What a Wonderful World (17 page)

If indeed the core is a plate graveyard, it could explain another phenomenon. Plumes of superhot mantle appear to rise from the core. They heat the underside of a plate like a blowtorch. There is such a superplume under the Hawaiian chain of islands. In fact, each island is a volcano born as the plate drifted across the blowtorch.

The core is at a temperature of about 5,000 °C, comparable to that of the surface of the Sun. It could be that the plate graveyard on the outer core permits heat from the core to escape only where there are gaps in the piles of plates, and that this is the origin of the superplumes.

Though we can never go there, the interior of the Earth is gradually yielding its secrets. ‘The world is the geologist’s great puzzle-box,’ said Swiss geologist Louis Agassiz in 1856. ‘He stands before it like the child to whom the separate pieces of his puzzle remain a mystery till he detects their relation and sees where they fit, and then his fragments grow at once into a connected picture beneath his hand.’
8

‘The thickness of the Earth's atmosphere, compared with the size of the Earth, is in about the same ratio as the thickness of a coat of shellac on a schoolroom globe is to the diameter of the globe,' said Carl Sagan.
1
Yet this insignificant sliver of haze makes life on our planet possible. Not only does it act like a blanket,
trapping
precious warmth, it evens out wild extremes in temperature between night and day. Without the atmosphere, the Blue Planet would not be blue. It would be a dazzlingly white ball of ice with an average temperature of -18 °C.

About 4.55 billion years ago, when the Earth was newly minted, the atmosphere is believed to have been made mostly of carbon dioxide, spewed from volcanoes. But about 3.8 billion years ago, the planet came under violent and sustained
bombardment
by city-sized asteroids. This Late Heavy Bombardment not only turned the surface molten but blasted into space the early atmosphere and all the water.
2
Evidence points to icy comets later bringing much of the water that today covers 71 per cent of the planet's surface.
3

Today's atmosphere consists of about one-fifth oxygen and four-fifths nitrogen, with a few trace gases such as argon, water vapour and carbon dioxide. In marked contrast with its
primordial
antecedent, it is almost entirely the creation of life. For aeons, blue-green algae, or cyanobacteria, pumped oxygen – the waste product of photosynthesis – into the air. This combined
with the planet's plentiful iron to make iron oxide, creating tremendous deposits of reddish-brown rocks, which can be seen in Australia today. When the iron could soak up no more, oxygen built up to catastrophic levels in the atmosphere. It poisoned large numbers of organisms. Crucially, however, oxygen provided the super-charged energy source for animals and, one day, humans.
4

But the atmosphere is more than an oxygen-rich blanket that shrouds the world. It is a layer of air in ceaseless motion, driven by solar energy. The Sun heats the equatorial regions more than it does the poles, making the equator hotter than the poles.
5
Since heat always flows from a hot body to a cold body in order to even out the temperature, heat flows from the equator to the poles through the atmosphere in an attempt to iron out the global temperature. ‘The Earth and its atmosphere constitute a vast
distilling
apparatus in which the equatorial ocean plays the part of the boiler, and the chill regions of the poles the part of the condenser,' wrote the nineteenth-century English physicist John Tyndall.
6

If the Earth was not spinning – or it was spinning very slowly, like Venus – it would be particularly simple for heat to travel from the equator to the poles.
7
Hot air, being lighter than cold air – think of hot-air balloons – would rise at the equator and travel towards the poles.
8
There, it would lose its heat, sink down, returning to the equator closer to the surface. Such a continuous conveyor belt of air is known as a Hadley cell after George Hadley, the English lawyer and meteorologist who proposed it in 1735. In fact, a non-rotating Earth would support
two
Hadley
circulation cells – one in the northern hemisphere and one in the southern hemisphere.

The Earth, however,
is
spinning rapidly – once every 24 hours. Consequently, the ground – and therefore the air – is moving most quickly at the equator and most slowly at the poles. This is why NASA, the American space agency, launches from Florida, and ESA, the European Space Agency, from Kourou in French Guyana. By lifting off as close as possible to the equator, rockets get the maximum boost from the Earth's rotation.

Without even knowing it, people at the equator are travelling at almost
twice
the speed of a Boeing 747 – about 1,670 kilometres per hour.
9
The consequence for hot air, travelling away from the equator, is that it finds itself continually travelling faster than the ground below it. From the point of view of someone on that ground, the air therefore appears deflected in the direction of the Earth's rotation – to the right, or
east
, in the northern hemisphere and to the left, or
west
, in the southern hemisphere.
10

So extreme is the deflection of north- and south-travelling air that there is no simple way for heat to get from the equator to the poles. The circulation, instead of forming a simple Hadley
circulation
cell, splits into
three
– in other words, in each
hemisphere
, there are
three
overturning cells of air. Think of them as three parallel bands, each spanning about a third of the distance between the equator and the pole.

On a faster-spinning planet, the circulation splits into
even more
bands. Jupiter, for instance, rotates in a mere 10 hours despite having an equatorial diameter about 11 times that of the Earth. This super-fast spin – the planet's equator is moving about 25 times faster than the Earth's – causes its circulation to split into about 15 bands – 7 on each side of the equator band.
While the lighter bands are called zones, the darker bands are called belts.

In the circulation band in the polar regions of our planet, relatively warm air moves towards the pole at a high altitude. Because it finds itself moving faster than the ground, it appears to someone on the ground to be deflected in the direction of the Earth's rotation. Consequently, the high-altitude winds are westerly; they blow
from
the west, in the same sense as the Earth's rotation. When the air reaches the poles, it cools and sinks. It then returns, at a lower altitude, whence it came. Because it now finds itself moving more slowly than the ground, from the point of view of someone on the ground, it appears deflected in the direction opposite to that of the Earth's rotation. This means that the winds close to the ground near the polar caps blow mainly easterly – that is,
from
the east, which is against the sense of the Earth's rotation.

Something very similar happens in the closest of the
circulation
bands to the equator. High-altitude air flowing away from the equator appears from the ground to be deflected in the
direction
of the Earth's rotation. Such winds in the tropics therefore blow
westerly
. Low-altitude air flowing back to the equator, on the other hand, appears to an observer on the ground to be deflected against the Earth's rotation. This is why the winds near sea level in the tropics – known as the trade winds – blow primarily easterly.

The most interesting of the three terrestrial circulation bands, however, is the middle one, halfway between the polar band and
the tropical band. Here, at mid-latitudes,
11
the Earth's rotation has its biggest effect on the air moving north and south.
12
This makes the circulation inherently unstable, leading to the constant spawning of eddies, or vortices. The middle circulation cell is also the domain of super-fast westerly winds at high altitude. This jet stream can blow at more than 400 kilometres per hour and steers weather systems. It is why flying from America to Europe is quicker than flying in the opposite direction.

This is a good place to dispel an old wives' tale. People often say water swirls down a plughole consistently one way in the northern hemisphere and the other way in the southern
hemisphere
. It does not. The water can swirl either way, depending on the initial oomph it gets – from the flow from the tap or from any unevenness in the sink itself. Differences in the speed of the Earth's surface due to the planet's rotation are simply too small across the tiny span of sink to have any effect on the water. But this is not true for an air mass that is hundreds or more kilometres across. In marked contrast with water swirling in a sink, these
do
indeed spin differently in the northern and southern hemispheres.

It works in this way. Imagine a region of low pressure, known as a cyclone, in the northern hemisphere.
13
Surrounding air rushes in from all sides to try to equalise the pressure. Air rushing in in a northerly direction finds itself moving faster than the ground – that is, from the ground it appears to be deflected
eastward
, in the direction of the Earth's spin. Air rushing in in a southerly direction finds itself moving more slowly than the ground – that is, it appears deflected westward, in an opposite sense to the Earth's spin. The effect of this is to spin the air mass anticlockwise. (In the southern hemisphere, a cyclone spins clock wise.) For a high-pressure system, known as an anticyclone,
the opposite reasoning applies. An anticyclone spins clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.

Weather is loosely defined as ‘day-to-day variations in
atmospheric
conditions'. It occurs in the lowest layer of the
atmosphere
, or troposphere. In principle, the weather ought to be entirely predictable. There is, for instance, a mathematical formula called the Navier–Stokes equation that determines completely the future evolution of a fluid such as the Earth's atmosphere. In practice, however, what the Navier–Stokes
equation
predicts depends enormously on the initial conditions. Plug into the equation two sets of temperatures at different locations around the world and the result, within a week, will be two entirely different weather systems.

American meteorologist and broadcaster Robert T. Ryan puts in a nutshell the challenge faced every day by weather forecasters: ‘Imagine a rotating sphere that is 8,000 miles in diameter, with a bumpy surface, surrounded by a 25-mile-deep mixture of
different
gases whose concentrations vary both spatially and over time, and is heated, along with its surrounding gases, by a nuclear reactor 93 million miles away. Imagine also that this sphere is revolving around the nuclear reactor and that some locations are heated more during parts of the revolution. And imagine that this mixture of gases receives continually inputs from the surface below, generally calmly but sometimes through violent and highly localized injections. Then, imagine that after watching the gaseous mixture, you are expected to predict its state at one location on the sphere one, two, or more days into the future.'
14

It is often said, in fact, that the weather is chaotic. This is a type of behaviour that shows infinite sensitivity to initial
conditions
.
‘Does the flap of a butterfly's wings in Brazil set off a tornado in Texas?' asked Edward Lorenz, one of the pioneers of the mathematical theory of chaos.
15
The answer appears to be yes and no. Certainly, the atmosphere – particularly the
circulation
band at mid-latitudes – has the unpredictability of water boiling in a saucepan. However, it appears to hover somewhere between predictability and chaos. After all, if this were not the case, and Lorenz's butterfly effect held sway, weather forecasters would have no success at all. To many this is cold comfort. ‘The trouble with weather forecasting', said the American financial analyst Patrick Young, ‘is that it's right too often for us to ignore it and wrong too often for us to rely on it.'

I have not mentioned the oceans. This is a big omission. The oceans are responsible for transporting about half of the heat from the equator to the poles, making them as important as the atmosphere.

In the North Atlantic, for instance, warm water from the Gulf of Mexico travels north past the west coast of Europe, boosting the region's temperature significantly above that of other
land-masses
at comparable latitudes, such as Canada. Near the pole, some of the water freezes into sea ice, in the process of which salt is driven out, making the sea water saltier. Since salt is
relatively
heavy, the water sinks to the bottom of the ocean. There, it flows along the sea floor back to the Gulf of Mexico. The result is a conveyor belt in the ocean reminiscent of the Hadley cell
conveyor
in the atmosphere, with warm water flowing north, cooling and sinking, then returning south.

But the oceans do more than transport heat from the equator to the poles. They also
store
heat, which they later slowly release. This evens out variations in the temperature between, for instance, summer and winter.

The seasons arise because the Earth does not spin with its equator always pointing towards the Sun. It spins tilted at 23.5° to the vertical. This means that, at one point in the Earth's orbit, the northern hemisphere is tipped towards the Sun, creating
summer
(
winter
in the southern hemisphere) and, six months later, tilted away from the Sun, creating winter (
summer
in the southern hemisphere). The Earth's orbit is not circular but elliptical, and summer in the south coincides with the time when the Earth is at its closest to the Sun.
16

Because the equator does not always point towards the Sun, the hottest point on the surface of the Earth is not always the equator. In fact, the subsolar point migrates north and south with the seasons, and, with it, migrate the whole system of three circulation bands in each hemisphere.