Climate change isn't a new idea. In the 4th century BC, the Greek philosopher Theophrastus believed that the number of spots he counted on the Sun's surface could explain the changing rainfall. During the Age of Enlightenment in the 18 th century, gentlemen scholars noticed that many Classical writers described a climate different from their own day; Edward Gibbon, for instance, noted in his 1776 weighty tome The Decline and Fall of the Roman Empire that the 3rd century AD must have been cooler, citing the frequently frozen Rhine and Danube rivers in support of his idea. After this, things really gathered pace. By the mid-19th century it was realized that ice ages had been common in our planet's past. The nice cosy image of a stable world disappeared forever.

It's hard to imagine that a world of ice can help us understand future climate. With concerns of global warming it might seem academic to try to understand ice age climates. After all, it is the opposite of what we're anticipating. We'll look a little later at how ice ages were discovered, but for now let's just take it as read that they're a fact and look at how they might be useful for understanding what lies ahead.

Although we often associate ice ages with mammoths and strategically clothed cave people, the frozen spells of our imagination happened relatively recently. For a planet that's 4.6 billion years old, hairy jumbos and cavern dwellers were just yesterday. There was a time, however, when things were far worse. Much earlier, a series of global catastrophes seem to have taken place, each cocooning the world with ice. Seen from space, our planet would have appeared as an enormous iceball. No one is seriously suggesting the world is going to turn into a global winter wonderland in the near future, but this severe chill gives us an idea of how far the world can go in the stakes of extreme climate.

A crucial element of all this is identifying what happened, and when, in our early planet's history. It's an issue fraught with problems. Not least of these is finding parts of the world that preserve the past. The further back you go, the less there is. There are a whole host of reasons why this is the case but the bottom line is the world is a dynamic place. The best you can often hope for is an exposure of rock or mud that somehow escaped the ravages of time; preserving in it what happened at the moment of formation. How to date these records of the past plagued early scientists until it was realized that many contained fossils of ancient plants and animals. Over millions of years, the different fossils preserved a record of evolution and extinction: essentially the story of life on our planet. Although fossils don't give a direct age for a rock, they can be placed in an order relative to one another. The geological scale was born. A plethora of weird and wonderful names were conjured up to chronicle the different times of our planet's history. With the development of scientific dating techniques it's now possible to put ages to these past times. We now know for instance, that the explosion of life recorded by fossils in the Cambrian - after the Roman name for Wales where it was first discovered - began 542 million years ago. But for a lesson in future climate change we can go further back in time to when little life existed: the so-called Neoproterozoic.

Although it's a dreadful mouthful, the Neoproterozoic represents a fascinating era that spans a vast 460 million years. In spite of the huge amount of time involved, it was realized late in the 19th century that some parts of the world preserved evidence of early ice ages. In 1871, James Thomson reported ancient rubble at Port Askaig in Islay, Scotland, that had been laid down by glaciers. In 1891, the Norwegian geologist Hans Henrik Reusch followed this up with the discovery of ridges and mounds containing rock and debris that marked the outer limit of an ancient ice field.

This period of advancing ice was named after the site where it was first discovered. Found along Varangerfjord within the Norwegian Arctic Circle, Reusch called his ice age the Varangian. It might sound a bit like we're in fantasyland but bear with me.

The idea that the world was covered by ice at the dawn of time was really first touted by one of the great scientists and adventurers of the early 20th century: Sir Douglas Mawson. Mawson was a Boy's Own hero. In 1911, he led an Australian expedition to Antarctica's George V Land to collect scientific samples, reach the Magnetic South Pole and see at first hand how ice might mould the landscape. The team suffered a series of disasters that would be more fitting on the pages of a Hollywood script. After over-wintering at Cape Denison, Mawson led a three-man team to explore the eastern lands, crossing what are now known as the Mertz and Ninnis glaciers. The conditions were particularly harsh at this time and a combination of crevasses, lack of supplies, vitamin A poisoning (from eating the livers of their sledge dogs) and exhaustion resulted in the deaths of Mawson's less fortunate friends and colleagues, Mertz and Ninnis. Against all the odds Mawson struggled back alone for 24 days and reached Cape Denison to be greeted with smoke on the horizon; his ship had sailed away only hours earlier. Fortunately, a small group had volunteered to stay in case of his return and Mawson was forced to spend another winter in Antarctica. His discoveries and tales of endurance rank alongside those of his contemporaries but were eclipsed by the death of Scott in 1912.

When Mawson wasn't leading heroic expeditions to Antarctica, he spent a large part of his career looking at the early geology of the Earth. As the evidence for ancient ice ages mounted up, Mawson pulled all the data together. In 1949 he recognized over twenty Neoproterozoic sites around the world, stretching from the Arctic to the Equator, and argued for the first time that that this widespread pattern showed that the world had 'experienced its greatest Ice-Age'. This was revolutionary stuff, but Mawson wasn't the only one touting new ideas.

Between 1912 and 1915, a young German scientist called Alfred Wegener published scientific papers and a landmark book called The Origin of Continents and Oceans in which he attempted to solve a centuries-old conundrum. In 1596, the Dutchman Abraham Ortelius, and later the Briton Francis Bacon, had spotted how easily South America could snuggle up alongside Africa. Charles Darwin had also been intrigued and speculated 'on land being grouped towards centres near Equator in former periods and then splitting off'. Wegener had looked into this further. He realized that rocks found on one continent matched another; for instance, the geology of the Scottish Highlands was identical to the Appalachian Mountains in North America. He also found fossils of tropical species in the Arctic. On the basis of all this and more, Wegener proposed that the continents ploughed their way through the oceans, changing their location on the surface. By 1922, Wegener had developed his ideas further and was arguing that around 300 million years ago the continents had formed one large landmass - a supercontinent - called Pangaea. According to his argument, Pangaea had subsequently split up and formed the world we see today. The critics were scathing of Wegener, largely because he had no real idea of how the continents could move about on the surface; 'purely fantastic' and 'German pseudoscience' were typical of some opinions given of Wegener's ideas. Mawson certainly didn't believe a word of it and took the location of his ice age sites at face value.

Although flawed at one level, Wegener's notion of drift led to the realization in the 1960s that the continents form plates which float on a buoyant layer in the Earth: this idea of plate tectonics explained how new continents and oceans were created, destroyed or rubbed along uncomfortably together. More importantly for our story, it explained the jigsaw puzzle seen today; since at least 3.8 billion years ago, plates have migrated around the world, rejigging their position relative to one another. Ultimately, if we took the latest in time travelling technology to visit the Neoproterozoic, the world's surface wouldn't look like it does today. Mawson's idea of a world of ice was suddenly in trouble. The simplest explanation was that tropical sites showing ice had once been near the poles and subsequently travelled towards the Equator. If so, the world's climate back then might not have been any different from today; the same sort of trend from tropical heat to icy poles probably existed. But to test this it was necessary to delve into the rocks themselves and look for a faint magnetic signal.

It might seem odd that magnetism can help us understand past climate change but it does so at several different levels. Back in the 15 th century, there was an amazing mixture of practical benefits and quack urban myths linked to magnets - in many ways we haven't really moved on today. Magnets had been known about since at least the 7 th century BC and were widely used as a navigational aid at sea. But it was only in 1600 that the Englishman William Gilbert undertook one of the first methodological studies of magnets. Although he later went on to become physician to Queen Elizabeth I, he is best remembered for his book De Magnete. Written in Latin, On the Magnet was a huge success. In it, Gilbert debunked a whole host of tall tales, including the belief that garlic disrupted magnetic fields; in spite of this it remained a flogging offence for over another century for British naval helmsmen to eat garlic. Importantly, Gilbert recognized that magnets had poles and argued the Earth was one giant magnet; if a handheld magnet was freely suspended, he noted it should point towards the ground. We now know the magnetic field is created by the molten iron-rich part of the Earth's core and that a compass will align itself to the magnetic field when it is free to do so. This change in inclination gives a strong clue as to where you are on the Earth's surface; a magnet will lie parallel to the ground at the Equator, but at the poles will dip at a right angle.

When volcanic rocks are thrust onto the Earth's surface or sediments are laid down at the bottom of a lake or on the seabed, any magnetic particles that are present will align themselves to the Earth's field. Most of the time this involves iron, and although the signal is tiny, the orientation of these particles can be measured; essentially they're compasses frozen in time. From this it's possible to work out where on the Earth's surface rocks or sediment were first laid down. It doesn't matter where the land goes afterwards, the signal should be preserved; a memory of a past latitude. By the 1960s, measurements of the Earth's changing magnetic field had made plate tectonics mainstream. It was clear that the continents had roamed over the world's surface. Mawson's intransigence on continental drift had lost a lot of support for the idea of an icy world; it seemed more likely that his tropical sites had been at the poles and later migrated towards the Equator. It seemed farcical to many early critics that the whole world could have been covered in a lot of ice. But not everyone thought it was a mad idea.

Working in the Arctic, British geologist Brian Harland worked on ice deposits that were Neoproterozoic in age. He analyzed the rocks and measured their magnetic properties, providing some early support for the idea of plate tectonics. Through the 1950s and 1960s he concluded that the ice age deposits he was finding in the Arctic were formed at a latitude where the magnetic field was parallel to the ground. They had to have originated from near the tropics. All hell broke loose. The critics countered him. This couldn't be right. The magnetic signal was so faint that any slight problem in the lab could screw up the measurement. And if this wasn't enough, water flowing through rocks and sediments can lay down extra magnetic particles, overprinting the original signal. The measurements made by Harland had to be an artefact; something had happened to the magnetism in his ice age deposits since they were formed. An icy world looked to be one big red herring. Sure there was ice over 540 million years ago but it seemed pretty unlikely it could have reached the tropics. The world's climate just couldn't have done that. The idea seemed doomed.

To prove that glaciers had indeed got down to the tropics, an ice age deposit needs to show an unaltered low-latitude magnetic signal. This is easier said than done. It's difficult to demonstrate that a signal has been preserved over hundreds of millions of years. It took some three decades before the evidence was found in the Flinders Ranges of South Australia. Within the beautiful Pichi Richi Pass is a rather odd looking set of rocks that seem to be covered with parallel lines. It looks like someone has come along and compulsively drawn a great number of strokes on the stone. But this couldn't be further from the truth; these markings reveal the presence of an ancient ice age estuary.

Making up part of what is known as the Elatina Formation, the conspicuous rocks in the Flinders Ranges are packed with information, preserving a record of the changing tide some 635 million years ago. Each day the sea came in and out, and stacked layer upon layer of sand and silt on the estuary floor. By good fortune, these layers have survived at Pichi Richi, and perversely it's because of their great age. Today the sea is a living frenzy, with small creatures thick on the ground, churning up the ocean floor to salvage anything that can be consumed; fortunately, back at the time when the Elatina sediments were being laid down, little life existed to mix up the layers. By measuring the changing thickness of the layers, it's possible to calculate the length of day when the sediments were laid down. 635 million years ago, a day was somewhere around 22 hours long. From similar sediments dating back 900 million years ago, a day was shorter still at 21 hours. The cause of the ever-lengthening day is the increasing distance the Moon is putting between us - today it's nearly 4 centimetres a year - causing the Earth to spin ever slower on its axis.

Critical to our story is that some of the layers in the Elatina Formation contain stones that couldn't have got there from the scenario painted above; the layers are too delicate to have formed within an ocean capable of transporting big stones. They must have been delivered by melting ice passing overhead, dropping any transported rubble into the estuary sediments below. There are quite a few of these dropstones in Elatina, showing that around 635 million years ago there was enough ice about to form glaciers that reached out over the sea. The question is: where on the Earth's surface were the tidal Elatina rocks formed?

Looking at the orientation of the magnetic particles in the rocks seemed to suggest they had been laid down in the tropics. But was this real? It could easily have been the case that the magnetic signal had been overwritten. Joseph Kirschvink at the California Institute of Technology concocted a great test of this challenge. Some of the rocks from Elatina were clearly folded, indicating that after the estuarine sediments had been laid down they'd slumped and then hardened. If the magnetic signal had been overwritten, then all the particles would be parallel to one another, regardless of where they were relative to the layers. But if the particles were aligned parallel to the layers, following the folds and bends, it would show that the signal was authentic and hadn't been changed since the sediments were laid down. Convincingly, when the samples were analyzed, the particles were shown to be parallel to the layers. The estuarine muds and dropstones had been formed very near the tropics. Kirschvink was inspired and in 1992 coined the phrase 'Snowball Earth' to describe an ice-smothered planet. The idea, however, continued to lie on the fringe of acceptability.

Things really only hotted up in 1998 when Harvard University's Paul Hoffman and colleagues described a sequence of rocks in Namibia that appear to have formed around the same time as those in the Flinders Ranges. In a major article published in the journal Science, Hoffman's team recognized a sequence of carbonates and glacial debris, capped by yet more carbonate. Here, the exposed rocks appeared to have formed on the edge of a former sea in a setting akin to the Bahamas. Crucially, the magnetic characteristics of the rocks showed that the ice had been around when the site was at 12° S. The debris contained dropstones, similar to those found in the Elatina Formation, but was also made up of lumps of carbonate gouged from older rocks as the glacier had migrated over the land. Just as Mawson had realized that much of the action driven by the Antarctic ice was happening under his feet and out at sea, most of the evidence for the Namibian ice was preserved in what had been an offshore environment. Here was crucial support for the idea that ice had existed in the tropics.

Suddenly a planet of ice seemed possible; the world's climate actually seemed capable of going to such an extreme. Since the work in Namibia, there has been a plethora of studies. At least 16 sites have now been found with evidence for ice, most capped with a thick layer of carbonates many metres in thickness. Importantly, all of them seem to have formed close to the Equator, with none found at latitudes greater than 60°. But it doesn't seem to have happened once; there were at least three immense events between 710 and 580 million years ago. The ice ages preserved at Pichi Richi and Namibia look like they were formed during the same glaciation, known as the Marinoan. But there were at least another two ice ages: the Varangian described earlier and another called the Sturtian.

But this all raises the obvious question: what impact did these enormous upheavals have on early life? We now know that the earliest multicellular life arose before the Cambrian. In 1946, evidence for soft-bodied organisms was discovered in the Ediacara Hills of South Australia; these fossils have been dated to between 635 and 542 million years ago, predating the explosion of live during the Cambrian. That the Cambrian was thought to hold the earliest evidence for life isn't surprising; the hard-shelled organisms of this period were ideal for preservation. The parts of soft-bodied organisms like those in the Ediacaran, however, were rarely preserved in the geological record. Instead, their tracks, recorded on soft sediments, have come down to us as fossils. It's still not clear what their relationship is to the major groups living today; the Ediacarans don't appear to have had any major limbs, and almost certainly absorbed nutrients from the seawater in which they lived.

Crucially, algae and bacteria were common in the rocks before the ice ages. After the Marinoan, Ediacaran life suddenly blossomed. Mawson spotted this early on and was one of the first to suggest that warming after a global ice age might have driven the flowering of life on our planet.

You might reasonably assume a global ice age would snuff out all life. Yet here we are. There must have been some refuges for living things to continue. Life might have conceivably struggled on around hydrothermal vents at the bottom of the sea. Alternatively, perhaps enough light could have penetrated the ice so as to allow photosynthesizers to survive in the oceans. Conversely, some mountains may have been high enough to get above the ice and provide asylum. We know these sort of environments are capable of supporting some forms of life today. We're still not sure what happened during the Marinoan, but it is tantalizing that such extreme conditions might have given life a kickstart.

This all sounds great but let's just think what we're saying here. Sometime in the past the world was almost entirely covered in ice. How could this be? The results from Pichi Richi show that our world was rotating faster than it does today; this would have meant that much of the heat received from the Sun in the tropics would have stayed in the tropics. Although this should mean that ice ages were common at the poles, the tropics should have been relatively warm. There shouldn't have been ice everywhere. What on Earth was going on?

How could a Snowball Earth have come about? At the time, the Sun was 6% less bright than it is today. This would have helped with the cooling, but it immediately throws up another conundrum: how did the Earth ever escape the ice? To all intents and purposes, once a planet becomes fully frosted, it should stay as such. Yet manifestly this is not the case. The whole thing seemed to be a big puzzle.

So what could have caused a Snowball Earth? One possibility that was touted early on was that the Earth may have rotated at a different angle compared with today. This might sound a bit bizarre but it's a thought-provoking idea. If you visit a map shop or department store, you'll often see globes for sale, ranging from the cheap and cheerful through to the expensive, heavy-duty affairs. The one thing they should all have in common is that the axis of the globe is set at an angle of 23.5° from the vertical (Figure 2.1). It's this angle that gives us the seasons. When one hemisphere points towards the Sun, it's summertime; six months later, the other hemisphere takes pole position. We'll see why this feature of the Earth is important in explaining more recent ice ages, but for the moment let's just accept that we know that the angle can change within a few degrees.

If you've got an old battered globe packed away in the kids' cupboards or suddenly get the urge to buy something more presidential, try a little experiment. Get a torch and shine the light beam on different parts of the surface, making sure the torch is level with the floor. You should notice that in the tropics, the beam of light is tightly focused on one spot. When you switch to the poles, the light should 'spread' out on the surface, reducing the amount of light (and heat) that falls on one area. Now if you increase the angle of

Figure 2.1 The tilt of the Earth.

the globe slightly and repeat the exercise you'll notice little change in the tropics; the light stays tightly concentrated on one spot. Importantly though, over the poles the torch beam should become less spread out and begin to focus on a smaller spot. A similar effect can be felt when you go outside with the Sun low in the sky: because of the angle, the Sun's rays are spread out so you don't feel much heat; when the Sun moves overhead, the rays become more concentrated and it gets warmer. The practical upshot is that the high latitudes receive more heat from the Sun as the angle increases; later on we'll see that this plays a rather important role in reducing the temperature difference across the globe.

But there's no way that a shift of a few degrees could explain all the ice seen in the tropics more than 580 million years ago. One possibility is that around 4.5 billion years ago, something large struck the Earth, creating the Moon. Such a large impact might conceivably have also made the tilt in the Earth's spin axis much larger than it is today; possibly reaching a massive 54° or more. It's hugely contentious that the Earth was at such a large angle but let's just assume for a moment that this happened. What would be the effect of such a gargantuan angle? If you tilt your globe at home and repeat the exercise, things go a bit funny: the tropics receive a lot less heat from the Sun than the poles. The result of this is that the low latitudes would be in a constant ice age, while the poles would become positively temperate.

One type of rock provides an excellent test of whether the tilt was so drastically different from today: evaporites. These are deposits of salt formed when a lake of saline water evaporates. If it's hot and dry enough, thick layers of salt can form. Today evaporites are found on the tropical side of 30° latitude; few, if any, evaporites form over the Equator because the associated high rainfall instantly dissolves any salt that does form. A study reported by David Evans at Yale University looked at the magnetic signal in ancient evaporites that were formed over the past 2.5 billion years. Importantly, he found that the evaporites were consistently laid down between 10 and 35°, exactly where we'd expect to find them today. It means that when the Earth wasn't experiencing raging blizzards, the low latitudes were hot and dry. The angle of the Earth must have been similar to today.

If this is the case then we're left with the inevitable conclusion that temperatures had dropped enough for glaciers to be found in the tropics; average surface temperatures may have got as low as -50 °C. When Joseph Kirschvink first coined the term Snowball Earth in 1992 he also put forward some ideas as to how an icy world might end. Critically it all comes down to feedbacks. We'll read a lot about feedbacks through this book; they crop up in climate change all the time, exaggerating some aspect of the planet's climate and making changes either larger or smaller. The proportion of the Sun's radiation that's reflected off the surface - the albedo - can be a major climate feedback. An extreme version of albedo is when you're dazzled by light reflecting off any bright object. Freshly fallen snow has a high albedo and can reflect up to 90% of the radiation that falls on it; that which isn't reflected is absorbed and heats the surface.

Under early Snowball Earth conditions, Kirschvink envisaged high albedo as a positive feedback: any ice cover would reflect heat from the Sun, helping cool the planet. As the Earth got colder, the icy areas became larger, reflecting ever more sunlight back into space. Calculations suggested that if more than half of the world's surface was covered in ice you'd get a runaway positive feedback, driven by this ice-albedo effect: the world would become a Snowball Earth.

This positive feedback would have been helped by the distribution of the continents at this time. We know the continents were concentrated over the low and mid-latitudes - forming a supercontinent called Rodinia (Figure 2.2) - and this would have added to the albedo effect. Today we find that more heat is received from the Sun than lost to space within the latitudes of 37° north and south; polewards of these latitudes, more heat is lost through reflectance and scattering as the Sun's rays penetrate our atmosphere. If none of this extra heat in the tropics were

Figure 2.2 The continent of Rodinia 750 million years ago.

moved to the north and south, the temperature in the polar regions would plunge more than 20 °C while the tropics would become about 10 °C warmer. The critical thing here is that the distribution of ice, land and ocean over the surface has a big impact on how much heat is taken up by the Earth in the first place. Although land has a lower albedo than ice (it can be anywhere from 10 to 40%), it's a lot higher than the oceans (which can get as low as 4%). The practical upshot of this is that if there is proportionately more land in the tropics than ocean, a greater amount of the Sun's radiation would be reflected. If we butchered our globe and covered a large area of the low and mid-latitudes in grey coloured paper (to represent Rodinia), we'd find that a lot of the light from our torch would be reflected; a lot less heat would be taken up by the planet. It would have all helped develop a Snowball Earth.

But this can't have been the full story. Snowball Earths had to end. But how? The most obvious candidate is warming driven by greenhouse gases, most probably carbon dioxide. If the levels built up high enough, there would be enough heat in the atmosphere to melt our planet's icy shell. A likely source is volcanic activity. The amounts involved are hard to quantify this far back, but today's volcanoes contribute somewhere between 0.1 and 0.3 gigatonnes of carbon to the atmosphere each year; although this is less than 1% of what we're putting into the atmosphere, it would add up over millions of years. To finish off a Snowball Earth, carbon dioxide in the air would have had to have reached around 350 times modern levels. This is devastatingly high; it would mean carbon dioxide was somewhere around 120,000 ppm compared to today's concentration of 380 ppm and rising.

Today, carbon dioxide is naturally stripped from the air. This can happen in a number of different ways. The world's oceans are one good absorber of carbon dioxide. Perhaps surprisingly, so too are mountains. Carbon dioxide reacts with moisture in the air to form carbonic acid, and this can eat away at carbonate and silicate rocks; the more mountains there are, the more rocks are exposed and the more carbon dioxide is taken out of the atmosphere. If the sea and land were covered in snow and ice, these natural sinks for carbon dioxide would have effectively stopped working, allowing greenhouse gas levels to build up in the air. At first, a runaway ice-albedo would have kept the Earth in a near-permanent winter wonderland. But after around 10 million years or so, the carbon dioxide levels would have got high enough to override the cold. Once the ice had started to melt, a landscape of smashed rock would have suddenly been exposed to the elements. There would have been an all-out assault, with massive amounts of rock being broken down by a rain of carbonic acid, making the acid rain of the 1970s seem like a Sunday picnic. The broken down carbonate rock would have been washed into the world's oceans, making them extremely alkaline and forming the thick cap carbonates seen in Namibia and other Snowball Earth sites.

The critical point that Kirschvink made was that, given enough time, the carbon dioxide levels in the air would have become large enough to override the ice-albedo and ultimately shift temperatures the other way. Once the temperatures had become high enough, the snow would begin to melt and the albedo would crash. Wet snow has a far lower albedo than the freshly fallen stuff (it's about 40%), allowing more heat to be absorbed by the surface. The snow would melt and the exposed land would shift to an albedo as low as 10%, helping drive an enhanced greenhouse effect.

Needless to say, not everyone is convinced. Some researchers have argued that it's more likely the tropical oceans were actually ice free when the continents were experiencing surging glaciers. This has been likened to one enormous slushball, where the open oceans would have allowed some form of life to continue. The flip side of all this is that with large expanses of open water it would have been hard to keep a slushball going as long as a Snowball Earth. Because more ocean would have been exposed to sunlight, the planet could have responded more quickly to the carbon dioxide gas being belched out by the world's volcanoes. Given this, a slushball should have only lasted around a million or so years.

It's possible to test this by using an exotic element only found in abundance within meteorites and deep in the Earth: iridium. Bernd Bodiselitsch and colleagues of the University of Vienna looked at several cores of sedimentary rock spanning the end of the Marinoan Snowball Earth. They reasoned that if iridium is constantly being showered onto the Earth's surface as meteorites burn up in the upper atmosphere, most of this element should have been locked up in the snow and ice of a Snowball Earth. With warming, however, the snow and ice would melt, flushing all the iridium on the surface into the oceans to become locked up in the sediments as a distinct layer. The longer the icy conditions lasted, the more iridium should be present in the spike. By measuring the amount of iridium through the cores, Bodiselitsch's team found a huge spike of iridum within the sediments at the end of the ice age. Other elements within the level of the spike showed that meteorites were the most likely source of the iridium; volcanic eruptions have a distinctly different suite of elements. Using the known rate of meteorite strikes on Earth over the last 80 million years, the amount of iridium falling through from the sky could be used to calculate how long the ice had covered the surface. The iridium in the spikes suggests that the Marinoan lasted around 12 million years; far too long for a slushball. It looks like snowballs ruled.

We know that there were at least three ice ages through the Neoproterozoic, suggesting that the climate went from extreme warmth to extreme cold and back again. The most likely explanation is that once much or all of the snow and ice had melted, the high albedo brought on by the low latitudes of the continents would have started to reflect the sunlight off the planet. The high levels of erosion caused by the presence of continents in the tropics would have helped bring the carbon dioxide levels back down from their mega levels. All these factors would have conspired to kickstart yet another Snowball Earth. The Earth only seems to have got off this extreme climate helter skelter when Rodinia broke up and some of the continents migrated to higher latitudes, calming everything down.

The conditions during Snowball Earth show just how far our planet can go when feedbacks start to kick in. Fortunately, a return to these extreme cycles seems pretty unlikely in the near future; the continents are unlikely to start converging on the tropics any day soon. The critical thing is that the combination of greenhouse gases and albedo play a major role in controlling the temperature of our planet. As we've just seen, albedo can have a massive effect on the amount of energy our planet reflects and absorbs from the Sun, while greenhouse gases played a pivotal role in ramping up the temperature enough to break the icy deadlock of a Snowball Earth. It's a sobering thought that although carbon dioxide is naturally stripped from the air by the weathering of rocks, it's not likely to help us much today; it's been estimated that this process would take around 80,000 years to get the amount of carbon dioxide in the air down from 500 to 400 ppm. But could these sorts of feedbacks conspire against us in the future? To answer this we need to start looking at the more recent past.

Chapter 3

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