The Science of Discworld Revised Edition

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