These expansive ice sheets and glaciers locked up huge amounts of water, and the sea levels around the world dropped by up to 120 metres, exposing much of the continental shelves around the margins of the great land masses as dry ground. The North American, Greenland and Scandinavian ice sheets spread all the way to the lip of these continental shelves, and the seas around them would have been covered with floating ice layers.6
As well as being punishingly cold close to the ice sheets, reduced evaporation from the frigid seas would have made the world much drier. Howling winds drove fierce dust storms across the arid plains.7 Much of Europe and North America’s landscape would have been tundra-like, with the underlying soil frozen all year round (permafrost), and dry, grassy steppes stretching as far as the eye could see further south. Many of the trees that grow across Europe today survived only in isolated refuges around the Mediterranean. Twenty thousand years ago, the dense forests and woodlands of today’s Central Europe would have instead resembled present-day Northern Siberia.8
With the end of each ice age, the oceans rose again and flooded the continental shelves. The returning interglacial climate saw the ecosystems around the world slowly spread back towards the poles, following the ameliorating conditions behind the retreating ice sheets. Migrations are common within the animal world – birds flying south for the winter or the great herds of wildebeest surging like a tide across the Serengeti – but forests also migrate. Of course, individual trees cannot uproot and move, but as the climate gets warmer, seeds and saplings survive a little further north each year, and over time the forest genuinely marches (like the prophecy in Macbeth). After the last ice age, tree species in Europe and Asia are estimated to have migrated north at an average rate of over 100 metres every year.9 Animals followed them – herbivores feeding directly on plants and the predators in turn tracking them. Recurring ice ages have forced the movement of plant and animal life to sweep north and south like a living tide.
Ice ages vary in their intensity, and interglacial periods are also not all alike.10 The last interglacial, which occurred about 130,000–115,000 years ago, was generally warmer than our current one. Temperatures were up at least 2 °C from today, sea levels around 5 metres higher, and the sort of animals you would normally associate with Africa were tramping through Europe. When construction workers were digging in Trafalgar Square in the centre of London in the late 1950s, they discovered the remains of a range of large animals – rhinoceros, hippopotamus and elephants, as well as lions – all dating from this previous interglacial period.11 Standing in the shadow of Nelson’s Column today, tourists are eager to snap selfies with the bronze lion statues sitting on guard at the corners. How many of them realise that during the last interglacial they would have had to keep an eye out for the real deal?
Ice Age Earth, showing the major continental ice sheets, and sea levels 120 metres lower than today.
But despite the brief warmer periods that allowed these animals to spread, the Quaternary is essentially one long ice age; even during the interglacial periods thick ice caps still smother the poles. Let’s turn now to what has been going on with the Earth over recent planetary history to create such a cold, fluctuating climate. It turns out that the recurring pattern of these ice ages has cosmic causes: it can be explained by shifts in the tilt of the Earth relative to the Sun and in its orbital path.
CELESTIAL CLOCKWORK
If the Earth rotated perfectly upright there would be no seasons. It is the tilt of the planet’s axis which means that for half the year the Northern Hemisphere receives more warmth than its southern counterpart as it leans towards the Sun – which appears high in the sky for its rays to shine more directly onto the surface – to create summer. The situation is reversed six months later to create the northern winter and, correspondingly, southern summer. The Earth also does not move around the Sun in a perfect circle: its orbital path is elongated slightly into an egg shape known as an ellipse. At one point in its year-long orbit the Earth is slightly nearer the Sun, and six months later it’s slightly further away.fn1
To make things more complicated, these features of our world and its orbit also change over time, nudged by the gravitational effects of the other planets in the solar system (especially the giant Jupiter). There are three significant ways in which the Earth’s cosmic circumstances vary, resulting in the set of celestial cycles which I briefly introduced in the last chapter. First, our orbit varies between a more circular and a more elongated shape over a roughly 100,000-year ‘eccentricity’ cycle. Second, over about 41,000 years the tilt of the Earth relative to the Sun sways back and forth between 22.2° and 24.5°, tipping the poles towards or away from the Sun. This tilt has a strong effect on the intensity of the seasons, and so even a tiny change in the angle means that the Arctic receives slightly more or less warmth in summer. And the third and shortest cycle is the 26,000 years over which the axis of the planet rolls round in a circle like a wobbling spinning top, a process known as precession. Precession changes the times of year when the Northern and Southern hemispheres are tilted towards the sun, and thus the timing of the seasons (it’s also called precession of the equinoxes). At the moment, the North Pole just so happens to be pointing towards the star called Polaris – which makes it very useful for navigators as we’ll see in Chapter 8 – but in about 12,000 years’ time the Earth’s spin axis will have rolled around to point towards a new north star, Vega, and summer in the Northern Hemisphere will fall in what we currently call December.
So the stretch, tilt and wobble of the Earth and its orbit all have effects on the planet’s climate, and they vary cyclically over time. These periodic variations are the Milankovitch cycles that I mentioned briefly in the last chapter, named after the Serbian scientist who first worked out how these cosmic periodicities change the climate on Earth. Milankovitch cycles don’t on the whole reduce the total amount of sunlight warming the surface of the Earth over a year’s orbit: they change the distribution of the sun’s heat between the Northern and Southern hemispheres, and therefore the intensity of the seasons.
The Milankovitch cycles: variations in Earth’s orbit and axis that affect our climate.
Contrary to what you might intuitively think, the key driver for triggering ice ages is not how chilly the Arctic gets in winter, but how cool the summer is. With a period of colder summers in the north, each winter’s new snowfall is not completely melted away again and so can build up year on year.12 A cooler Arctic summer also often means a warmer winter, and this too can favour the build-up of ice sheets: greater evaporation from warmer seas will deliver more snowfall. The eccentricity of Earth’s orbit in particular acts as an amplifier on the effects of the direction of Earth’s axis as it precesses round in a loop. For example, whenever these two cycles fall into step with each other, so that the point in our orbit when the North Pole tilts towards the Sun happens to coincide with Earth’s furthest point in its elliptical circuit, then the arctic will receive unusually cool summers. And as a result, the winter’s ice growth won’t completely melt away and will start to accumulate. The planet begins to slide into another ice age.
The Earth remains stuck in this whitened state, reflecting away much of the sun’s heat, until the Milankovitch rhythms cycle back to deliver more warmth to the north and the ice sheets thaw and retreat again.13 The thawing at the end of each glacial period is always much quicker than the freezing at the beginning. As the Milankovitch cycles act to warm the Northern Hemisphere again, the ocean releases more carbon dioxide and more water vapour, both of which are greenhouse gases and so amplify the warming. The rising sea levels also undermine the edges of the ice sheets, and as these melt away, a greater surface area of land and sea is exposed that absorbs more sunlight than bright white ice.14 Hence the rhythm of the ice ages is marked by a slow descent into frozen conditions, followed by rapid deglaciation.
From the start of this icehouse period about 2.6 million years ago the pulse of ice ages followed the 41,000-year
beat of the Milankovitch cycle of the Earth’s tilt, but for reasons that aren’t yet clear about 1 million years ago this transitioned into slower but more extreme swings following the 100,000-year eccentricity cycle of Earth’s stretching orbit.15 The ice ages fell into synch with the beat of a different drum – a drum that beats slower but louder.16 Each ice age became more intense and lasted longer: major ice sheets from the North Pole were able to advance right down the Eurasian and North American land masses, and didn’t completely melt away again during interglacial warm periods.17 (The Antarctic ice cap also waxes and wanes, although to a much lesser extent.18)
In this sense, then, astrologers are right – just not in the way they think. The motion of the other planets through the heavens won’t determine your mood or luck, but their gravitational effects on our world do influence something far more profound: the climate of the Earth itself. Understanding the celestial clockwork regulating the tick of these ice ages over the past few million years is pretty straightforward. But the subtle effects of the Milankovitch cycles can only trigger repeated swings between ice age and interglacial phases if the world’s climate is already unstably perched right on the brink of glaciation. So the bigger question is: what caused these icehouse conditions in the first place?
FROM HOTHOUSE TO ICEHOUSE
At the moment, the Earth is in something of a weird period of its lifetime. For around 80–90 per cent of its existence our planet has been significantly hotter than it is today; periods with ice caps at the poles are in fact something of a rarity.19 Over the last 3 billion years there have been perhaps only six eras with significant ice on the planet.20 Yet over the past 55 million years, the Earth has experienced a continued chilling and the global climate has shifted from hothouse to icehouse. This is known as the Cenozoic cooling, after the geological era during which it occurred.
The layer cake of different rocks beneath our feet enables geologists to divide the long history of our planet into different eras, periods and epochs, often by referring to the kinds of fossils found within them, like chapters and paragraphs within the book of time. The current era dominated by mammals and angiosperm plants – we will return to our planet’s fauna and flora in Chapter 3 – is called the Cenozoic (meaning ‘new life’) and began 66 million years ago with the mass extinction of species that ended the Mesozoic (‘middle life’) Era characterised by the dinosaurs. The most recent period within the Cenozoic is the Quaternary, defined by the fluctuating climate of glaciations and interglacial phases that we have just encountered. Slicing up time even more finely, the latest epoch of the Quaternary is the Holocene: our current interglacial that holds the entire history of human civilisation.
At the end of the Cretaceous Period, just before the dinosaur-killing mass extinction 66 million years ago, the world was hot and humid with lush forests growing even in polar regions. Sea levels were perhaps as much as 300 metres higher than today and submerged half the continental area on the planet – only 18 per cent of the Earth would have been dry land back then.21 This warm phase continued for the next 10 million years, peaking with the Palaeocene–Eocene Thermal Maximum (we’ll explore its significance in Chapter 3) 55.5 million years ago, before the global climate began a sustained cooling. About 35 million years ago the first permanent ice sheets appeared on Antarctica,22 20–15 million years ago ice sheets began forming over Greenland, and by the beginning of the Quaternary the cooling had passed the threshold for the North Pole’s ice cap to begin expanding. We entered the current phase of pulsing ice ages.23
It seems the Earth has been committed to a concerted effort towards cooling down. What grand-scale planetary processes have been conspiring to drive this global chilling?
Gases like carbon dioxide and methane, as well as water vapour, in the atmosphere act like the panes of glass in a greenhouse: they allow the short-wavelength visible sunlight to shine right through and heat the Earth, but block the longer-wavelength infrared light given off by the warm planet surface. The effect of these greenhouse gases is to trap heat energy from escaping back into space, and so insulate the planet, leading to higher temperatures. Any mechanism that reduces the amount of these greenhouse gases in the air will therefore drive a global cooling.
Divisions of Earth’s geological history
Fifty-five million years ago, as we saw in the last chapter, the dance of the continents drove India to begin to crash into Eurasia and thrust up the huge Himalayas. Ever since, this towering mountain range has been vigorously eroded by high-altitude glaciers and rain. The minerals of the rocks react with the carbon dioxide dissolved in rainwater, which then flows in rivers to the ocean where it is used by marine life to build their calcium carbonate shells. When these creatures die, their shells drift to the seafloor and become buried. Thus the Himalayas are being gradually disassembled grain by grain and in the process carbon dioxide is locked away from the atmosphere. While this is a powerful mechanism for effectively sucking CO2 out of the air, it still took around 20 million years to reduce the high levels of this greenhouse gas from the Cretaceous Period to below the threshold where the world became cool enough for ice to start forming at the poles.24
While the young Himalayas eroded, continental drift carried Antarctica to its current position over the South Pole, and Australia and South America rode north. This isolated Antarctica and opened up an unobstructed sea path right around the pole, a great oceanic moat completely surrounding the southern continent. A strong ocean current circling Antarctica became established, and this blocked warm ocean currents from the equator reaching Antarctic shores, keeping the continent chilled. The first permanent ice cap began to form on Antarctica about 35 million years ago.25
Plate tectonics also rearranged the other continents to shove most of the land mass into the Northern Hemisphere, while the southern half of the world became mostly open ocean (a feature we’ll return to when we come to look at the powerful Roaring Forties winds in Chapter 8). For the past 30 million years or so, 68 per cent of the northern hemisphere has been continents, with only a third of the Earth’s land south of the equator.26
This yin-and-yang division of the world – a land-dominated Northern Hemisphere and an oceanic southern half – amplifies the effects of seasonal variations in warmth from the sun. Land cools down far more quickly in winter than the turbulent ocean water, and is much better able to support growing, thick ice sheets. Yet while it is true in general that there is more land mass in the Northern Hemisphere, the pole in the Southern Hemisphere happens to have a continent – Antarctica – currently sitting right over it, whereas the North Pole is sea. This explains why the South Pole became smothered in an ice cap much earlier than its northern counterpart. At the North Pole, where ice melts more easily in the ocean, it wasn’t until 2.6 million years ago that the climate became cool enough for ice no longer to melt away each summer and to accumulate year on year.
The final geological factor that created the icehouse conditions of today was the formation of the Isthmus of Panama. This narrow thread of land joining North and South America was also the result of continental collision, the plate subduction first producing a string of volcanic islands and then lifting the seafloor up above the waves. The closure of the connection between the Pacific and Atlantic oceans occurred 2.8 million years ago and deflected the equatorial current to the north, strengthening the Gulf Stream that delivers warm water to the landmasses around the North Atlantic.27 While this current of warm water may have slightly delayed glaciation in the north, overall the extra moisture in the air from evaporation produced greater snowfall in winter and so encouraged the growth of ice sheets in the Northern Hemisphere.28
As the ice caps formed, first at the South and then at the North Pole, they contributed to further cooling as their bright white surface reflected more sunshine back into space – a snowball effect that scientists term a feedback loop. And as the seas became cooler they could hold more dissolved carbon dioxide from the air, further pulling down atmospheric leve
ls and reducing the warming greenhouse effect.29
The effects of mountain-building and subsequent erosion removing atmospheric carbon dioxide, plate tectonics isolating Antarctica on the South Pole and forming the Panama Isthmus to alter ocean circulation patterns, continental drift shunting most of the rest of the land masses into one hemisphere – all these factors combined to propel us into icehouse conditions. The cooling of our planet to the stage where large ice sheets formed in the north 2.6 million years ago was a critical threshold and the entire climate tipped into an unstable state. Now, whenever the Milankovitch cycles acted to cool the North Pole slightly, the ice cap expanded over Europe, Asia and North America, and these large northern continents were able to support thick ice sheets. Even a small increase in the expanse of white ice would reflect more of the sunlight away, causing further cooling, and so set in motion a runaway process that saw the ice sheets expand ever further, and locking up more water out of the oceans leading to a drop in sea levels.
This sustained chilling trend of the world over the past 55 million years of the Cenozoic Era has had a profound influence on the planet, and our own evolution. As we saw in the last chapter, the shift to cooler, drier conditions shrank the forests of East Africa and replaced them with grasslands, driving the development of hominins. And the rapid fluctuations of the Rift Valley’s amplifier lakes, which drove us to become a highly versatile and intelligent species, were caused by the precession rhythm of the Milankovitch cycles.
Beginning around 100,000 years ago a planetary alignment was sliding into place. The tilt of the Earth’s axis causing summer in the Northern Hemisphere started to coincide with the time when the planet was furthest from the sun in its elliptical orbit, meaning that northern summers became ever cooler. The ice from each winter didn’t melt away again but accumulated. The northern ice sheets began to grow and expand southwards as the Earth slid into another ice age.
Origins Page 4