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 around 2 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. 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 six warmest summers in Britain this century have 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 II 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 II 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 II. 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 itfrom 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 temperature, 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.
It may well have rained for a million years. So it’s not surprising that Rincewind noticed that it was a bit wet.
Thanks to gravity, water goes downhill, so all that rain accumulated in the lowest depressions in the Earth’s irregular surface. Because the atmosphere had a lot of carbon dioxide in it, those early oceans contained a lot of dissolved carbon dioxide, making the water slightly acidic. There may have been hydrochloric and sulphuric acids too. The acid ate away at the surface rocks, causing minerals to dissolve in the oceans; the sea began to get salty.
At first the amount of oxygen in the atmosphere increased slowly, because the effect of incoming sunlight isn’t particularly dramatic. But now life got in on the act, bubbling off oxygen as a byproduct of photosynthesis. The oxygen combined with any remaining hydrogen in the atmosphere, whether on its own or combined inside methane, to produce more water. This also fell as rain, and increased the amount of ocean, leading to more bacteria, more oxygen - and so it continued until the available hydrogen pretty much ran out.
Originally it used to be thought that the oceans just kept dissolving the rocks of the continents, accumulating more and more minerals, getting saltier and saltier until the amount of salt reached its current value of about 3.5%. The evidence for this is the percentage of salt in the blood of fishes and mammals, which is about 1%. In effect, it was believed that fish and mammal blood were ’fossilized’ ocean. Today we are still often told that we have ancient seas in our blood. This is probably wrong, but the argument is far from settled. It is true that our blood is salty, and so is the sea, but there are plenty of ways for biology to adjust salt content. That 1% may just be whatever level of salt makes best sense for the creature whose blood it is. Salt - more properly, the ions of sodium and chlorine into which it decomposes - have many biological uses: our nervous systems, for instance, wouldn’t work without them. So while it is entirely believable that evolution took advantage of the existence of salt in the sea, it need not be st
uck with the same proportion. On the other hand, there is good reason to think that cells first evolved as tiny free-floating organisms in the oceans, and those early cells weren’t sophisticated enough to fight against a difference in salt concentration between their insides and their outsides, so they may well have settled on the same concentration because that was all they could initially manage - and having done so, they were rather stuck with it.
Can we decide by taking a more careful look at the oceans? Oceans have ways to lose salt as well as gaining it. Seas can dry out; the Dead Sea in Israel is a famous example. There are salt mines all over the place, relics of ancient dried-up seas. And just as living creatures - bacteria - took out carbon dioxide, turning it into oxygen and sugar, so they can take out other dissolved minerals too. Calcium, carbon and oxygen go into shells, for instance, which fall to the ocean floor when their owner dies. The clincher is ... time. The oceans are thought to have reached their current composition, and in particular their current degree of saltiness, about 2 to 1.5 billion years ago. The evidence is the chemical composition of sedimentary rocks rocks formed from deposits of shells and other hard parts of organisms - which seems not to have changed much in the interim. (Though in 1998 Paul Knauth presented evidence that the early ocean may have been more salty than it is now, with somewhere between 1.5 to 2 times as much salt. His calculations indicate that salt could not have been deposited on the continents until about 2.5 billion years ago.) Simple calculations based on how much material dissolves in rivers and how fast rivers flow show that the entire salt content of the oceans can be supplied from dissolved continental rocks in twelve million years - the twinkling of a geological eye. If salt had just built up steadily, the oceans would now be far more salt than water So the oceans are not simply sinks for dissolved minerals, one-way streets into which minerals flow and get trapped. They are mineral-processing machines. The geological evidence of the similarity of ancient and modern sedimentary rocks suggests that the inflow and the outflow pretty much balance each other.
Terry Pratchett - The Science of Discworld Page 16
