What a Wonderful World (18 page)

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.
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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.
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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.
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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.
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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.
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In fact, it is now about 30 per cent brighter than it was at its birth, 4.55 billion years ago.
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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.
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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.'