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Snowball Earth
BBC2 9.00pm Thursday 22nd February 2001

Paul Hoffman has found clues in Namibia to Earth's frozen past NARRATOR (DILLY BARLOW): There is a theory that scientists once dismissed as absurd - that long ago the Earth became so cold every inch of it was entombed in ice. The whole planet became one vast snowball and it lasted for ten million years.

PROF. JOSEPH KIRSCHVINK (California Institute of Technology): The image of a frozen, white Earth was probably the worst nightmare I've personally ever come across. Death to an entire planet.

NARRATOR: For years the snowball theory has been dismissed as impossible. The critics said if the snowball had struck all life would have died and we would not exist.

PROF. GUY NARBONNE (Queen's University, Kingston, Ontario): The problem with the snowball is that this should have been the greatest environmental calamity of all time and yet we can't find the bodies.

NARRATOR: But a few defiant scientists have adopted the snowball theory with religious ferocity and if they can prove it, they may have found something to stun the world, a theory that may explain the origins of complex life on our planet. Earth has no more terrible force than an ice age. Only five times in its four billion year existence has our planet suffered this kind of catastrophe. No one is certain what started them or why they happened, but there is one thing about ice ages on which most scientists agree. The conventional view is that no matter how ferocious the advance of the cold in any ice age there will always be parts of the world that are safe and warm. The freeze spreads out slowly from the poles, but never reaches the hottest part of the planet, the tropics. It has long been assumed that this is a basic law of nature. The tropics can never freeze. But then this basic law of nature can't explain what happened 600 million years ago in Namibia. Paul Hoffman is one of those defiant geologists who disputes the conventional view. He has spent years trying to convince his colleagues about the snowball Earth.

PROF. PAUL HOFFMAN (Harvard University): As a scientist I was completely convinced it was right, but I knew I was going to have a struggle on my hands because any idea as radical as, as this is going to be difficult to get across and there are going to be sceptics for sure.

NARRATOR: The place Hoffman and his colleague, Dan Schrag, come back to study again and again is Namibia. It is not an obvious place in which to seek evidence of a catastrophic ice age. It is a land of relentless heat, one of those locations that has been in the ropics for hundreds of millions of years. The conventional laws of nature say an ice age could never have penetrated here, but lurking among the sun-baked rocks is something that the conventional laws of nature cannot explain away. 600 million years ago these hills were at the bottom of the ocean and lying on the old seabed are mysterious boulders. Geologists call them drop stones and drop stones could only have been brought here by one thing: a glacier. To recreate drop stones in the lab first you need ice.

PROF. DANIEL SCHRAG (Harvard University): This is going to take a while.

NARRATOR: The ice needs to float on shallow water above a sandy sea floor. This is to simulate a glacier as it flows off the land onto the surface of the sea. Because it has carved through the landscape a glacier always collects a huge amount of rock debris.

DANIEL SCHRAG: The ice would then get filled up with all sorts of boulders and cobbles and in this case we're just using some gravel which we can form on top of this layer of ice in our tank here.

NARRATOR: Having recreated a boulder-filled glacier, you need to melt the ice. As the ice melts the boulders are released.

DANIEL SCHRAG: When most of the ice has melted is a point you can see that all the rocks the glacier's carried out to the ocean are now dropped to the sea floor. In addition to all of this, in the real world there'd also be a lot of sand and a lot of much finer grain sediment and so these rocks would get filled in with very fine grain sediment as well.

NARRATOR: In a few thousand years the fine grain sediment will cement itself together to form rocks, with the boulders trapped inside and that's how drop stones are made. All you need is a glacier. The message from the drop stones suggests just one thing: Namibia in the tropics, a place that everyone says has been hot for hundreds of millions of years, must once have been covered in ice.

PAUL HOFFMAN: The most compelling evidence here is the juxtaposition of a large boulder like this enclosed within very fine and delicately layered sediment and so it's difficult, if not impossible, to imagine any way to account for this other than a very extensive glaciation.

NARRATOR: But for 50 years, since the idea that an ice age must have smothered the tropics was first proposed, it has been deemed so radical that most scientists have dismissed it as absurd. The theory has its roots in the 1940s when a young geologist called Brian Harland made a series of intrepid field trips deep into the Arctic.

BRIAN HARLAND (Cambridge University): Conditions were very different then - manhauling in the old Scots tradition. It was cold but you get used to it, you can get used to anything really.

NARRATOR: In the Arctic Harland had ample opportunity to learn all there was to learn about that telltale sign of glaciation - drop stones.

BRIAN HARLAND: Glacial deposits were very dramatic, currents very distinctive, easy to identify and when I saw them I knew from my own experience that this is glacial. I didn't have to think very hard about it.

NARRATOR: But it was when he returned from the Arctic, armed with this new-found knowledge, that Harland made his scientific breakthrough. With his now expert eye, he analysed rocks from all over the world. What he discovered amazed him. Almost everywhere he looked, in rocks formed about 600 million years ago, he found drop stones: classic evidence that there had once been thick ice even on the hottest continents.

BRIAN HARLAND: There's no continent really without some glacial record. You have them in different parts of Europe, in Africa, plenty in Africa, not so well known in South America, but they have been known later in North America.

NARRATOR: The drop stones were so widespread that Harland began to contemplate the unthinkable.

BRIAN HARLAND: The fact is we've got evidence of a global glaciation.

NARRATOR: He knew it made no sense. He knew that at the peak of the last Ice Age, which ended just 10,000 years ago, ice spread down to where New York is today, but no further, but the evidence was staring Harland in the face, evidence which seemed to flout the laws of nature. 600 million years ago the drop stones said ice had spread over every continent, right across the tropics to the Equator itself. It meant there must have been an ice age of unimaginable ferocity, in fact too unimaginable. When Harland presented his theory of global glaciation colleagues dismissed it as laughable.

BRIAN HARLAND: There was a lot of scepticism. I don't think I was ever shaken by it because I was pretty confident in my own observations, but I was interested and a bit annoyed in a way that, that so many people were hostile to the idea.

NARRATOR: The sceptics had a whole host of reasons why Harland had to be wrong, reasons that were utterly convincing, as they were based on the most basic rules about how the world works. For a start, it's given that the tropics, that wide belt around the Earth, has always received the full glare of the sun. As they have always basked in sunlight, so they must always have remained warm. Even during the last Ice Age geological evidence suggests the tropics were pleasantly hot and they had another reason why the snowball theory must be wrong. There was a far simpler explanation for drop stones in the tropics, one that was absolutely consistent with scientific knowledge and which didn't involve a ludicrous idea like the tropics freezing over. Continents drift. They scrape across the surface of the Earth at about the same pace that your fingernails grow. This means that in about 400 million years continents can shift from one end of the globe to the other. Obviously, the doubters said, 600 million years ago all the continents had drifted into icy, polar regions. Once there, glaciers would have formed and drop stones would be common in all rocks of this era, and after those drop stones had been formed at the poles the land, and the drop stones, would have been carried back down to the tropics by continental drift. For years this remained the accepted explanation for why drop stones appeared everywhere. The only way to make the theory of a global glaciation credible was for someone to make the impossible possible. They would need to find a way for the tropics to freeze over, and this is just what happened next. (RUSSIAN COUNTDOWN) It was the 1960s, the height of the Cold War. The world was obsessed with calculating the odds of surviving a nuclear Holocaust. It was known that a series of massive nuclear explosions would create clouds of dust, smoke and soot. Sunlight would be blocked out. Hypothetically the Earth would enter a nuclear winter, a man-made ice age. In the Soviet Union finding out how severe this man-made ice age could be became the task for one climatologist. Mikhail Budyko was that man.

PROF. MIKHAIL BUDYKO (State Hydrological Institute, St. Petersburg): Long ago, probably 25 or 30 years, I compiled a number of studies which could be used to describe origin of ice ages.

NARRATOR: What Budyko was to uncover would fly in the face of conventional wisdom. He would show how those predictions that the tropics couldn't freeze over were complacent and unfounded. Budyko knew that because the land and oceans are dark they absorb most of the heat coming from the Sun's rays and that is how our planet is warmed up; but sheets of ice are white. They reflect sunlight like a mirror, so an ice-covered Earth absorbs far less solar heat. During an ice age as the freeze spreads the Earth grows whiter, more heat is reflected away so less and less heat is absorbed and so the Earth grows ever colder. It means that potentially the Earth could be caught in a vicious circle of unstoppable freezing. Budyko converted this hypothetical scenario into a mathematical formula and that formula produced a terrifying prediction: the Earth's climate has a theoretical breaking point. As long as the ice sheets remain close to the poles the Earth is safe, but if the freeze continues, such as might happen in a nuclear winter, they could advance down to about where Texas is today. Once the freeze had reached that point so much of the Earth would be covered in white ice that over half the solar heat that normally warms the planet would be reflected back into space. At that point there wouldn't be enough heat left to warm up the Earth. Once this happens there could, in theory, be a runaway freeze, a freeze that nothing can stop. Temperatures plummet, ice sheets spread across all the continents, the oceans, and eventually even the tropics. If this was ever to happen the entire planet would be trapped in ice. There would be a snowball Earth. What was most disturbing about Budyko's calculations was that an Earth encased in ice would reflect so much solar heat it could never warm up enough to thaw, ever. A snowball Earth would mean a world entombed in ice for eternity.

MIKHAIL BUDYKO: It was my opinion twenty years ago that such a system will be stable for very long time and possibly all life will disappear.

NARRATOR: This was Budyko's paradox. He had shown that a runaway freeze could ice over the tropics in theory, but he had also shown that it could never have happened in practice because, if it had, the world would still be a snowball today and that would mean we would never have existed. For 15 years nobody could solve the paradox of the runaway freeze. The snowball Earth theory remained a logical impossibility. To resurrect the theory believers would have to do two things. First, they had to prove that ice really had reached the hottest parts of the planet, the tropics, and that the drop stones hadn't arrived there merely through continental drift. Second, they had to come up with a theory that offered an escape from the runaway freeze. That was when a new scientist entered the picture. In the late 70s Joe Kirschvink was one of geology's rising stars and back then he thought the idea of a snowball Earth was mad.

JOSEPH KIRSCHVINK: The willingness to accept a nutty idea like freezing the whole planet really needs to be supported by incredibly strong evidence and overall if you had asked me to place a wager back in the late 70s I would have been on the conservative side on that one.

NARRATOR: Kirschvink's conservatism came about because he believed the simple, conventional explanation for why there were drop stones at the Equator today. 600 million years ago, he thought, all the continents must have been up near the icy poles when the drop stones were formed. There was, however, only one technique for testing their theory and it just so happened that Kirschvink had dedicated his entire professional career to that technique. It involved magnets and Kirschvink is a man in love with magnets.

JOSEPH KIRSCHVINK: I have wiped out my credit cards so many times by playing with strong magnets I, I don't like to remember it.

NARRATOR: Few people realise it, but every rock is a weak magnet. Each one contains magnetic minerals and the direction these minerals pull in is fixed for ever the moment the rock is first formed and what gives these minerals their magnetic direction is the magnetic field round the Earth's core. This means that rocks formed at the poles will always have a magnetic direction that points up and down. Rocks formed at the Equator will always have a magnetic direction that points from side to side.

JOSEPH KIRSCHVINK: Rocks have the ability to preserve magnetic direction because of magnetic minerals in them. They act like little compass needles and will line themselves up.

NARRATOR: By finding out a rock's magnetic direction Kirschvink is able to identify where in the world it was originally formed, no matter where continental drift might have taken it in the millions of years that followed. In the 1980s Kirschvink heard of a team of Australians who had run tests which, they claimed, showed evidence of glaciers in the tropics. Ever the sceptic, Kirschvink decided to duplicate their tests on his own, more sophisticated equipment. It meant examining drop stones from some of the hottest parts of the Earth. The question was: were they already at the Equator when they were formed by the ice, or had they been created at the poles and then carried to the tropics by continental drift? To answer the question Kirschvink turned to his pride and joy: a custom built, hypersensitive magnetometer.

JOSEPH KIRSCHVINK: It's, it's an incredible instrument. You lower the sample into the actual magnetometer chamber. The weak magnetic field of the sample generates an electric current and so it allows us to measure the magnetic moment of an extraordinarily weak magnet, one billionth, say, the level of an ordinary hand magnet I think.

NARRATOR: Attached to the magnetometer was a computer which plotted each rock's origin on an electronic outline of Earth as seen from above. The centre of the circle represented the poles, the circumference the Equator. If the drop stones had been formed at the poles, as Kirschvink and everyone else thought, they would show up at the centre of the circle on the computer. If they had been formed at the Equator they would show up at the circle's edge. The machine gave its answer: the drop stones had been formed in the tropics. 600 million years ago there must have been a snowball Earth. In the late 80s Kirschvink adopted the snowball Earth theory with fanaticism and if his findings flew in the face of existing theories about the laws of nature then stuff those theories.

JOSEPH KIRSCHVINK: There's a fundamental theorem in science: if the theory is in conflict with data modify the theory. There's so many wonderful, beautiful theories that have been slaughtered by a nasty little fact.

NARRATOR: But Kirschvink was racked by the insurmountable paradox of the runaway freeze, that if a snowball Earth had ever happened then science said we should still be entombed in ice today.

JOSEPH KIRSCHVINK: How do you get out of it? Obviously the climate modellers had assumed that that was an irreversible step and that you would never get out of it and yet we're out of it now and if we had been in it before some point we must have gotten out of it.

NARRATOR: To get out of the deep freeze what Kirschvink needed was a power that would stay hot, even when the whole planet had frozen over, something that Budyko hadn't thought of, something that could burn for ever, something like hell.

JOSEPH KIRSCHVINK: Looking at an active volcano you realise that magma tens or hundreds of kilometres below the surface couldn't care less whether there was a thin layer of ice over the oceans. It will still emerge.

NARRATOR: Volcanoes survive ice ages because they have a direct channel to the molten rock deep within the Earth, rock that reaches temperatures of over 1,000 degrees, but that would only melt ice in their immediate area. Kirschvink had spotted something else about volcanoes: they also produce gas, ten billion tons a year. One gas volcanoes emit in huge quantities is carbon dioxide, a gas that causes the greenhouse effect and global warming. Today carbon dioxide is being pumped into the atmosphere by both volcanoes and industrial activity, but what stops the Earth from overheating is that we have a natural way of removing the excess carbon dioxide from the atmosphere. Rain is the Earth's natural cleaning agent. As it falls through the atmosphere each droplet of rain absorbs carbon dioxide and cleans the air, but Kirschvink realised that on a snowball Earth there could have been no rain. The snowball was so cold all the water on the planet's surface was frozen solid. Without liquid water nothing could have evaporated into the air, so there would have been no clouds and without clouds there can be no rain and without rain, Kirschvink realised, there would have been nothing to cleanse the atmosphere of carbon dioxide.

JOSEPH KIRSCHVINK: You can't have rain if you don't have evaporation, so I couldn't see anything that would scrub out the carbon dioxide from the atmosphere under those conditions.

NARRATOR: It meant there would have been nothing to stop the carbon dioxide from the volcanoes from building up over millions of years. It would have caused global warming on an inconceivable scale. Kirschvink came across calculations showing that after ten million years without rain the atmosphere would have been 10% carbon dioxide. Today it is far less than 1%. This extra carbon dioxide would have created a greenhouse effect that raised the temperature to an average of 50 degrees Centigrade, hotter than the Earth has ever been, hot enough to melt the ice.

JOSEPH KIRSCHVINK: And that seemed to be a natural and possible escape, certainly enough to break the snowball, the ice condition.

NARRATOR: Joe Kirschvink had cracked it. He had found the way out of the runaway freeze, a way that made perfect scientific sense, a way that was consistent with the laws of nature.

JOSEPH KIRSCHVINK: The realisation that we may have found the way out of the snowball was wonderful.

NARRATOR: By 1990 Kirschvink had evidence that the tropics had frozen over for ten million years and he'd come up with a theoretical escape route from the runaway freeze, but the problem was it was just a theory. He had no physical evidence to prove the ice had melted because of an extreme greenhouse effect and without that evidence the snowball Earth theory continued to be ignored. But all that was about to change. In 1992 Paul Hoffman entered the story. One of geology's most respected, but open-minded figures, he was to turn into one of the snowball Earth theory's most fervent disciples.

PAUL HOFFMAN: I love the snowball Earth theory. I mean it's almost a religious ferocity with which I want to get across this, this saga of Earth history with so many twists and turns and, and events that, that are, are, are so far outside the, our own experience.

NARRATOR: Hoffman's mission was to find that hard evidence that had alluded Kirschvink, the final piece of the puzzle that would prove the snowball had ended because of the greenhouse effect and he knew just where to start looking: Namibia. Hoffman was drawn to Namibia first by the drop stones, but even more by what sat immediately on top of them. Towering directly above were huge formations called cap carbonates, calcium carbonate crystallised into rock. These bizarre structures seemed to appear right out of the blue.

PAUL HOFFMAN: Above this line right here... creates... on top no more stones whatsoever, so the glaciation must have come to an abrupt end and sitting directly on top is a thick pile of carbonate and it's puzzled geologists for generations.

NARRATOR: What really puzzled geologists was that caps made from calcium carbonate are usually formed in warm water, but in Namibia they had suddenly appeared on top of an ice-cold glacier.

PAUL HOFFMAN: What this indicates is that you go from glacial conditions to an ocean that is warm in a flash.

NARRATOR: To most geologists this instant change from cold to hot rock defied their understanding, but Hoffman suspected it might be a huge clue, that the sudden appearance of mountains of calcium carbonate must somehow be linked to the death of the snowball in a colossal greenhouse effect, but how? Hoffman needed someone who could explain why calcium carbonate would be formed not just because of warm conditions in general, but specifically in the conditions created by a melting snowball Earth, so he turned to a geochemist, Dan Schrag.

PAUL HOFFMAN: I talked about this problem with a number of people but the one person who grabbed this problem with both hands right from the outset was my colleague, Dan Schrag.

NARRATOR: Painstakingly they analysed the snowball's theoretical final moments stage by stage, working out what chemical and climatic processes were at work. First they realised that as the ice melted it would produce liquid water. The water would then evaporate and create clouds and those clouds would cause a change in the weather the likes of which has never been seen by human eyes.

DANIEL SCHRAG: When the ice is retreating this is probably the most spectacular climate change in our history. In some ways we're going from the coldest climate the Earth's ever experienced to the warmest climate the Earth's ever experienced. We immediately said that this was the mother of our climate changes.

NARRATOR: The most elemental powers of nature would be unleashed upon the Earth.

PAUL HOFFMAN: One would predict hurricanes of such intensity that our unimaginable, so it's quite possibly you could have had waves of, you know, 100 metres in, in, in amplitude coming in and crashing in on the shoreline, so it would just be unbelievably violent.

NARRATOR: And above all there would be rain, the first rain storms for millions of years, rainstorms that would last at least a century and it wouldn't be just any rain. The rainwater would react with the vast quantities of carbon dioxide in the atmosphere and form carbonic acid. This acid rain would then deluge the Earth. The acid rain would pound the exposed rocks, producing a violent chemical reaction. It would break the rocks down into their consistuent parts, one of which was calcium. This would then fuse with the carbon in the acid rain. The result: mountains of calcium carbonate exactly as they could see right above the drop stones in Namibia.

DANIEL SCHRAG: Suddenly it became clear that the natural expectation of a prolonged global glaciation ending in extremely high levels of carbon dioxide was that you would expect these very unusual thick carbonate rocks should immediately follow the glacial deposits.

NARRATOR: Hoffman and Schrag had found the final evidence.

PAUL HOFFMAN: The beauty of the snowball Earth theory is exactly that it links the chemistry, the geology, the planetary science and bam-bam-bam, all of the facts are, are consistent with the predictions of the theory.

NARRATOR: In 1998 Hoffman and Schrag made a triumphal tour of the world's universities promoting the snowball theory, and then someone threw a spanner in the works. An entire scientific discipline, the biologists, took one look at the ten million year long snowball Earth and said it was quite simply impossible. One scientist who had problems with the snowball was Guy Narbonne. He, like many other biologists, could see there was a fundamental problem reconciling the snowball Earth theory with what he knew.

GUY NARBONNE: The geological evidence speaks very strongly of ice sheets marching on the Equator. The biological evidence speaks very strongly of open water being available at this whole time.

NARRATOR: Narbonne's problem was another set of nature's laws, nothing less than the laws that govern life itself. 600 million years ago, when the snowball was said to have struck, there was nothing living on the land. It all existed in the sea, and it was not life as we know it. The most common organism on the planet was this: cyano-bacteria. This mass of fossilised cyano-bacteria is so ancient it predates even the snowball.

GUY NARBONNE The oldest fossils that we have in the world are stromatalytes and these are colonies like you see here and they're made out of literally billions of cyano-bacteria.

NARRATOR: What made cyano-bacteria and more sophisticated ancient organisms like green algae central to the snowball debate was that they photosynthesised. Just like plants today under the water these life forms absorbed sunlight and converted it into energy, so these species' chances of surviving the snowball would have depended on whether there was enough sunlight for photosynthesis in the snowball seas at a time when every inch of the planet was allegedly covered in thick ice.

GUY NARBONNE: Light penetration through ice is not great if the ice is more than a couple of tens of metres thick.

NARRATOR: And that was the problem biologists pointed to in Hoffman and Schrag's work. Their calculations had reckoned on the snowball producing ice sheets up to a kilometre thick and even in the warmest places around the Equator they would have been tens of metres thick, more than enough, biologists believe, to block out any sunlight from the oceans and that meant death to most things that depended on photosynthesis for life.

GUY NARBONNE: You cut off photosynthesis and within a few years, let alone a few million years, mass extinction.

NARRATOR: It meant that many of the photosynthesising species should have become extinct 600 million years ago and the trouble was they hadn't. Cyano-bacteria grow in rock-like colonies all over the tropics today, as do other photosynthetic plants dating from before the snowball like green algae. For many biologists there seemed to be only one possible conclusion: there may have been a severe ice age, but not a fully fledged snowball that covered all the oceans because life forms that should have died out are still with us. Unless someone could find a way for marine life to survive under a thick sheet of ice the whole theory would be back in the bin. Then along came the man from NASA, Chris McKay.

DR. CHRIS McKAY (Space Science Division, NASA): I'm a planetary scientist with an interest in life in extreme environments, in particular environments that are cold and dry. Several years ago I saw the papers about the snowball Earth indicating the geological evidence that the Earth was ice-covered and had been ice-covered for millions of years, plus the evidence that there were organisms, bacteria and green algae, that survived this ice-covered state and that's the puzzle: how could these photosynthetic organisms survive?

NARRATOR: McKay was intrigued because it just so happened he had visited a place on Earth today that was so cold it could actually be compared with the snowball of 600 million years ago: Antarctica, the coldest place on Earth. Here temperatures can fall to -30%ordm;C, almost as severe as the snowball at its warmest point (the Equator) and deep in Antarctica are the dry valleys where there are lakes smothered by an ice sheet many metres thick.

CHRIS McKAY: We think these little lakes represent a good model for looking at the snowball Earth - could there by life underneath the ice cover on a snowball Earth?

NARRATOR: Scientists were eager to discover if there was enough light for life to survive in the water below the thick ice. They were about to be astonished. There was light, lots of it, even though overhead the ice was five metres thick. There was so much light the water was filled with plant life and bacteria including two organisms dating from the time of the snowball that would have needed sunlight to live.

CHRIS McKAY: When we're diving in the lakes and swimming around there's life in the water, there's bacteria, cyano-bacteria, but there's also green algae.

NARRATOR: And McKay had a hunch as to why this photosynthetic life could survive. It was something about the nature of the ice itself. It was like glass. It didn't block out sunlight at all.

CHRIS McKAY: The transparency of this ice is very high and it's due to the fact that it's freezing very slowly underneath this thick ice cover. Now let's imagine the snowball Earth where there's also a thick ice cover. The ice on the bottom is freezing very slowly as well, yet you have the same optical properties as this clear, clean ice.

NARRATOR: By freezing slowly, water rejects impurities such as salt or dirt which make it cloudy, so the ice becomes clean and transparent. Because of this transparency life-giving sunlight would have been reaching down into the snowball seas. McKay realised that even at its height there would have been havens around the snowball's Equator where the ice was thin enough for photosynthetic life to cling on below. But there was more. Although disciples of the theory accept the snowball would have been death to most things, they argue whatever life did survive would have emerged into a new world almost devoid of competition, the perfect conditions for an explosion of evolutionary change. These fossils show the new species which appeared soon after the time of the snowball. All display a massive increase in size and complexity from the simple organisms that lived before. These are the first multi-cellular life forms. Even those with some questions about the snowball theory recognise a new climate after a mass extinction could be a spur for evolution.

GUY NARBONNE: All we can say for sure is that the complexity of life dramatically increases after the snowball glaciations. What we can infer is that the greatest environmental catastrophe to ever strike the Earth is probably in some way related to the greatest evolutionary explosions that have ever characterised the Earth.

NARRATOR: It may just be that the snowball saga was the biggest force for the development of life the world has ever seen. But if the snowball can change the course of life on the planet there remains one final question. No one knows why the snowball happened or if it could happen again. The sun is hotter than it was 600 million years ago and is getting hotter all the time. Climate trends would have to change dramatically. If it ever did happen again it almost certainly wouldn't be for millions of years, but if it ever did that would mean death to almost everything, including us.

Back to 'Snowball Earth' programme page

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