Supercontinent: Ten Billion Years in the Life of Our Planet (12 page)

Six years seems long enough, but the rate at which the process was completed is not important when we are confronted by rocks in
outcrop
. Face to face with the record of events at a single locality, the question is: how fast could
any single dune
have been covered by water? Even using our conservative estimate of influx rate, it would seem that in what is now Yorkshire, the sea would have risen by some tens of centimetres a day – enough to bury a fifty-metre-high dune in about eight months. One season and the dune sea below you would have become the sea bed, its once sunstruck curves draped in black mud.

The suddenness of the inundation also explains other features characteristic of the dune sandstones underneath the first marine
sediment
. Dunes have a characteristic internal structure, formed as sand grains are blown over the crests to cascade down the lee slope as the dune migrates downwind. This creates a large-scale form of ‘
cross-bedding
’, measurable in metres; the fossil dune surfaces forming great rococo festoons and swags.

The odd thing about these particular dunes is how many of them now appear to lack this characteristic bedform, especially at their
centres
. Here the laminae of sand are often either contorted and chaotic or have vanished completely. For a long time geologists were at a loss to explain this; but the emerging tale of the dunes’ sudden inundation provided an explanation. As the dunes were buried, large pockets of air became trapped at their hearts. Eventually, as the water got deeper, this trapped gas would eventually overcome the strength of the
sediment
confining it and be released suddenly, disrupting all the original bedding of the sand as it escaped.

So, as you watch the advancing tide from your vantage point on the slopes of the Pennine mountains, and see the dune tops slowly vanish beneath a scummy, turbid tide of thick, slimy, bitter water, you will be rewarded from time to time by the sight and sound of sudden bubbling.

Because at least part of this transgression of the sea was caused by the global rise of sea levels, this story was repeated all over the edges of the fragmenting supercontinent. But this particular example, which geologists call the Zechstein Sea, was (like many of the others) not stable. Like the modern Mediterranean, it could not exist for long without being connected to the global ocean. But because the Zechstein was a shelf sea underlain by continental crust, it was much shallower than the Mediterranean, which is a true ocean floored by dense ocean crust that sits low on the Earth’s surface. This made the Zechstein especially vulnerable to drying.

This cannot have been a simple ‘on-off’ process. Zechstein
sediments
, now buried deep below the bed of the North Sea, are hundreds of metres thick. To make just thirty centimetres of evaporite (as
minerals
produced this way, including anhydrite and gypsum, are known), you need to drive off five hundred metres of seawater: twice the depth of the Zechstein Sea at its deepest. Clearly, fresh supplies of ocean water had to be entering the sea continuously, over long periods, and evaporating under the intense heat of the Permian desert.

And this, of course, is where Ripon’s troublesome soluble gypsum comes from. The Zechstein Sea may have dried up almost completely as many as five times in its relatively short lifespan of barely ten
million
years. In doing so, it left behind regular cycles of chemical deposits, each series beginning with the most insoluble minerals (which precipitate first) and ending with those that crystallize only when there is hardly any water left to be dissolved in. So the first
minerals
to appear are limestone (calcium carbonate, which precipitates readily in warm, saturated water and may do so in your kettle) and dolomite, an impure limestone made of a chemical mixture of
calcium
and magnesium carbonate.

The first such rock to be deposited, known generally as the
Magnesian Limestone, was the very rock chosen to build the grand new Palace of Westminster, home of the British Parliament, which was rising as the Blanford brothers were leaving for India in 1856. Called Anstone, it came from quarries near Worksop, and proved a disaster in the metropolis’s acid rain. Alas, despite its workability and lovely biscuit colour, the Mother of Parliaments’ new home soon began dissolving before its builders’ eyes, giving rise to a lot of
amusing
but chemically suspect jokes about why the geologists advising the Parliamentary commission had suggested building the Palace of Westminster out of laxative (Epsom Salt is magnesium
sulphate
).

The new sea brought some relief to the barren heart of northern Pangaea, and it is likely that around its edges the land grew green, or at least greener than it had been. But the supercontinent that enclosed it turned the Earth into a very different world from ours.

The Earth’s climate is largely controlled by a set of fairly simple physical constants, but as scientists are increasingly finding, the
combination
of simple things can have results of almost unpredictable complexity.

As it orbits the Sun, at a distance of ninety-four million miles, the Earth receives a certain amount of radiation from it, known as
insolation
. The Sun’s output has been increasing with time, and over hundreds of millions of years this small increase – brought about by the gradual exhaustion of its primary fuel, hydrogen – is significant enough to have to be taken into account. It is one of those secular changes of which Sir Charles Lyell would not have approved.

This radiation hits the Earth and warms it up, and the atmosphere of the Earth keeps the heat in by the well-known ‘greenhouse effect’, and moves it around. By and large, the average energy received at the top of
Earth’s atmosphere is a fairly constant 343-watts per square metre: a bit more than three lightbulbs’worth. But the complex interaction of axial tilt and other superimposed cycles made the distribution of heat over the surface of the planet a very complex thing to model.

Seasons, the most obvious climate changes of which we are aware, are caused by the tilt of the Earth’s axis relative to the Sun, which
currently
stands at about 23.4 degrees from the ‘vertical’ (defined as the right angle to the plane of the Earth’s orbit around the Sun, called the ecliptic). Thus, as the Earth revolves around the Sun, for half the year the Northern Hemisphere is tilted towards it, while for the other six months it’s the turn of the Southern Hemisphere. This tilting
effectively
concentrates the Sun’s heat first in one hemisphere and then in another, just like leaning towards the fire to warm your face (Northern Hemisphere Summer) and then walking around to the other side, so that you face away from the fire and the heat warms your bottom instead (Southern Hemisphere Summer). At the Equator, of course, this axial tilt has little effect and seasonality is less noticeable.

But there’s much more to it than that. If you have ever watched a spinning top that isn’t moving perfectly, or the behaviour of a double pendulum, you will have a feeling for the complex way in which
harmonic
systems behave. There are also eccentricities in the system to consider, and cycles that affect the degrees of eccentricity. Several such long-period cycles affect the orbit of the Earth around the Sun, and these in turn change the climate because they affect the amount of insolation: how much heat hits a unit area of Earth in any one place. How all these cycles interact, sometimes reinforcing one another, sometimes cancelling one another out, creates a highly complex system that means that the Earth’s climate is never constant.

Take the Earth’s elliptical orbit. The Sun does not sit at the centre of the ellipse, so the Earth–Sun distance that every schoolboy thinks he knows is actually only an average. However, this ellipticity varies
(over a period of 98,500 years) from very elliptical to almost circular. At its most elliptical, the extra distance from the Sun can cut the amount of insolation by as much as 30 per cent from when the Earth is closest. This cycle has almost no effect at all on the
total
amount of heat received by the Earth per year, because it all averages out. However, it does increase ‘seasonality’ (the contrast between the
seasons
) in one hemisphere, while reducing it in the other.

The inclination of the Earth’s axis to the ecliptic varies (over a timespan of 41,000 years), between extreme values of 21.39 (
nearest
to ‘vertical’) and 24.36 degrees (most inclined). This cycle also affects the length of the dark polar winters, and has a marked effect on climate in high latitudes. In addition, the Earth’s axis of spin describes a circle (over a period of 21,700 years). You can make a spinning top do this very easily by giving it a nudge. This is called precession.

If you combine two of these factors – the 98,500-year cycle in orbital ellipticity with the 21,700-year cycle in the Earth’s axial tilt (precession) – you generate a harmonic interference between the two cycles: they produce another cycle. At one extreme the Earth will come closest to the Sun during the Southern Hemisphere Summer; and at the other it will come closest during the Northern Hemisphere Summer. At these extreme points in the cycle, the additive effect of axial tilt and proximity to the Sun makes the summer more intense (and, six months later, the winters deeper, occurring as they will, when the Earth is at its farthest from the Sun). Intense summer conditions will increase the heating of land areas (land heats up and cools down much more quickly than the more even-tempered ocean), with
striking
effects on rainfall, as we shall see. The overall effect creates cycles of seasonality. But by the same token, when the seasons are at their most contrasted in one hemisphere, they will be at their least
contrasted
in the other.

Orbital climate-forcing effects were first described by Scots
geologist
James Croll (1821–90) and later developed by Serb mathematician Milutin Milankovich (1879–1958), and for this reason they are known collectively as Croll–Milankovich cycles. But the climate is not all about angles of tilt and rays per square metre. The Earth’s fluid shells – the air and water – are what make it completely different from any other space rock struck by starlight. Earth’s atmosphere and the hydrosphere absorb and transport the Sun’s heat around, creating an equable average temperature at surface (currently about 25 degrees Celsius). In the oceans this circulation is achieved by a set of
interlocking
convection-driven cells called gyres; there’s one in the North Atlantic and one in the South Atlantic, for example. But they are not discrete: they mesh like cogs in a gearbox, shunting water (and heat) from one gyre to another. In fact, the ocean basins are connected by a three-dimensional ‘global conveyor’, as it has become known, refreshing and warming bottom waters, creating fertile upwellings of cold, mineral-rich waters elsewhere, and preventing stratification: the tendency of warm water to float on cold, light on dense. This keeps the whole ocean system oxygenated and healthy.

Oceanic convection cells are very much dependent on the shape of the ocean basins – and hence on the distribution of continents. But if you want stability, look to the atmosphere. Here three huge,
sausage-like
convection cells sit around each hemisphere like the folds of rubber flesh surrounding M. Michelin. They are invisible of course, though the cloud patterns give them away – if you know what to look for. They have existed for billions of years and continue their
convection
more or less irrespective of what the orbit is doing, or where the continents happen to lie on the shifting surface of the globe. Behind the fickle airs there is a dynamic stability that has easily
outlasted
the transient continents.

These great convection tubes create the major climatic zones of the
Earth, which like them lie in belts parallel to the Equator. The cells exist as the stable answer to the need to dissipate heat from where it is most plentiful – at the Equator – to the poles. At the Earth’s
waistline
, hot air rises, creating more or less permanent low pressure and rain as moisture condenses. The rising air hits the upper edge of the atmosphere and splits in two, some going south, some north. We shall follow the northern limb.

This air travels north high up at the top of the atmosphere until it meets more – circulating in the next cell – coming in the opposite direction. The two currents collide and sink back to Earth again. This falling air is dry and creates permanent high pressure. Where it hits land it produces desert conditions everywhere on land except near coasts, where some moisture can blow a little way inland. Thus on either side of the wet equatorial region you find bands of deserts. They stand out well on those ‘where is the plane?’ simulations
provided
on long-haul flights.

On hitting the Earth, the air splits again. Some goes back south, to pick up moisture and rise again at the Equator. The rest travels north along the Earth’s surface and does not rise again until it meets cold air travelling Equatorwards from the pole. The two then meet and rise, creating another line of low-pressure systems, and rain. Over the poles, in the final or Polar cell, cold dry air sinks, creating high
pressure
with (usually) relatively low evaporation – the dry arctic air of the tundra.

A complication, introduced by the Earth’s rotation, is the Coriolis effect, named after French mathematician Gustave-Gaspard de Coriolis (1792–1843), who worked out the mathematics governing it. This is the apparent force, acting on all objects moving on the Earth’s
rotating surface, that tends to deflect them to the right in the Northern Hemisphere and to the left in the Southern. This is why weather systems (and, allegedly, water disappearing down plugholes) rotate clockwise north of the Equator and anticlockwise south of it. On air moving in the cells, it acts to change the simple circular, ‘
up-across
-down’ convections I have just hinted at into helical ones.