The Science of Discworld Revised Edition

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 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,