  Fortunately for us, the big freeze wasn’t quite long enough, or cold enough, to kill off everything. (Bacteria in volcanic vents on the ocean floor and in the Earth’s crust would have survived no matter what, but evolution would have been set back a long, long way.) So when the Earth warmed, life exploded into a fresh, competition-free world. Paradoxically, a major reason why we are here today may be that we very nearly weren’t. Our entire evolutionary history is full of these good news–bad news scenarios, where life leaps forward joyously over the bodies of the fallen …

  Rincewind can be forgiven for feeling that Roundworld has it in for him. Life has suffered from many different kinds of natural disaster. Here are two more. In the Permian/Triassic extinction of 250 million years ago, 96% of all species died within the space of a few hundred thousand years.1 William Hobster and Mordeckai Magaritz think this happened because they suffocated. Carbon isotopes show that a lot of coal and shale oxidized in the run-up to the extinction, probably because of a fall in sea level, which exposed more land. The result was a lot more carbon dioxide and a lot less oxygen, which was reduced to half today’s level. Land species were especially badly affected.

  Another global extinction, though less severe, occurred 55 million years ago: the Palaeocene/Eocene boundary. In cores of sediment drilled from the Antarctic, James Kennett and Lowell Stott discovered evidence of the sudden death of a lot of marine species. It seemed that trillions of tons (tonnes) of methane had burst from the ocean, sending temperatures through the roof, methane being a powerful greenhouse gas. Jenny Dickens suggested that the methane was released from deposits of methane hydrates in permafrost and on the seabed. Methane hydrates are a crystal lattice of water enclosing methane gas: they are created when bacteria in mud release the gas and it becomes trapped.

  Coincidentally, one of the main results of the Palaeocene/Eocene extinction was a burst of evolutionary diversity, leading in particular to the higher primates – and us. Whether something is a disaster depends on your point of view. Rocks may not have a point of view, as Ponder Stibbons pointed out, but we certainly do.

  1 To the best of our knowledge, based on deduction from the available evidence. Certainly it was a big extinction – far bigger than the one that killed off (or helped to kill off) the dinosaurs. We remember the dinosaur one because they’ve had such good PR people.

  TWENTY-NINE

  GOING FOR A PADDLE

  I THINK IT looks more like a Hogswatchnight ornament,’ said the Senior Wrangler later, as the wizards took a pre-dinner drink and stared into the omniscope at the glittering white world. ‘Quite pretty, really.’

  ‘Bang go the blobs,’ said Ponder Stibbons.

  ‘Phut,’ said the Dean, cheerfully. ‘More sherry, Archchancellor?’

&n
bsp; ‘Perhaps some instability in the sun …’ Ponder mused.

  ‘Made by unskilled labour,’ said Archchancellor Ridcully. ‘Bound to happen sooner or later. And then it’s nothing but frozen death, the tea-time of the gods and an eternity of cold.’

  ‘Sniffleheim,’ said the Dean, who’d got to the sherry ahead of everyone else.

  ‘According to HEX, the air of the planet has changed,’ said Ponder.

  ‘A bit academic now, isn’t it?’ said the Senior Wrangler.

  ‘Ah, I’ve got an idea!’ said the Dean, beaming. ‘We can get HEX to reverse the thaumic flow in the cthonic matrix of the optimized bi-direction octagonate, can’t we?’

  ‘Well, that’s the opinion of four glasses of sherry,’ said the Archchancellor briskly, to break the ensuing silence. ‘However, if I may express a preference, something that isn’t complete gibberish would be more welcome next time, please. So, Mister Stibbons, is this the end of the world?’

  ‘And if it is,’ said the Senior Wrangler, ‘are we going to have a lot of heroes turning up?’

  ‘What are you talking about, man?’ said Ridcully.

  ‘Well, the Dean seems to think we’re like gods, and a great many mythologies suggest that when heroes die they go to feast forever in the halls of the gods,’ said the Senior Wrangler. ‘I just need to know if I should alert the kitchens, that’s all.’

  ‘They’re only blobs,’ said Ridcully. ‘What can they do that’s heroic?’

  ‘I don’t know … stealing something from the gods is a very classical way,’ the Senior Wrangler mused.

  ‘Are you saying we should check our pockets?’ said the Archchancellor.

  ‘Well, I haven’t seen my penknife lately,’ said the Senior Wranger. ‘It was just a thought, anyway.’

  Ridcully slapped the despondent Stibbons on the back.

  ‘Chin up, lad!’ he roared. ‘It was a wonderful effort! Admittedly the outcome was a lot of blobs with the intelligence of pea soup, but you shouldn’t let utter hopeless failure get you down.’

  ‘We never do,’ said the Dean.

  It was after breakfast next day when Ponder Stibbons wandered into the High Energy Magic building. A scene of desolation met his eye. There were cups and plates everywhere. Paper littered the floor. Forgotten cigarettes had etched their charred trails on the edge of desks. A half-eaten sardine, cheese and blackcurrant pizza, untouched for days, was inching its way to safety.

  Sighing, he picked up a broom, and went over the tray containing HEX’s overnight write-out.

  It seemed a lot fuller than he would have expected.

  ‘Not just blobs – there’s all sorts of stuff! Some of it’s wiggling …’

  ‘Is that a plant or is it an animal?’

  ‘I’m sure it’s a plant.’

  ‘Isn’t it … walking … rather fast?’

  ‘I don’t know. I’ve never seen a plant walking before.’

  The wizardery of UU was filtering back in the building as the news got around. The senior members of the faculty were clustered around the omniscope, explaining to one another, now that the impossible had happened, that of course it had been inevitable.

  ‘All those cracks under the sea,’ said the Dean. ‘And the volcanoes, of course. Heat’s bound to build up over time.’

  ‘That doesn’t explain all the different shapes, though,’ said the Senior Wrangler. ‘I mean, the whole sea looks like somebody had just turned over a very big stone.’

  ‘I suppose the blobs had time to consider their future when they were under the ice,’ said the Dean. ‘It suppose you could think of it as a very long winter evening.’

  ‘I vote for lavatories,’ said the Lecturer in Recent Runes.

  ‘Well, I’m sure we all do,’ said Ridcully. ‘But why at this point?’

  ‘I mean that the blobs were … you know … excusing themselves for millions and millions of years, then you’re get a lot of, er, manure …’ the Lecturer ventured.

  ‘A shitload,’ said the Dean.

  ‘Dean! Really!’

  ‘Sorry, Archchancellor.’

  ‘… and we know dunghills absolutely teem with life …’ the Lecturer went on.

  ‘They used to think that rubbish heaps actually generated rats,’ said Ridcully. ‘Of course, that was just a superstition. It’s really seagulls. But you saying life is, as it were, advancing by eating dead men’s shoes? Or blobs, in this case. Not shoes, of course, because they didn’t have any feet. And wouldn’t have been bright enough to invent shoes even if they did. And even if they had been, they couldn’t have done. Because there was, at that time, nothing from which shoes might be made. But apart from that, the metaphor stands.’

  ‘There still are blobs in there,’ said the Dean. ‘There’s just lots of other things, too.’

  ‘Any of it lookin’ intelligent?’ said Ridcully.

  ‘I’m not certain how we’d spot that at this stage …’

  ‘Simple. Is anything killing something it doesn’t intend to eat?’

  They stared into the teeming broth.

  ‘Bit hard to define intentions, really,’ said the Dean, after a while.

  ‘Well, does anything look as if it is about to become intelligent?’

  They watched again.

  ‘That thing like two spiders joined together?’ said the Senior Wrangler after a while. ‘It looks very thoughtful.’

  ‘I think it looks very dead.’

  ‘Look, I can see how we can settle this whole evolution business once and for all,’ said Ridcully, turning away. ‘Mister Stibbons, can HEX use the omniscope to see if anything changes into anything else?’

  ‘Over a moderately sized area, I think he probably can, sir.’

  ‘Get it to pay attention to the land,’ said the Dean. ‘Is there anything happening on the land?’

  ‘There’s a certain greenishness, sir. Seaweed with attitude, really.’

  ‘That’s where the interesting stuff will happen, mark my words. I don’t know what this universe is using for narrativium, but land’s where we’ll see any intelligent life.’

  ‘How do you define intelligence?’ said Ridcully. ‘In the long term, I mean.’

  ‘Universities are a good sign,’ said the Dean, to general approval.

  ‘You don’t think that perhaps fire and the wheel might be more universally indicative?’ said Ponder carefully.

  ‘Not if you live in the water,’ said the Senior Wrangler. ‘The sea’s the place here, I’ll be bound. On this world practically nothing happens on the land.’

  ‘But in the water everything’s eating each other!’

  ‘Then I’ll look forward to seeing what happens to the last one served,’ said the Senior Wrangler.

  ‘No, when it comes to universities, the land’s the place,’ said the Dean. ‘Paper won’t last five minutes under water. Wouldn’t you say so, Librarian?’

  The Librarian was still staring into the omniscope.

  ‘Ook,’ he said.

  ‘What’s that he said?’ said Ridcully.

  ‘He said “I think the Senior Wrangler might be right”,’ said Ponder, going over to the omniscope. ‘Oh … look at this …’

  The creature had at least four eyes and ten tentacles. It was using some of the tentacles to manœuvre a slab of rock against another slab.

  ‘It’s building a bookcase?’ said Ridcully.

  ‘Or possibly a crude rock shelter,’ said Ponder Stibbons.

  ‘There we are, then,’ said the Senior Wrangler. ‘Personal property. Once something is yours, of course you want to improve it. The first step on the road to progress.’

  ‘I’m not sure it’s got actual legs,’ said Ponder.

  ‘The first slither, then,’ said the Senior Wrangler, as the rock slipped from the creatures tentacles. ‘We should help it,’ he added firmly. ‘After all, it wouldn’t be here if it wasn’t for us.’

  ‘Hold on, hold on,’ said the Lecturer in Recent Runes. ‘It’s only
making a shelter. I mean, the Bower Bird builds intricate nests, doesn’t it? And the Clock Cuckoo even builds a clock for its mate, and no one says they’re intelligent as such.’

  ‘Obviously not,’ said the Dean. ‘They never get the numerals right, the clocks fall apart after a few months, and they generally lose two hours a day. That doesn’t sound like intelligence to me.’

  ‘What are you suggesting, Runes?’ said Ridcully.

  ‘Why don’t we send young Rincewind down again in that virtually-there suit? With a trowel, perhaps, and an illustrated manual on basic construction?’

  ‘Would they be able to see him?’

  ‘Er … gentlemen …’ said Ponder, who had been letting the eye of the omniscope drift further into the shallows.

  ‘I don’t see why not,’ said Ridcully.

  ‘Er … there’s a … there’s …’

  ‘It’s one thing to push planets around over millions of years, but at this level we couldn’t even give our builder down there a heavy pat on the back,’ said the Dean. ‘Even if we knew which part of him was his back.’

  ‘Er … something’s paddling, sir! Something’s going for a paddle, sir!’

  It was probably the strangest cry of warning since the famous ‘Should the reactor have gone that colour?’ The wizards clustered around the omniscope.

  Something had gone for a paddle. It had hundreds of little legs.

  THIRTY

  UNIVERSALS AND PAROCHIALS

  CHANCE MAY HAVE played a greater role than we imagine in ensuring our presence on the Earth. Not only aren’t we the pinnacle of evolution: it’s conceivable that we very nearly didn’t appear at all. On the other hand, if life had wandered off the particular evolutionary track that led to us, it might well have blundered into something similar instead. Intelligent crabs, for example. Or very brainy net-weaving jellyfish.