‘The Bursar, for example?’
‘Not even the Bursar, sir.’
‘He’s not very complicated, though. If only we could find a parrot that was good at sums, we could pension the old chap off.’
‘No, sir. There’s nothing like the Bursar. Not even an ant or a blade of grass. You might as well try to tune a piano by throwing rocks at it. Life does not turn up out of nowhere, sir. Life is a lot more than just rocks moving in circles. The one thing we’re not going to run into is monsters.’
Two minutes later Rincewind blinked and found, when he opened his eyes, that they were somewhere else. There was a rather grainy redness in front of them, and he felt rather warm.
‘I don’t think it’s working,’ he said.
‘You should be seeing a landscape,’ said Ponder, in his ear.
‘It’s all just red.’
There was the sound of distant whispering. Then the voice said, ‘Sorry. The aim wasn’t very good. Wait a moment and we’ll soon have you out of that volcanic vent.’
In the HEM Ponder took the ear trumpet away from his ear. The other wizards heard it sizzling, as if a very angry insect was trapped therein.
‘Curious language,’ he said, in mild surprise, ‘well, let’s raise him somewhat and let time move on a little …’
He put the trumpet to his ear and listened.
‘He says it’s pissing down,’ he announced.
EIGHTEEN
AIR AND WATER
IT’S CERTAINLY A surprise that the rigid rules of physics permit anything as flexible as life, and the wizards can hardly be blamed for not anticipating the possibility that living creatures might come into being on the barren rocks of Roundworld. But Down Here is not as different from Up There as it seems. Before we can talk about life, though, we need to deal with a few more features of our home planet: atmosphere and oceans. Without them, life as we know it could not have arisen; without life as we know it, our oceans and atmosphere would be distinctly different.
The story of the Earth’s atmosphere is inextricably intertwined with that of its oceans. Indeed, the oceans can reasonably be viewed as just a rather damp, dense layer of the atmosphere. The oceans and the atmosphere evolved together, exerting strong influences on each other, and even today such an ‘obviously’ atmospheric phenomenon as weather turns out to be closely related to what happens in the oceans. One of the main recent breakthroughs in weather prediction has been to incorporate the oceans’ ability to absorb, transport, and give off heat and moisture. To some extent, the same point can be made about the solid regions of the Earth, which also co-evolved with the air and the seas, and also interact with them. But the link between oceans and atmosphere is stronger.
The Earth and its atmosphere condensed together out of the primal gascloud that gave rise to the Sun and to the solar system. As a rough rule of thumb, the denser materials sank to the bottom of the condensing clump of matter that we now inhabit, and the lighter ones floated to the top. Of course there was, and still is, a lot more going on than that, so the Earth is not just a series of concentric shells of lighter and lighter matter, but the general distribution of solids, liquids, and gases makes sense if you think about it that way. And so, as the molten rocks of Earth began to cool and solidify, the nascent planet found itself already enveloped in a primordial atmosphere.
It was almost certainly very different from the atmosphere today, which is a mixture of gases, the main ones being the elements nitrogen, oxygen and the inert gas argon, and the compounds carbon dioxide and water (in the form of vapour). The primordial atmosphere also differed considerably from the gas cloud out of which it condensed – it wasn’t just a representative sample of what was around. There are several reasons for this. One is that a solid planet and a gas cloud retain different gases. Another is that a solid planet can generate gases, by chemical or even nuclear reactions, or by other physical processes, which can escape from its interior into its atmosphere.
The early cloud was rich in hydrogen and helium, the lightest of elements. The speed with which a molecule moves becomes slower as the molecule gets heavier – a molecule with one hundred times the mass moves at about one-tenth the speed. Anything that moves faster than the Earth’s escape velocity, about 7 miles per second (11 km/sec), can overcome the planet’s gravity and disappear into space. Molecules in the atmosphere whose molecular weight – what you get by adding up the atomic weights of the component atoms – is less than about 10 should therefore disappear into the void. Hydrogen has molecular weight 2, helium 4, so neither of these otherwise abundant gases should be expected to hang around. The most abundant molecules in the primal gas cloud, with molecular weight greater than 10, are methane, ammonia, water, and neon. This is similar to what we find today on the gas giants Jupiter, Saturn, Uranus, and Neptune – except that they are more massive, so have a greater escape velocity, and can retain lighter gases such as hydrogen and helium as well. We can’t be certain that the Earth of 4 billion years ago possessed a methane-ammonia atmosphere, because we don’t know exactly how the primal gas cloud condensed, but it is clear that if the ancient Earth ever possessed such an atmosphere, it lost nearly all of it. Today there is little methane or ammonia, and what there is has a biological origin.
Shortly after the Earth was formed, the atmosphere contained very little oxygen. Around 2 billion years ago, the proportion of oxygen in the atmosphere increased to about 5%. The most likely cause of this change – though perhaps not the only one – was the evolution of photosynthesis. At some stage, probably soon after 4 billion years ago, bacteria in the oceans evolved the trick of using the energy of sunlight to turn water and carbon dioxide into sugar and oxygen. The oxygen that they produced did not show up in the atmosphere, in any appreciable amount, until two billion years ago. A lot of gases and minerals had to be oxidised first. Plants use the same trick today, and they use the same molecules as one of the early bacteria did: chlorophyll. Animals proceed in pretty much the opposite direction: they power themselves by using oxygen to burn food, producing carbon dioxide instead of using it up. Those early photosynthesizing bacteria used the sugar for energy, and multiplied rapidly, but to them the oxygen was just a form of toxic waste, which bubbled up into the atmosphere. The oxygen level then stayed roughly constant until about 600 million years ago, when it underwent a rapid increase to the current level of 21%.
The amount of oxygen in today’s atmosphere is far greater than could ever be sustained without the influence of living creatures, which not only produce oxygen in huge quantities but use it up again, in particular locking it up in carbon dioxide. It is startling how far ‘out of balance’ the atmosphere is, compared to what would happen if life were suddenly removed and only inorganic chemical processes could act. The amount of oxygen in the atmosphere is dynamic – it can change on a timescale that by geological standards is extremely rapid, a matter of centuries rather than millions of years. For example, if some disaster occurred which killed off all the plants but left all the animals, then the proportion of oxygen would halve in about 500 years, to the level on mountain peaks in the Andes today. The same goes for the scenario of ‘nuclear winter’ introduced by Carl Sagan, in which clouds of dust thrown into the atmosphere by a nuclear war stop most of the sunlight from reaching the ground. In this case, plants may still eke out some kind of existence, but they don’t photosynthesize: they do use oxygen, though, and so do the microorganisms that break down dead plants.
The same screening effect could also occur if there were unusual numbers of active volcanoes, or if a big meteorite or comet hit the Earth. When comet Shoemaker-Levy 9 hit Jupiter in 1994, the impact was equivalent to half a million hydrogen bombs.
The ‘budget’ of income and expenditure for oxygen, and the associated but distinct budget for carbon, is still not understood. This is an enormously important question because it is vital background to the debate about global warming. Human activities, such as electrical power plants, industry, use of
cars, or simply going about one’s usual business and breathing while one does so, generate carbon dioxide. Carbon dioxide is a ‘greenhouse gas’ which traps incoming sunlight like the glass of a greenhouse. So if we produce too much carbon dioxide, the planet should warm up. This would have undesirable consequences, ranging from floods in low-lying regions such as Bangladesh to big changes in the geographical ranges of insects, which could inflict serious damage on crops. The question is: do these human activities actually increase the Earth’s carbon dioxide, or does the planet compensate in some way? The answer makes the difference between imposing major restrictions on how people in developed (and developing) countries live their lives, and letting them continue along their current paths. The current consensus is that there are clear, though subtle, signs that human activities do increase the carbon dioxide levels, which is why major international treaties have been signed to reduce carbon dioxide output. (Actually taking that action, rather than just promising to do so, may prove to be a different matter altogether.)
The difficulties involved in being sure are many. We don’t have good records of past levels of carbon dioxide, so we lack a suitable ‘benchmark’ against which to assess today’s levels – although we’re beginning to get a clearer picture thanks to ice cores drilled up from the Arctic and Antarctic, which contain trapped samples of ancient atmospheres. If ‘global warming’ is under way, it need not show up as an increase in temperature anyway (so the name is a bit silly). What it shows up as is climatic disturbance. So even though the eight warmest summers in Britain in the 20th century all occurred in the nineties, we can’t simply conclude that ‘it’s getting warmer’, and hence that global warming is a fact. The global climate varies wildly anyway – what would it be doing if we weren’t here?
A project known as Biosphere 2 attempted to sort out the basic science of oxygen/carbon transactions in the global ecosystem by setting up a ‘closed’ ecology – a system with no inputs, beyond sunlight, and no outputs whatsoever. In form it was like a gigantic futuristic garden centre, with plants, insects, birds, mammals, and people living inside it. The idea was to keep the ecology working by choosing a design in which everything was recycled.
The project quickly ran into trouble: in order to keep it running, it was necessary to keep adding oxygen. The investigators therefore assumed that somehow oxygen was being lost. This turned out to be true, in a way, but for nowhere near as literal a reason. Even though the whole idea was to monitor chemical and other changes in a closed system, the investigators hadn’t weighed how much carbon they’d introduced at the start. There were good reasons for the omission – mostly, it’s extremely difficult, since you have to estimate carbon content from the wet weight of live plants. Not knowing how much carbon was really there to begin with, they couldn’t keep track of what was happening to carbon monoxide and carbon dioxide. However, ‘missing’ oxygen ought to show up as increased carbon dioxide, and they could monitor the carbon dioxide level and see that it wasn’t going up.
Eventually it turned out that the ‘missing’ oxygen wasn’t escaping from the building: it was being turned into carbon dioxide. So why didn’t they see increased carbon dioxide levels? Because, unknown to anybody, carbon dioxide was being absorbed by the building’s concrete as it ‘cured’. Every architect knows that this process goes on for ten years or so after concrete has set, but this knowledge is irrelevant to architecture. The experimental ecologists knew nothing about it at all, because esoteric properties of poured concrete don’t normally feature in ecology courses, but to them the knowledge was vital.
Behind the unwarranted assumptions that were made about Biosphere 2 was a plausible but irrational belief that because carbon dioxide uses up oxygen when it is formed, then carbon dioxide is opposite to oxygen. That is, oxygen counts as a credit in the oxygen budget, but carbon dioxide counts as a debit. So when carbon dioxide disappears from the books, it is interpreted as a debt cancelled, that is, a credit. Actually, however, carbon dioxide contains a positive quantity of oxygen, so when you lose carbon dioxide you lose oxygen too. But since what you’re looking for is an increase in carbon dioxide, you won’t notice if some of it is being lost.
The fallacy of this kind of reasoning has far wider importance than the fate of Biosphere 2. An important example within the general frame of the carbon/oxygen budget is the role of rainforests. In Brazil, the rainforests of the Amazon are being destroyed at an alarming rate by bulldozing and burning. There are many excellent reasons to prevent this continuing – loss of habitat for organisms, production of carbon dioxide from burning trees, destruction of the culture of native Indian tribes, and so on. What is not a good reason, though, is the phrase that is almost inevitably trotted out, to the effect that the rainforests are the ‘lungs of the planet’. The image here is that the ‘civilized’ regions – that is, the industrialized ones – are net producers of carbon dioxide. The pristine rainforest, in contrast, produces a gentle but enormous oxygen breeze, while absorbing the excess carbon dioxide produced by all those nasty people with cars. It must do, surely? A forest is full of plants, and plants produce oxygen.
No, they don’t. The net oxygen production of a rainforest is, on average, zero. Trees produce carbon dioxide at night, when they are not photosynthesizing. They lock up oxygen and carbon into sugars, yes – but when they die, they rot, and release carbon dioxide. Forests can indirectly remove carbon dioxide by removing carbon and locking it up as coal or peat, and by releasing oxygen into the atmosphere. Ironically, that’s where a lot of the human production of carbon dioxide comes from – we dig it up and burn it again, using up the same amount of oxygen.
If the theory that oil is the remains of plants from the carboniferous period is true, then our cars are burning up carbon that was once laid down by plants. Even if an alternative theory, growing in popularity, is true, and oil was produced by bacteria, then the problem remains the same. Either way, if you burn a rainforest you add a one-off surplus of carbon dioxide to the atmosphere, but you do not also reduce the Earth’s capacity to generate new oxygen. If you want to reduce atmospheric carbon dioxide permanently, and not just cut short-term emissions, the best bet is to build up a big library at home, locking carbon into paper, or put plenty of asphalt on roads. These don’t sound like ‘green’ activities, but they are. You can cycle on the roads if it makes you feel better.
Another important atmospheric component is nitrogen. It is a lot easier to keep track of the nitrogen budget. Organisms – plants especially, as every gardener knows – need nitrogen for growth, but they can’t just absorb it from the air. It has to be ‘fixed’ – that is, combined into compounds that organisms can use. Some of the fixed nitrogen is produced as nitric acid, which rains down after thunderstorms, but most nitrogen fixation is biological. Many simple lifeforms ‘fix’ nitrogen, using it as a component of their own amino-acids. These amino-acids can then be used in everybody else’s proteins.
The Earth’s oceans contain a huge quantity of water – about a third of a billion cubic miles (1.3 billion cubic km). How much water there was in the earliest stages of the Earth’s evolution, and how it was distributed over the surface of the globe, we have little idea, but the existence of fossils from about 3.3 billion years ago shows that there must have been water around at that time, probably quite a lot. As we’ve already explained, the Earth – along with the rest of the solar system, Sun included – condensed from a vast cloud of gas and dust, whose main constituent was hydrogen. Hydrogen combines readily with oxygen to form water, but it also combines with carbon to form methane and with nitrogen to form ammonia.
The primitive Earth’s atmosphere contained a lot of hydrogen and a fair quantity of water vapour, but initially the planet was too hot for liquid water to exist. As the planet slowly cooled, its surface passed a critical temperature, the boiling point of water. That temperature was probably not exactly the same as the one at which water boils now; in fact even today it’s not one inflexible tempe
rature, because the boiling point of water depends on pressure and other circumstances. Nor was it just a simple matter of the atmosphere’s getting colder: its composition also changed because the Earth was spouting out gases from its interior through volcanic activity.
A crucial factor was the influence of sunlight, which split some of the atmospheric water vapour into oxygen and hydrogen. The hydrogen escaped from the Earth’s relatively weak gravitational field, so the proportion of oxygen got bigger while that of water vapour got smaller. The effect of this was to increase the temperature at which the water vapour could condense. So as the temperature of the atmosphere slowly fell, the temperature at which water vapour would condense rose to meet it. Eventually the atmosphere going down passed the boiling point of water going up, and water vapour began to condense into liquid water … and to fall as rain.
It must have absolutely bucketed down.
When the rain hit the hot rocks beneath, it promptly evaporated back into vapour, but as it did so it cooled the rocks. Heat and temperature are not the same. Heat is equivalent to energy: when you heat something, you input extra energy. Temperature is one of the ways in which that energy can be expressed: it is the vibration of molecules. The faster those vibrations are, the higher the temperature. Ordinarily, the temperature of a substance goes up if you heat it: all the extra heat is expressed as more vibration of the molecules. However, at transitions from solid to liquid, or liquid to vapour or gas, the extra heat goes into changing the state of the substance, not into making its temperature higher. So you can throw in a lot of heat and instead of the stuff getting hotter, it changes state – a so-called phase transition. Conversely, when a substance cools through a phase transition, it gives off a lot of heat. So the cooling water vapour put more heat back into the upper atmosphere, from which it could be radiated away into space and lost. When the hot rocks turned the water back into vapour, the rocks got a lot cooler very suddenly. In a geologically short space of time, the rocks had cooled below the boiling point of water, and now the falling rain no longer got turned back into vapour – at least, not much of it did.
The Science of Discworld Revised Edition Page 17
