The Science of Discworld Revised Edition

cycle. The third is a variation in the eccentricity of the Earth’s orbit – how oval it is – whose period is around 100,000 years. Putting all three cycles together, it is possible to calculate the changes in heat received from the sun. These calculations agree with the known variations in the Earth’s temperature, and it seems particularly likely that the Earth’s warming up after ice ages is due to increased warmth from the sun, thanks to these three astronomical cycles.
    It may seem unsurprising that when the Earth receives more heat from the sun, it warms up, and when it doesn’t, it cools down, but not all of the heat that reaches the upper atmosphere gets down to the ground. It can be reflected by clouds, and even if it gets to ground level it can be reflected from the oceans and from snow and ice. It is thought that during ice ages, this reflection causes the Earth to lose more heat than it would otherwise do, so ice ages automatically make themselves
worse
. We get kicked out of them when the incoming heat from the sun is so great that the ice starts to melt despite the lost heat. Or maybe the ice gets dirty, or … It’s not so clear that we get kicked into an ice age when less of the sun’s warmth reaches the Earth – indeed the slide into an ice age is usually more gradual than the climb back out of it.
    All of which makes one wonder whether global warming caused by gases excreted from animals might be partly responsible. When gases such as carbon dioxide and methane build up in the atmosphere, they cause the famous ‘greenhouse effect’, trapping more sunlight than usual, hence more heat. Right now, most scientists have become convinced, the Earth’s supply of ‘greenhouse gases’ is growing faster than it would otherwise do thanks to human activities such as farming (burning rainforests to clear land), driving cars, burning coal and oil for electricity, and farming again (cows produce a lot of methane: grass goes in one end and methane emerges at the other). And how could we forget the carbon dioxide breathed out by people? One person is equivalent to half a car, maybe more.
    Maybe in the past there were vast civilizations of which we now know nothing, all traces having vanished – except for their effect on the global temperature. Maybe the Earth seethed with vast herds of cattle, buffalo, elephants busily excreting methane. But most scientists think that climate change results from variations in five different factors: the sun’s output of radiant heat, the Earth’s orbit, the composition of the atmosphere, the amount of dust produced by volcanoes, and levels of land and oceans resulting from movement of the Earth’s crust. We can’t yet put together a really coherent picture in which the measurements match the theory as closely as we’d like, but one thing that
is
becoming clear is that the Earth’s climate has more than one ‘equilibrium’ state. It stays in or near one such state for a while, then switches comparatively rapidly to another, and so on.
    The original idea was that one state was a warm climate, like the one we have now, and the other was a cold ‘ice age’ one. In 1998 Didier Paillard refined this idea to a three-state model: interglacial (warm), mild glacial (coldish), and glacial (very cold). A drop in heat received from the sun below some critical threshold, caused by those astronomical cycles, triggers a switch from warm to coldish. When the resulting ice builds up sufficiently, it reflects so much of the sun’s heat that this triggers another switch from coldish to very cold . But when the sun’s heat finally builds up again to another threshold value, thanks once more to the three astronomical cycles, then the climate switches back to warm. This model fits observations deduced from the amount of oxygen-18 (a radioactive isotope of oxygen) in geological deposits.
    Finally, some drama. About 700 million years ago there was an ice age so severe that it very nearly killed off all of the surface life on Earth. This ‘big freeze’ lasted for between 10 and 20 million years, the ice reached the equator, and it seems that the seas froze to a depth of half a mile (1 km) or more. According to the ‘Snowball Earth’ theory, proposed by Paul Hoffman and Daniel Schrag in 1998, ice covered the entire Earth at this time. However, if ice really covered the
whole
Earth, it should have done more damage than the fossil record indicates.
    And that’s not the only