The sun, moving as it does, sets up processes of change and becoming and decay, and by its agency the finest and sweetest water is every day carried up and is dissolved into vapour and rises to the upper region, where it is condensed again by the cold and so returns to the earth.
ARISTOTLE, Meteorology, 350 BC
It’s raining men! Hallelujah! It’s raining men!
THE WEATHER GIRLS
‘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.
The oceans play an important role in all of this because they store heat – a lot more than the atmosphere. Warmed up in summer, they then gradually release their heat during winter. This means that the coldest part of the winter in any hemisphere comes not at winter solstice – when that hemisphere is pointed away from the Sun – but several months later. Just as the atmosphere evens out extremes of temperature between day and night, the oceans even out extremes of temperature between summer and winter.
Climate, in contrast to weather – the day-to-day variation in atmospheric conditions – is defined as ‘the average state of the atmosphere and oceans over longer periods of time than associated with weather’. Typically, this is of thirty years or more. ‘Climate is what we expect,’ said Mark Twain. ‘Weather is what we get.’
One of the striking discoveries of science is that the climate of the Earth has not always been as it is. For instance, a whopping 90 per cent of the past 1 million years has been an ice age, a period of depressed global temperatures characterised by extensive ice sheets in the northern and southern hemispheres.
One of the triggers of ice ages is believed to be natural cycles in the Earth’s orbit around the Sun caused by the gravitational tug of the Sun, Moon and other planets. Over a period of 100,000 years, for instance, the Earth’s elliptical orbit becomes more stretched out than squashed up. Over a period of 42,000 years, the Earth’s spin axis, currently tilted at 23.5° from the vertical, tips over further, then rears up closer to the vertical. And, over a period of 26,000 years, the Earth’s spin axis changes its orientation in space, rotating through a full circle about the vertical.17 These cycles, known as Milanković cycles, vary the amount of sunlight falling on the Earth’s surface.
But, it is not only variations in the amount of sunlight intercepted by the Earth that are thought to cause ice ages. Intrinsic variations in the Sun may also play a role. The Sun is actually remarkably steady and alters its heat output by less than 1 per cent over the course of a solar cycle.18 This is too little to have much effect on the Earth’s climate. However, the small fluctuation in total heat output is accompanied by a variation of as much as 100 per cent in solar ultraviolet radiation.19 Such high-energy light shatters high-altitude molecules such as ozone in the stratosphere, the layer above the weather, or troposphere. Since these molecules play an important role in transporting heat down through the atmosphere, the boost in solar ultraviolet can have an appreciable effect on the Earth’s climate.
But it is not simply changes in the amount of sunlight falling on the Earth’s surface that play a role in triggering ice ages. There are more down-to-earth things – literally – such as the movement of continents.20 Once upon a time, for instance, South America was connected to Antarctica. Warm water flowed from the equator directly down the coast of South America, keeping Antarctica ice-free. About 33 million years ago, however, the two continents broke apart. With the opening up of the Drake Passage between South America and Antarctica, it was suddenly possible for water to circulate in a west-to-east direction between the Pacific and Atlantic oceans. With water now largely flowing from west to east, rather than from north to south, the flow of heat towards Antarctica was significantly reduced and Antarctica, as a consequence, froze.
A similar change to the ocean circulation could be triggered in the North Atlantic by human-induced global warming. Currently, warm water flows from the Gulf of Mexico up past the coast of western Europe. There it cools, sinks and returns. This conveyor belt of warm water keeps the coast of western Europe relatively warm. However, melting sea ice near the pole could disrupt the flow of heat from the equator to the pole. This is because sea ice, when it initially forms, expels salt. The melting of sea ice near the pole will therefore make the water less salty and, crucially, less heavy, so that it no longer sinks (melting fresh-water ice from Greenland will do the same). The result could be that the North Atlantic conveyor will to some extent shut down, plunging the temperature off the coast of Europe to a level more typical of its latitude – that is, more like Winnipeg in Canada. The Earth will, of course, still have to transport heat from the equator to the poles. But air currents and east–west flows, through the unfrozen Arctic sea, might take over that role, much as they did 33 million years ago after the split of South America from Antarctica.
Recent ice ages, however, are nothing compared with ancient ice ages. The world is believed to have gone through two periods when ice stretched in an unbroken sheet all the way from the poles to the equator. These episodes, known as Snowball Earths, occurred about 650 million years ago and 2.2 billion years ago, respectively. The causes are disputed. But a plausible explanation of the first episode is that it was caused by blue-green algae suddenly evolving the ability to split water molecules and release oxygen in photosynthesis. This happened about 2.3 billion years ago. The oxygen from such cyanobacteria destroyed methane – an abundant greenhouse gas in the atmosphere – which had been keeping the planet warm.
A planet covered entirely in ice reflects sunlight back into space. For this reason, Earth is likely to have remained locked in each of its Snowball states for millions of years. What brought each super-cold spell to an end was probably volcanic eruptions, which pumped more and more carbon dioxide back into the atmosphere until, finally, its warming effect was enough to thaw out the Earth.
Carbon dioxide is of course the gas that is produced by the burning of fossil fuels such as oil and coal and whose concentration in the atmosphere has been increasing since the beginning of the industrial age. Over precisely the same period the global temperature has been steadily rising – exactly what would be expected since carbon dioxide is known to trap heat in the atmosphere.
It works this way. Carbon dioxide – and the rest of the gases that compose the atmosphere – are transparent to visible light from the Sun (if they were not, we would not be able to see the Sun). Sunlight therefore passes through the air unhindered and heats the ground. The ground, in turn, heats the air, which is why the temperature is highest near the ground and steadily decreases with altitude all the way to the top of the troposphere, the domain of weather.
To be precise, the ground glows with heat radiation typical of a body at about 20 °C. Crucially, such far infrared is absorbed by carbon dioxide in the atmosphere. In other words, the Earth’s heat is prevented from escaping into space and is instead trapped in the atmosphere. This is not quite what happens in a greenhouse, where glass is transparent to sunlight but provides a physical barrier to the escape of rising, or convecting, warm air. Despite this, however, carbon dioxide is widely known as a greenhouse gas.
Actually, by far the most important greenhouse gas in the atmosphere is water vapour. This is responsible for about 75 per cent of the warming effect of the atmosphere compared with only 20 per cent for carbon dioxide. We should on the whole be grateful for greenhouses gases since, without them, the average temperature of the Earth would be a super-chilly -18 °C.
However, if humans continue adding more and more carbon dioxide to the atmosphere, the global temperature will continue to rise. ‘Geological change usually takes thousands of years to happen but we are seeing the climate changing not just in our lifetimes but also year by year,’ warned the English chemist James Lovelock.
The Greenland ice sheet and Antarctic ice sheet are already melting. But the melting will accelerate, significantly raising the sea level globally and inundating low-lying coastal areas. The circulation of the ocean and atmosphere will change unpredictably, with worrying implications for the Earth’s 7 billion people. Nobody knows where it will all end. However, nature has conveniently shown us one possibility: Venus.
Being about two-thirds of the Earth’s distance from the Sun, Venus lost its water early on in its history. Basically, the extra heat from the Sun caused its primordial oceans to begin evaporating away. Water vapour, being a potent greenhouse gas, warmed the planet more, which evaporated more of the oceans, which warmed it even more, and so on. This runaway greenhouse effect, first proposed by Carl Sagan and William Kellogg in 1961, eventually boiled away Venus’s oceans entirely. We see no sign of them today because, at the top of the atmosphere, high-energy ultraviolet from the Sun split water molecules into their constituent hydrogen and oxygen atoms, which then wafted away from the planet on the wind from the sun. Ultimately, Venus lost its oceans to space.
On Earth, carbon dioxide from volcanoes is washed out of the atmosphere by rain. But this could not happen on waterless Venus. Instead, the level of carbon dioxide in the atmosphere rose and rose. Today, the planet has about 92 Earth-atmospheres-worth of carbon dioxide. Not only does this create a crushing pressure on the surface – equivalent to the pressure almost a kilometre down in the Earth’s oceans – but the warming effect of the greenhouse gas creates a temperature hot enough to melt lead. The whole planet is shrouded in impenetrable sulphuric-acid clouds, made from sulphur dioxide vomited from volcanoes. Venus, in short, is hell.
Since the Earth is further from the Sun than Venus, it is not clear whether our warming of the planet will eventually trigger the catastrophe of a runaway greenhouse. But, whether we are responsible or not, one thing is sure: one day it will happen naturally.
The reason is that the Sun is slowly growing hotter as it burns through its hydrogen fuel.21 In fact, it is now about 30 per cent brighter than it was at its birth, 4.55 billion years ago.22 In the future, as the Sun continues to get more luminous, more and more water from the oceans will turn into water vapour, which will trap more heat in the atmosphere, which will turn more of the oceans into water vapour, and so on. On the sweltering planet, carbon dioxide, locked up in carbonate rocks such as chalk cliffs, will begin leaking into the atmosphere, trapping more heat, which will create more heating, which will drive more carbon dioxide into the atmosphere. Eventually, by about AD 1 billion, the oceans will have boiled away entirely into space and the atmosphere will be made mostly of carbon dioxide. Coincidentally, the Earth has pretty much the same amount of carbon dioxide locked up in carbonate rocks as Venus currently has in its super-dense atmosphere. So, when it all floods out into the atmosphere, the Earth will be almost exactly like Venus.
But this will not be the end of the Earth’s ordeal. In 5 billion years’ time, the Sun will run out of hydrogen fuel in its core. It will swell into a monstrous super-luminous red giant, pumping out 10,000 times as much heat as it does today. If this bloated star does not completely swallow our planet – and it will definitely envelop the close-in worlds of Mercury and Venus – it will certainly reduce the Earth to a burnt and blackened lump of slag.23
Long before that time, however, our descendants – should any still survive – will have to leave the Solar System and find another planet to live on. ‘Earth is the cradle of humanity,’ said Siberian rocket pioneer Konstantin Tsiolkovsky. ‘But mankind cannot stay in the cradle for ever.’
1 James Hutton, ‘Theory of the Earth; or an Investigation of the Laws Observable in the Composition, Dissolution, and Restoration of Land upon the Globe’, Transactions of the Royal Society of Edinburgh, vol. 1 ( 1788), pp. 209–304.
2 Most sedimentary rocks are actually created in the oceans not in lakes. However, the pioneer geologists of the eighteenth century had essentially got it right.
3 L. P. Hartley, The Go-Between.
4 The first transatlantic telegraph cable was laid by Isambard Kingdom Brunel’s ship, the Great Eastern, in 1866. For his part in the feat, physicist William Thomson was knighted by Queen Victoria, finally becoming Lord Kelvin.
5 The Solar System is believed to have formed from the shrinkage under gravity of a cold cloud of interstellar gas and dust. The cloud, just like the Galaxy, was spinning. Consequently, it shrank faster between its poles than around its waist, where centrifugal force was opposing gravity. The result was a pancake-shaped cloud, with the Sun forming at the centre and leftover debris orbiting in a disc around it. Dust grains in the debris disc stuck together to make bigger dust grains in a runaway process that resulted in a vast number of kilometre-sized bodies. It was the collision of these planetesimals that gradually built up the planets, including the Earth. The final stages of this accretion process are recorded in the giant impact basins on the Moon.
6 See Chapter 13, ‘Earth’s aura: The atmosphere’.
7 Why Venus has no plate tectonics is not clear. But water is necessary to create the granite out of which continental crust is made. And Venus, being closer to the Sun than the Earth is, is believed to have lost its water to space early in its history.
8 Louis Agassiz, Geological Sketches.