The Science of Discworld Revised Edition

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,