WEBVTT 99:59:59.999 --> 99:59:59.999 Narrator: There is a theory that scientists once dismissed as absurd: 99:59:59.999 --> 99:59:59.999 that long ago the earth became so cold, every inch of it was entombed in ice. 99:59:59.999 --> 99:59:59.999 The whole planet became one vast snowball, 99:59:59.999 --> 99:59:59.999 and it lasted for 10 million years. 99:59:59.999 --> 99:59:59.999 [Theme Music] 99:59:59.999 --> 99:59:59.999 Earth has no more terrible force than an ice age. 99:59:59.999 --> 99:59:59.999 Only 5 times in its 4 billion year existance 99:59:59.999 --> 99:59:59.999 has our planet suffered this kind of catastrophe. 99:59:59.999 --> 99:59:59.999 No one is certain what started them, or why they happened, 99:59:59.999 --> 99:59:59.999 but there is one thing about ice ages on which most scientists agree: 99:59:59.999 --> 99:59:59.999 the conventional view is that no matter how ferocious the advance of the cold in any ice age, 99:59:59.999 --> 99:59:59.999 there will always be parts of the world that are safe and warm. 99:59:59.999 --> 99:59:59.999 The freeze spreads out slowly from the poles, 99:59:59.999 --> 99:59:59.999 but never reaches the hottest part of the planet, the tropics. 99:59:59.999 --> 99:59:59.999 It has long been assumed that this is a basic law of nature. 99:59:59.999 --> 99:59:59.999 The tropics can never freeze. 99:59:59.999 --> 99:59:59.999 But then, this basic law of nature can't explain what happened 600 million years ago in Namibia. 99:59:59.999 --> 99:59:59.999 [Jazz Music] 99:59:59.999 --> 99:59:59.999 Paul Hoffman is one of those defiant geologists who disputes the conventional view. 99:59:59.999 --> 99:59:59.999 He has spent years trying to convince his colleagues about the snowball earth. 99:59:59.999 --> 99:59:59.999 As a scientist, I was completely convinced it was right. 99:59:59.999 --> 99:59:59.999 But I knew I was going to have a struggle on my hands. 99:59:59.999 --> 99:59:59.999 Because any idea as radical as this is going to be difficult to get across, 99:59:59.999 --> 99:59:59.999 and there are going to be skeptics for sure. 99:59:59.999 --> 99:59:59.999 Narrator: The place Hoffman and his colleague, Dan Schrag, 99:59:59.999 --> 99:59:59.999 come back to study again and again is Namibia. 99:59:59.999 --> 99:59:59.999 It is not an obvious place in which to seek evidence of a catastrophic ice age, 99:59:59.999 --> 99:59:59.999 it is a land of relentless heat. 99:59:59.999 --> 99:59:59.999 One of those locations that has been in the tropics for hundreds of millions of years. 99:59:59.999 --> 99:59:59.999 The conventional laws of nature say an ice age could never have penetrated here. 99:59:59.999 --> 99:59:59.999 But lurking among these sun-baked rocks is something that the conventional laws of nature 99:59:59.999 --> 99:59:59.999 cannot explain away. 99:59:59.999 --> 99:59:59.999 600 million years ago, these hills were at the bottom of the ocean, 99:59:59.999 --> 99:59:59.999 and lying on the old sea bed are mysterious boulders. 99:59:59.999 --> 99:59:59.999 [Eerie Music] 99:59:59.999 --> 99:59:59.999 Geologists call them drop stones, 99:59:59.999 --> 99:59:59.999 and drop stones could only have been brought here by one thing: a glacier. 99:59:59.999 --> 99:59:59.999 To recreate drop stones in the lab, first you need ice. 99:59:59.999 --> 99:59:59.999 This is going to take a while. 99:59:59.999 --> 99:59:59.999 Narrator: The ice needs to float on shallow water above a sandy sea floor. 99:59:59.999 --> 99:59:59.999 This is to simulate a glacier as it flows off the land, onto the surface of the sea. 99:59:59.999 --> 99:59:59.999 Because it is carved through the landscape, a glacier always collects a huge amount of rock debris. 99:59:59.999 --> 99:59:59.999 The ice would then get filled up with all sorts of boulders and cobbles, 99:59:59.999 --> 99:59:59.999 and in this case, we're just using some gravel, 99:59:59.999 --> 99:59:59.999 which we can pour on top of this layer of ice in our tank here. 99:59:59.999 --> 99:59:59.999 Narrator: Having recreated a boulder-filled glacier, you need to melt the ice. 99:59:59.999 --> 99:59:59.999 [Crashing sound] 99:59:59.999 --> 99:59:59.999 As the ice melts, the boulders are released. 99:59:59.999 --> 99:59:59.999 While most of the ice has melted at this point, 99:59:59.999 --> 99:59:59.999 you can see that all the rocks that the glaciers carried out to the ocean 99:59:59.999 --> 99:59:59.999 are now dropped to the sea floor. 99:59:59.999 --> 99:59:59.999 In addition to all of this--in the real world--boulders, 99:59:59.999 --> 99:59:59.999 there would also be a lot of sand, and a lot of much finer gained sediment. 99:59:59.999 --> 99:59:59.999 And so these rocks would get filled in with very fine-grained sediment as well. 99:59:59.999 --> 99:59:59.999 Narrator: In a few thousand years, the fine-grained sediment will cement itself together to form rock. 99:59:59.999 --> 99:59:59.999 With the boulders trapped inside. 99:59:59.999 --> 99:59:59.999 And that's how drop stones are made. 99:59:59.999 --> 99:59:59.999 All you need is a glacier. 99:59:59.999 --> 99:59:59.999 The message from the drop stones suggests just one thing: 99:59:59.999 --> 99:59:59.999 Namibia, in the tropics, a place that everyone says has been hot for hundreds of millions of years, 99:59:59.999 --> 99:59:59.999 must once have been covered in ice. 99:59:59.999 --> 99:59:59.999 The most compelling evidence here is the juxtaposition of a large boulder like this 99:59:59.999 --> 99:59:59.999 enclosed within very fine and delicately layered sediment. 99:59:59.999 --> 99:59:59.999 And so it's difficult, if not impossible to imagine any way to account for this 99:59:59.999 --> 99:59:59.999 other than a very extensive glaciation. 99:59:59.999 --> 99:59:59.999 But for 50 years, since the idea that an ice age must have smothered the tropics was first proposed, 99:59:59.999 --> 99:59:59.999 it has been deemed so radical that most scientists have dismissed it as absurd. 99:59:59.999 --> 99:59:59.999 The theory has its roots in the 1940s, when a young geologist called Brian Harland, 99:59:59.999 --> 99:59:59.999 made a series of intrepid field trips deep into the arctic. 99:59:59.999 --> 99:59:59.999 Conditions were very different then. 99:59:59.999 --> 99:59:59.999 Man hauling in the old Scotch tradition. 99:59:59.999 --> 99:59:59.999 It was cold, but, um, you get used to it. 99:59:59.999 --> 99:59:59.999 You can get used to anything, really. 99:59:59.999 --> 99:59:59.999 Narrator: In the arctic, Harland had ample opportunity to learn all there was to learn 99:59:59.999 --> 99:59:59.999 about that tell-tale sign of glaciation, drop stones. 99:59:59.999 --> 99:59:59.999 Glacier deposits were a very dramatic occurrence, very distinctive, easy to identify. 99:59:59.999 --> 99:59:59.999 And when I saw them, I knew from my own experience that this is glacial. 99:59:59.999 --> 99:59:59.999 I didn't have to think very hard about it. 99:59:59.999 --> 99:59:59.999 Narrator: But it was when he returned from the arctic, armed with his new-found knowledge, 99:59:59.999 --> 99:59:59.999 that Harland made his scientific breakthrough. 99:59:59.999 --> 99:59:59.999 With his now expert eye, he analyzed rocks from all over the world. 99:59:59.999 --> 99:59:59.999 What he discovered amazed him. 99:59:59.999 --> 99:59:59.999 Almost everywhere he looked, in rocks formed about 600 million years ago, he found drop stones. 99:59:59.999 --> 99:59:59.999 Classic evidence that there had once been thick ice, even on the hottest continents. 99:59:59.999 --> 99:59:59.999 The drop stones were so wide-spread that Harland began to contemplate the unthinkable. 99:59:59.999 --> 99:59:59.999 The fact is that we've got evidence of a global glaciation. 99:59:59.999 --> 99:59:59.999 Narrator: He knew it made no sense. 99:59:59.999 --> 99:59:59.999 He knew that at the peak of the last ice age, which ended just 10,000 years ago, 99:59:59.999 --> 99:59:59.999 ice spread down to where New York is today, but no further. 99:59:59.999 --> 99:59:59.999 But the evidence was staring Harland in the face. 99:59:59.999 --> 99:59:59.999 Evidence which seemed to flout the laws of nature. 99:59:59.999 --> 99:59:59.999 600 million years ago, the drop stones said, ice had spread over every continent. 99:59:59.999 --> 99:59:59.999 Right across the tropics, to the equator itself. 99:59:59.999 --> 99:59:59.999 It meant there must have been an ice age of unimaginable ferocity. 99:59:59.999 --> 99:59:59.999 In fact, too unimaginable. 99:59:59.999 --> 99:59:59.999 When Harland presented his theory of global glaciation, colleagues dismissed it as laughable. 99:59:59.999 --> 99:59:59.999 The skeptics had a whole host of reasons why Harland had to be wrong. 99:59:59.999 --> 99:59:59.999 Reasons that were utterly convincing, 99:59:59.999 --> 99:59:59.999 as they were based on the most basic rules about how the world works. 99:59:59.999 --> 99:59:59.999 For a start, it's a given that the tropics, that wide belt around the earth, 99:59:59.999 --> 99:59:59.999 has always received the full glare of the sun. 99:59:59.999 --> 99:59:59.999 As they have always basked in sunlight, so they must always have remained warm. 99:59:59.999 --> 99:59:59.999 Even during the last ice age, geological evidence suggests the tropics were pleasantly hot. 99:59:59.999 --> 99:59:59.999 And they had another reason why the snowball theory must be wrong. 99:59:59.999 --> 99:59:59.999 There was a far simpler explanation for drop stones in the tropics, 99:59:59.999 --> 99:59:59.999 one that was absolutely consistent with scientific knowledge, 99:59:59.999 --> 99:59:59.999 and which didn't involve a ludicrous idea like the tropics freezing over. 99:59:59.999 --> 99:59:59.999 Continents drift. 99:59:59.999 --> 99:59:59.999 They scrape across the surface of the earth at about the same pace that your fingernails grow. 99:59:59.999 --> 99:59:59.999 This means that in about 400 million years, continents can shift from one end of the globe to the other. 99:59:59.999 --> 99:59:59.999 Obviously, the doubters said, 600 million years ago all the continents had drifted into icy polar regions. 99:59:59.999 --> 99:59:59.999 Once there, glaciers would have formed, and drop stones would be common in all rocks of this area. 99:59:59.999 --> 99:59:59.999 And after those drop stones had been formed at the poles, 99:59:59.999 --> 99:59:59.999 the land and the drop stones would have been carried back down to the tropics by continental drift. 99:59:59.999 --> 99:59:59.999 For years, this remained the accepted explanation for why drop stones appeared everywhere. 99:59:59.999 --> 99:59:59.999 The only way to make the theory of a global glaciation credible 99:59:59.999 --> 99:59:59.999 was for someone to make the impossible possible. 99:59:59.999 --> 99:59:59.999 They would need to find a way for the tropics to freeze over. 99:59:59.999 --> 99:59:59.999 And this is just what happened next. 99:59:59.999 --> 99:59:59.999 [Counting in German] 99:59:59.999 --> 99:59:59.999 [Explosion] 99:59:59.999 --> 99:59:59.999 It was the 1960s, the height of the cold war. 99:59:59.999 --> 99:59:59.999 The world was obsessed with calculating the odds of surviving a nuclear holocaust. 99:59:59.999 --> 99:59:59.999 It was known that a series of massive nuclear explosions 99:59:59.999 --> 99:59:59.999 would create clouds of dust, smoke and soot. 99:59:59.999 --> 99:59:59.999 Sunlight would be blocked out. 99:59:59.999 --> 99:59:59.999 Hypothetically, the earth would enter a nuclear winter. 99:59:59.999 --> 99:59:59.999 A man-made ice age. 99:59:59.999 --> 99:59:59.999 In the Soviet Union, finding out how severe this man-made ice age could be 99:59:59.999 --> 99:59:59.999 became the task for one climatologist. 99:59:59.999 --> 99:59:59.999 Mikhail Budyko was that man. 99:59:59.999 --> 99:59:59.999 Long ago, probably 25 or 30 years, 99:59:59.999 --> 99:59:59.999 I compiled a number of studies which could be used to describe origin of ice ages. 99:59:59.999 --> 99:59:59.999 Narrator: What Budyko was to uncover would fly in the face of conventional wisdom. 99:59:59.999 --> 99:59:59.999 He would show how those predictions, that the tropics couldn't freeze over, 99:59:59.999 --> 99:59:59.999 were complacent and unfounded. 99:59:59.999 --> 99:59:59.999 Budyko knew that because the land and oceans are dark, 99:59:59.999 --> 99:59:59.999 they absorb most of the heat coming from our sun's rays, 99:59:59.999 --> 99:59:59.999 and that is how our planet is warmed up. 99:59:59.999 --> 99:59:59.999 But sheets of ice are white. 99:59:59.999 --> 99:59:59.999 They reflect sunlight like a mirror. 99:59:59.999 --> 99:59:59.999 So an ice-covered earth absorbs far less solar heat. 99:59:59.999 --> 99:59:59.999 During an ice age, as the freeze spreads, the earth grows whiter. 99:59:59.999 --> 99:59:59.999 More heat is reflected away, so less and less heat is absorbed. 99:59:59.999 --> 99:59:59.999 And so the earth grows ever colder. 99:59:59.999 --> 99:59:59.999 It means that potentially, the Earth could be 99:59:59.999 --> 99:59:59.999 caught in an vicious circle of unstoppable freezing. 99:59:59.999 --> 99:59:59.999 Budyko converted this hypothetical scenario into a mathematical formula, 99:59:59.999 --> 99:59:59.999 and that formula produced a terrifying prediction. 99:59:59.999 --> 99:59:59.999 The Earth's climate has a theoretical breaking point. 99:59:59.999 --> 99:59:59.999 [Dramatic Music] 99:59:59.999 --> 99:59:59.999 Narrator: As long as the ice sheets remain close to the poles, 99:59:59.999 --> 99:59:59.999 the Earth is safe but if the freeze continues, 99:59:59.999 --> 99:59:59.999 such as might happen in an nuclear winter, 99:59:59.999 --> 99:59:59.999 they could advance down to about where Texas is today. 99:59:59.999 --> 99:59:59.999 Once the freeze had reached that point, 99:59:59.999 --> 99:59:59.999 so much of the Earth would be covered in white ice 99:59:59.999 --> 99:59:59.999 but over half the solar heat that normally warms the planet 99:59:59.999 --> 99:59:59.999 would be reflected back into space. 99:59:59.999 --> 99:59:59.999 At that point, there wouldn't be enough heat left to warm up the Earth. 99:59:59.999 --> 99:59:59.999 Once this happens, there could in theory, be a runaway freeze. 99:59:59.999 --> 99:59:59.999 A freeze that nothing could stop. 99:59:59.999 --> 99:59:59.999 Temperatures plummet, ice sheets spread across all the continents, 99:59:59.999 --> 99:59:59.999 the oceans and eventually, even the tropics. 99:59:59.999 --> 99:59:59.999 If this was ever to happen, the entire planet would be trapped in ice. 99:59:59.999 --> 99:59:59.999 There would be a snowball Earth. 99:59:59.999 --> 99:59:59.999 What was most disturbing about Budyko's calculations 99:59:59.999 --> 99:59:59.999 was that an Earth encased in ice would reflect so much solar heat, 99:59:59.999 --> 99:59:59.999 it could never warm up enough to thaw, ever. 99:59:59.999 --> 99:59:59.999 It was one billion and twenty years ago that the system 99:59:59.999 --> 99:59:59.999 will be stable for a very long time and possibly, 99:59:59.999 --> 99:59:59.999 all life will disappear. 99:59:59.999 --> 99:59:59.999 Narrator: This was Budyko's paradox. 99:59:59.999 --> 99:59:59.999 He had shown that a runaway freeze could ice over the tropics in theory, 99:59:59.999 --> 99:59:59.999 but he had also shown that it could never have happened in practice 99:59:59.999 --> 99:59:59.999 because if it had, the world would still be a snowball today 99:59:59.999 --> 99:59:59.999 and that would mean we would never have existed. 99:59:59.999 --> 99:59:59.999 For 15 years, nobody could solve the paradox of the runaway freeze. 99:59:59.999 --> 99:59:59.999 The snowball Earth theory remained a logical impossibility. 99:59:59.999 --> 99:59:59.999 To resurrect the theory, believers would have to do two things. 99:59:59.999 --> 99:59:59.999 First, they had to prove that ice really had reached 99:59:59.999 --> 99:59:59.999 the hottest parts of the planet, the tropics and that the 99:59:59.999 --> 99:59:59.999 dropstones hadn't arrived there merely through continental drift. 99:59:59.999 --> 99:59:59.999 Second, they had to come up with a theory that offered 99:59:59.999 --> 99:59:59.999 an escape from the runaway freeze. 99:59:59.999 --> 99:59:59.999 [Music] 99:59:59.999 --> 99:59:59.999 That was when a new scientist had entered the picture. 99:59:59.999 --> 99:59:59.999 In the late 70s, Joe Kirschvink was one of geology's rising stars 99:59:59.999 --> 99:59:59.999 and back then, he thought the idea of a snowball Earth was mad. 99:59:59.999 --> 99:59:59.999 The willingness to accept a nutty idea like freezing the whole planet 99:59:59.999 --> 99:59:59.999 really needs to be supported by incredibly strong evidence 99:59:59.999 --> 99:59:59.999 and overall, if you had asked me to place a wager in the late 70s, 99:59:59.999 --> 99:59:59.999 I would've been on the conservative side on that one. 99:59:59.999 --> 99:59:59.999 Narrator: Kirschvink's conservatism came about because he believed 99:59:59.999 --> 99:59:59.999 the simple conventionalist explanation of why there were dropstones at the equator today. 99:59:59.999 --> 99:59:59.999 600 million years ago, he thought, all the continents must've been 99:59:59.999 --> 99:59:59.999 up near the icy poles when the dropstones were formed. 99:59:59.999 --> 99:59:59.999 There was however only one technique to test this theory, 99:59:59.999 --> 99:59:59.999 and it just so happened that Kirschvink had dedicated 99:59:59.999 --> 99:59:59.999 his entire professional career to that technique. 99:59:59.999 --> 99:59:59.999 It involved magnets and Kirschvink is a man in love with magnets. 99:59:59.999 --> 99:59:59.999 I have wiped out my credit cards so many times by playing with strong magnets, 99:59:59.999 --> 99:59:59.999 I don't like to remember it. 99:59:59.999 --> 99:59:59.999 Narrator: Few people realize it but every rock is a weak magnet. 99:59:59.999 --> 99:59:59.999 Each one contain magnetic minerals and the direction 99:59:59.999 --> 99:59:59.999 these minerals pull in is fixed forever the moment the rock is first formed and 99:59:59.999 --> 99:59:59.999 what gives these minerals their magnetic direction is the magnetic field around the Earth's core. 99:59:59.999 --> 99:59:59.999 This means that rocks formed at the poles will always have a 99:59:59.999 --> 99:59:59.999 magnetic direction that points up and down. 99:59:59.999 --> 99:59:59.999 Rocks formed at the equator will always have a 99:59:59.999 --> 99:59:59.999 magnetic direction that points from side to side. 99:59:59.999 --> 99:59:59.999 The rocks have the ability to preserve magnetic direction 99:59:59.999 --> 99:59:59.999 because of magnetic minerals in them. 99:59:59.999 --> 99:59:59.999 They act like little compass needles and will line themselves up. 99:59:59.999 --> 99:59:59.999 Narrator: By finding a rock's magnetic direction, Kirschvink is able to 99:59:59.999 --> 99:59:59.999 identify where in the world it was originally formed, 99:59:59.999 --> 99:59:59.999 no matter where continental drift might've taken it in the millions of years that followed. 99:59:59.999 --> 99:59:59.999 In the 1980s, Kirschvink heard of a team of Australians who had run 99:59:59.999 --> 99:59:59.999 tests which they claimed showed evidence of glaciers in the tropics. 99:59:59.999 --> 99:59:59.999 Ever the skeptic, Kirschvink decided to duplicate their tests on his own, more sophisticated equipment. 99:59:59.999 --> 99:59:59.999 It meant examining dropstones from some of the hottest parts of Earth. 99:59:59.999 --> 99:59:59.999 The question was were they already at the equator when they were 99:59:59.999 --> 99:59:59.999 formed by the ice or had they been created at the poles 99:59:59.999 --> 99:59:59.999 and then carried to the tropics by continental drift? 99:59:59.999 --> 99:59:59.999 To answer the question, Kirschvink turned to his pride and joy: 99:59:59.999 --> 99:59:59.999 a custom built, hyper-sensitive magnetometer. 99:59:59.999 --> 99:59:59.999 It's an incredible instrument. 99:59:59.999 --> 99:59:59.999 You lower the sample into the actual magnetometer chamber. 99:59:59.999 --> 99:59:59.999 The weak magnetic field of the sample generates an electric current 99:59:59.999 --> 99:59:59.999 and so it allows us to measure the magnetic moment of an extraordinarily weak magnet. 99:59:59.999 --> 99:59:59.999 One billionth say the level of a ordinary hand magnet type thing. 99:59:59.999 --> 99:59:59.999 Narrator: Attached to the magnetometer was a computer which plotted 99:59:59.999 --> 99:59:59.999 each rock's origin on an electronic outline of Earth as seen from above. 99:59:59.999 --> 99:59:59.999 The center of the circle represented the poles, the circumference, the equator. 99:59:59.999 --> 99:59:59.999 If the dropstones had been formed at the poles, as Kirschvink and everyone else thought, 99:59:59.999 --> 99:59:59.999 they would show up at the center of the circle on the computer. 99:59:59.999 --> 99:59:59.999 If they had been formed at the equator, they would show up at the circle's edge. 99:59:59.999 --> 99:59:59.999 The machine gave its answer. 99:59:59.999 --> 99:59:59.999 The dropstones had been formed in the tropics. 99:59:59.999 --> 99:59:59.999 600 million years ago, there must have been a snowball Earth. 99:59:59.999 --> 99:59:59.999 Here's a fundamental theorem in science. 99:59:59.999 --> 99:59:59.999 If a theory is in conflict with data, modify the theory. 99:59:59.999 --> 99:59:59.999 There's so many wonderful, beautiful theories that have been slaughtered by a nasty little fact. 99:59:59.999 --> 99:59:59.999 Narrator: But Kirschvink was racked by the insurmountable paradox of the runaway freeze. 99:59:59.999 --> 99:59:59.999 That if a snowball Earth had ever happened, 99:59:59.999 --> 99:59:59.999 then science said that we should still be entombed in ice today. 99:59:59.999 --> 99:59:59.999 How do you get out of it? 99:59:59.999 --> 99:59:59.999 Obviously, the climate modelers had assumed that that was 99:59:59.999 --> 99:59:59.999 a irreversible step and that you would never get out of it 99:59:59.999 --> 99:59:59.999 and yet, we're out of it now and if we'd been in it before, 99:59:59.999 --> 99:59:59.999 at some point, we must've gotten out of it. 99:59:59.999 --> 99:59:59.999 Narrator: To get out of the deep freeze, what Kirschvink needed was 99:59:59.999 --> 99:59:59.999 a power that would stay hot, even when the whole planet had frozen over. 99:59:59.999 --> 99:59:59.999 Something that Budyko hadn't thought of, something that could burn forever, something like hell. 99:59:59.999 --> 99:59:59.999 Looking at an active volcano, realize that magma tens or hundreds of kilometers 99:59:59.999 --> 99:59:59.999 below the surface couldn't care less whether there was a thin layer of ice over the oceans. 99:59:59.999 --> 99:59:59.999 It would still emerge. 99:59:59.999 --> 99:59:59.999 Narrator: Volcanos survive ice ages because they have a direct channel 99:59:59.999 --> 99:59:59.999 to the molten rock deep within the Earth, 99:59:59.999 --> 99:59:59.999 rock that reaches temperatures of over a thousand degrees but that would 99:59:59.999 --> 99:59:59.999 only melt ice in their immediate area. 99:59:59.999 --> 99:59:59.999 Kirschvink has spotted something else about volcanos. 99:59:59.999 --> 99:59:59.999 They also produce gas 10 billion times a year. 99:59:59.999 --> 99:59:59.999 One gas volcanos emit in huge quantities is carbon dioxide, 99:59:59.999 --> 99:59:59.999 a gas that causes the greenhouse effect and global warming. 99:59:59.999 --> 99:59:59.999 Today, carbon dioxide is being pumped into the atmosphere by both volcanos and industrial activities, 99:59:59.999 --> 99:59:59.999 but what stops the Earth from overheating is that we have a natural way 99:59:59.999 --> 99:59:59.999 of removing the excess carbon dioxide from the atmosphere. 99:59:59.999 --> 99:59:59.999 Rain is the Earth's natural cleaning agent. 99:59:59.999 --> 99:59:59.999 As it falls through the atmosphere, each droplet of rain 99:59:59.999 --> 99:59:59.999 absorbs carbon dioxide and cleans the air but Kirschvink realized 99:59:59.999 --> 99:59:59.999 that on a snowball Earth, there could've been no rain. 99:59:59.999 --> 99:59:59.999 The snowball was so cold, all the water on the planet's surface was frozen solid. 99:59:59.999 --> 99:59:59.999 Without liquid water, nothing could've evaporated into the air 99:59:59.999 --> 99:59:59.999 so there would've been no clouds and without clouds, 99:59:59.999 --> 99:59:59.999 there can be no rain. 99:59:59.999 --> 99:59:59.999 There would've been nothing to stop the carbon dioxide from the volcanos 99:59:59.999 --> 99:59:59.999 from building up over millions of years. 99:59:59.999 --> 99:59:59.999 It would've caused global warming on an inconceivable scale. 99:59:59.999 --> 99:59:59.999 Kirschvink came across calculations showing that after 10 million years without rain, 99:59:59.999 --> 99:59:59.999 the atmosphere would've been 10% carbon dioxide. 99:59:59.999 --> 99:59:59.999 Today, it's far less than 1%. 99:59:59.999 --> 99:59:59.999 This extra carbon dioxide would've created a greenhouse effect 99:59:59.999 --> 99:59:59.999 that raised the temperature to an average of 50 degrees Centigrade, 99:59:59.999 --> 99:59:59.999 hotter than the Earth has ever been. 99:59:59.999 --> 99:59:59.999 Hot enough to melt the ice. 99:59:59.999 --> 99:59:59.999 Kirschvink: And that seemed to be a natural and plausible escape. 99:59:59.999 --> 99:59:59.999 Certainly enough to break the snowball or the ice condition. 99:59:59.999 --> 99:59:59.999 Narrator: Joe Kirschvink had cracked it. 99:59:59.999 --> 99:59:59.999 He had found the way out of the runaway freeze, 99:59:59.999 --> 99:59:59.999 a way that made perfect scientific sense. 99:59:59.999 --> 99:59:59.999 A way that was consistent with the laws of nature. 99:59:59.999 --> 99:59:59.999 Kirschvink: The realization that we may have found 99:59:59.999 --> 99:59:59.999 the way out of the snowball, it was wonderful. 99:59:59.999 --> 99:59:59.999 Narrator: By 1990, Kirschvink had evidence that the tropics 99:59:59.999 --> 99:59:59.999 had frozen over for 10 million years and he had 99:59:59.999 --> 99:59:59.999 come up with a theoretical escape route from the runaway freeze, 99:59:59.999 --> 99:59:59.999 but the problem was it was just a theory. 99:59:59.999 --> 99:59:59.999 He had no physical evidence to prove the ice had melted 99:59:59.999 --> 99:59:59.999 because of an extreme greenhouse effect and without that evidence, 99:59:59.999 --> 99:59:59.999 the snowball Earth theory continued to be ignored. 99:59:59.999 --> 99:59:59.999 [Music] 99:59:59.999 --> 99:59:59.999 In 1992, Paul Hoffman entered the story. 99:59:59.999 --> 99:59:59.999 One of geology's most respected but open-minded figures. 99:59:59.999 --> 99:59:59.999 He was to turn into one of the snowball Earth 99:59:59.999 --> 99:59:59.999 theory's most fervent disciples. 99:59:59.999 --> 99:59:59.999 I loved the snowball Earth theory. 99:59:59.999 --> 99:59:59.999 I mean it's almost a religious ferocity with which 99:59:59.999 --> 99:59:59.999 I want to get across this saga of Earth history with 99:59:59.999 --> 99:59:59.999 so many twists and turns and events that are 99:59:59.999 --> 99:59:59.999 so far outside our own experience. 99:59:59.999 --> 99:59:59.999 Narrator: Hoffman's mission was to find that 99:59:59.999 --> 99:59:59.999 hard evidence that had eluded Kirschvink. 99:59:59.999 --> 99:59:59.999 The final piece of the puzzle that would 99:59:59.999 --> 99:59:59.999 prove the snowball had ended because of the greenhouse effect 99:59:59.999 --> 99:59:59.999 and he knew just where to start looking. 99:59:59.999 --> 99:59:59.999 Namibia. 99:59:59.999 --> 99:59:59.999 Hoffman was drawn to Namibia first by the dropstones 99:59:59.999 --> 99:59:59.999 but even more by what sat immediately on top of them. 99:59:59.999 --> 99:59:59.999 Towering directly above were huge formations called cap carbonates. 99:59:59.999 --> 99:59:59.999 Calcium carbonate crystalized into rock. 99:59:59.999 --> 99:59:59.999 These bizarre structures seemed to appear right out of the blue. 99:59:59.999 --> 99:59:59.999 Above this line right here, ??, no more stones whatsoever 99:59:59.999 --> 99:59:59.999 so the glaciation must've come to an abrupt end 99:59:59.999 --> 99:59:59.999 and sitting directly on top is a thick pile of carbonate 99:59:59.999 --> 99:59:59.999 and it's puzzled geologists for generations. 99:59:59.999 --> 99:59:59.999 [Dramatic Music] 99:59:59.999 --> 99:59:59.999 Narrator: What really puzzled geologists was that caps made 99:59:59.999 --> 99:59:59.999 from calcium carbonate are usually formed in warm water, 99:59:59.999 --> 99:59:59.999 but in Namibia, they had suddenly appeared on top 99:59:59.999 --> 99:59:59.999 of an ice cold glacier. 99:59:59.999 --> 99:59:59.999 What this indicates is that you go from glacial conditions 99:59:59.999 --> 99:59:59.999 to an ocean that is warm in a flash. 99:59:59.999 --> 99:59:59.999 Narrator: To most geologists, this instant change from 99:59:59.999 --> 99:59:59.999 cold to hot defied their understanding 99:59:59.999 --> 99:59:59.999 but Hoffman suspected it might be a huge clue. 99:59:59.999 --> 99:59:59.999 That the sudden appearance of mountains of 99:59:59.999 --> 99:59:59.999 calcium carbonate must somehow be connected to 99:59:59.999 --> 99:59:59.999 the death of the snowball in a colossal greenhouse effect. 99:59:59.999 --> 99:59:59.999 But how? 99:59:59.999 --> 99:59:59.999 Hoffman needed someone to explain why 99:59:59.999 --> 99:59:59.999 calcium carbonate would be formed not just because 99:59:59.999 --> 99:59:59.999 of warm conditions in general but specifically in the 99:59:59.999 --> 99:59:59.999 conditions created by a melting snowball Earth. 99:59:59.999 --> 99:59:59.999 So he turned to a geochemist, Dan Schrag. 99:59:59.999 --> 99:59:59.999 Painstakingly, they analyze the snowball's theoretical 99:59:59.999 --> 99:59:59.999 final moments, stage by stage, 99:59:59.999 --> 99:59:59.999 working out what chemical and climatic processes were at work. 99:59:59.999 --> 99:59:59.999 [Music] 99:59:59.999 --> 99:59:59.999 First, they realized that as the ice melted, 99:59:59.999 --> 99:59:59.999 it would produce liquid water. 99:59:59.999 --> 99:59:59.999 The water would then evaporate, and create clouds 99:59:59.999 --> 99:59:59.999 and those clouds would cause a change in the weather, 99:59:59.999 --> 99:59:59.999 the likes of which has never been seen by human eyes. 99:59:59.999 --> 99:59:59.999 Daniel Schrag: When the ice is retreating, 99:59:59.999 --> 99:59:59.999 this is probably the most spectacular climate change 99:59:59.999 --> 99:59:59.999 in Earth's history. 99:59:59.999 --> 99:59:59.999 In some ways, we're going from the coldest climate 99:59:59.999 --> 99:59:59.999 the Earth's ever experienced to the warmest climate 99:59:59.999 --> 99:59:59.999 the Earth's ever experienced. 99:59:59.999 --> 99:59:59.999 We immediately say that this was the mother of all climate changes. 99:59:59.999 --> 99:59:59.999 Narrator: The most elemental powers of nature 99:59:59.999 --> 99:59:59.999 would be unleashed upon the Earth. 99:59:59.999 --> 99:59:59.999 One would predict hurricanes of such intensity that are unimaginable 99:59:59.999 --> 99:59:59.999 so it's quite possible you could've had waves of 99:59:59.999 --> 99:59:59.999 100 meters in amplitude coming in and crashing 99:59:59.999 --> 99:59:59.999 in on the shore. 99:59:59.999 --> 99:59:59.999 It would be unbelievably violent. 99:59:59.999 --> 99:59:59.999 Narrator: And above all, there would be rain. 99:59:59.999 --> 99:59:59.999 The first rainstorms for millions of years. 99:59:59.999 --> 99:59:59.999 Rainstorms that would last at least a century 99:59:59.999 --> 99:59:59.999 and it wouldn't be just any rain. 99:59:59.999 --> 99:59:59.999 The rainwater would react with the vast quantites 99:59:59.999 --> 99:59:59.999 of carbon dioxide in the atmosphere and form carbonic acid. 99:59:59.999 --> 99:59:59.999 This acid rain would then deluge the Earth. 99:59:59.999 --> 99:59:59.999 The acid rain would pound the exposed rocks, 99:59:59.999 --> 99:59:59.999 producing a violent chemical reaction. 99:59:59.999 --> 99:59:59.999 It would break the rocks down into their constituent parts, 99:59:59.999 --> 99:59:59.999 one of which was calcium. 99:59:59.999 --> 99:59:59.999 This would then fuse with the carbon in the acid rain. 99:59:59.999 --> 99:59:59.999 The result, mountains of calcium carbonate, 99:59:59.999 --> 99:59:59.999 exactly as they could see right above the dropstones in Namibia. 99:59:59.999 --> 99:59:59.999 Suddenly, it became clear that the natural expectation 99:59:59.999 --> 99:59:59.999 of a prolonged global glaciation ending in extremely 99:59:59.999 --> 99:59:59.999 high levels of carbon dioxide was that you would expect 99:59:59.999 --> 99:59:59.999 these very unusual thick carbonate rocks should 99:59:59.999 --> 99:59:59.999 immediately follow the glacial deposits. 99:59:59.999 --> 99:59:59.999 [Music] 99:59:59.999 --> 99:59:59.999 Narrator: Hoffman and Schrag had found the final evidence. 99:59:59.999 --> 99:59:59.999 Schrag: The beauty of the snowball Earth theory is exactly that 99:59:59.999 --> 99:59:59.999 it links the chemistry, the geology, the planetary science 99:59:59.999 --> 99:59:59.999 and bam, bam, bam, all of the facts 99:59:59.999 --> 99:59:59.999 are consistent with the predictions of the theory. 99:59:59.999 --> 99:59:59.999 [Jazz Music] 99:59:59.999 --> 99:59:59.999 Narrator: In 1998, Hoffman and Schrag made an 99:59:59.999 --> 99:59:59.999 triumphal tour of the world's universities, 99:59:59.999 --> 99:59:59.999 promoting the snowball theory, 99:59:59.999 --> 99:59:59.999 and then someone threw a spanner in the works. 99:59:59.999 --> 99:59:59.999 An entire scientific discipline, the biologists took one look 99:59:59.999 --> 99:59:59.999 at the 10 million year long snowball Earth and said 99:59:59.999 --> 99:59:59.999 it was quite simply impossible. 99:59:59.999 --> 99:59:59.999 One scientist who had problems with the snowball was Guy Narbonne. 99:59:59.999 --> 99:59:59.999 He like many other biologists could see there was a 99:59:59.999 --> 99:59:59.999 fundamental problem reconciling the snowball Earth theory 99:59:59.999 --> 99:59:59.999 with what he knew. 99:59:59.999 --> 99:59:59.999 The geological evidence speaks very strongly 99:59:59.999 --> 99:59:59.999 of ice sheets marching on the equator. 99:59:59.999 --> 99:59:59.999 The biological evidence speaks very strongly of open water 99:59:59.999 --> 99:59:59.999 being available at this whole time. 99:59:59.999 --> 99:59:59.999 Narrator: Narbonne's problem was another set of nature's law. 99:59:59.999 --> 99:59:59.999 Nothing less than the laws that governed life itself. 99:59:59.999 --> 99:59:59.999 600 million years ago, when the snowball was set to have struck, 99:59:59.999 --> 99:59:59.999 there was nothing living on the land. 99:59:59.999 --> 99:59:59.999 It all existed in the sea and it was not life as we know it. 99:59:59.999 --> 99:59:59.999 The most common organism on the planet was this. 99:59:59.999 --> 99:59:59.999 Cyanobacteria. 99:59:59.999 --> 99:59:59.999 This mass of cyanobacteria is so ancient, 99:59:59.999 --> 99:59:59.999 it predates even the snowball. 99:59:59.999 --> 99:59:59.999 The oldest fossils that we have in the world 99:59:59.999 --> 99:59:59.999 are stromatolites and these are colonies like you see here. 99:59:59.999 --> 99:59:59.999 They're made out of literally billions of cyanobacteria. 99:59:59.999 --> 99:59:59.999 Narrator: What made cyanobacteria and more sophistocated 99:59:59.999 --> 99:59:59.999 ancient organisms like green algae essential to the snowball 99:59:59.999 --> 99:59:59.999 debate was that they photosynthesize. 99:59:59.999 --> 99:59:59.999 Just like plants today under the water, these lifeforms 99:59:59.999 --> 99:59:59.999 absorb sunlight and convert it into energy. 99:59:59.999 --> 99:59:59.999 So these species' chances of surviving the snowball 99:59:59.999 --> 99:59:59.999 would've depended on whether there was enough 99:59:59.999 --> 99:59:59.999 sunlight for photosynthesis in the snowball seas 99:59:59.999 --> 99:59:59.999 at a time when every inch of the planet was 99:59:59.999 --> 99:59:59.999 allegedly covered in thick ice. 99:59:59.999 --> 99:59:59.999 Light penetration through ice is not great if the ice 99:59:59.999 --> 99:59:59.999 is more than a couple tens of meters thick. 99:59:59.999 --> 99:59:59.999 Narrator: And that was the problem biologists pointed 99:59:59.999 --> 99:59:59.999 to in Hoffman and Schrag's work. 99:59:59.999 --> 99:59:59.999 Their calculations had reckoned on the snowball 99:59:59.999 --> 99:59:59.999 producing ice sheets up to a kilometer thick, 99:59:59.999 --> 99:59:59.999 and even in the warmest places around the equator, 99:59:59.999 --> 99:59:59.999 they would've been tens of meters thick. 99:59:59.999 --> 99:59:59.999 More than enough, biologists believe, to block out 99:59:59.999 --> 99:59:59.999 any sunlight from the oceans and that meant death 99:59:59.999 --> 99:59:59.999 to most things that depended on photosynthesis for life. 99:59:59.999 --> 99:59:59.999 Narbonne: You cut off photosynthesis and within a few years, 99:59:59.999 --> 99:59:59.999 let alone a few million years, mass extinction. 99:59:59.999 --> 99:59:59.999 Narrator: It meant that many of the photosynthesizing 99:59:59.999 --> 99:59:59.999 species should've become extinct 600 million years ago 99:59:59.999 --> 99:59:59.999 and the trouble was they hadn't. 99:59:59.999 --> 99:59:59.999 Cyanobacteria grow in rock-like colonies all over the tropics today 99:59:59.999 --> 99:59:59.999 as do other photosynthetic plants, dating from before 99:59:59.999 --> 99:59:59.999 the snowball like green algae. 99:59:59.999 --> 99:59:59.999 For many biologists, there seemed to be only one possible conclusion: 99:59:59.999 --> 99:59:59.999 there may have been a severe ice age but not a fully 99:59:59.999 --> 99:59:59.999 fledged snowball that covered all the oceans 99:59:59.999 --> 99:59:59.999 because lifeforms that should've died out are still with us. 99:59:59.999 --> 99:59:59.999 Unless someone could find a way for marine life 99:59:59.999 --> 99:59:59.999 to survive under a thick sheet of ice, 99:59:59.999 --> 99:59:59.999 the whole theory would be back in the bin. 99:59:59.999 --> 99:59:59.999 [Music] 99:59:59.999 --> 99:59:59.999 Then along came the man from NASA, Chris McKay. 99:59:59.999 --> 99:59:59.999 Chris McKay: I'm planetary scientist with an interest 99:59:59.999 --> 99:59:59.999 in life in extreme environments. 99:59:59.999 --> 99:59:59.999 In particular, environments that are cold and dry. 99:59:59.999 --> 99:59:59.999 Several years ago, I saw the papers about the snowball Earth, 99:59:59.999 --> 99:59:59.999 indicating the geological evidence that the Earth was ice covered 99:59:59.999 --> 99:59:59.999 and had been ice covered for millions of years. 99:59:59.999 --> 99:59:59.999 Plus the evidence that there are organisms (bacteria and green algae) 99:59:59.999 --> 99:59:59.999 that survived this ice covered state and that's the puzzle. 99:59:59.999 --> 99:59:59.999 How could these photosynthetic organisms survive? 99:59:59.999 --> 99:59:59.999 Narrator: McKay was intrigued because it just so happened 99:59:59.999 --> 99:59:59.999 he had visited a place on Earth today that was so cold, 99:59:59.999 --> 99:59:59.999 it could actually be compared with the snowball of 600 million years ago. 99:59:59.999 --> 99:59:59.999 Antarctica, the coldest place on Earth. 99:59:59.999 --> 99:59:59.999 Here, temperatures can fall to -30. 99:59:59.999 --> 99:59:59.999 Almost as severe as the snowball at its warmest point, the equator 99:59:59.999 --> 99:59:59.999 and deep in Antarctica are the dry valleys where there are lakes 99:59:59.999 --> 99:59:59.999 smothered by an ice sheet many meters thick. 99:59:59.999 --> 99:59:59.999 McKay: We think these little lakes represent a good 99:59:59.999 --> 99:59:59.999 model for looking at the snowball Earth. 99:59:59.999 --> 99:59:59.999 Could there be life underneath the ice cover on a snowball Earth? 99:59:59.999 --> 99:59:59.999 Narrator: Scientists were eager to discover if there was 99:59:59.999 --> 99:59:59.999 enough light for life to survive in the water below the thick ice. 99:59:59.999 --> 99:59:59.999 They were about to be astonished. 99:59:59.999 --> 99:59:59.999 [Music] 99:59:59.999 --> 99:59:59.999 There was light, lots of it 99:59:59.999 --> 99:59:59.999 even though over head, 99:59:59.999 --> 99:59:59.999 the ice was five meters thick. 99:59:59.999 --> 99:59:59.999 There was so much light, 99:59:59.999 --> 99:59:59.999 the water was filled with plant life and bacteria, 99:59:59.999 --> 99:59:59.999 including two organisms dating from the time 99:59:59.999 --> 99:59:59.999 of the snowball that would've needed sunlight to live. 99:59:59.999 --> 99:59:59.999 McKay: When we're diving in the lakes 99:59:59.999 --> 99:59:59.999 and swimming around, there's life in the water. 99:59:59.999 --> 99:59:59.999 There's bacteria, cyanobacteria 99:59:59.999 --> 99:59:59.999 but there's also green algae. 99:59:59.999 --> 99:59:59.999 Narrator: And McKay had a hunch as to why this 99:59:59.999 --> 99:59:59.999 photosynthetic life could survive. 99:59:59.999 --> 99:59:59.999 It was something about the nature 99:59:59.999 --> 99:59:59.999 of the ice itself. 99:59:59.999 --> 99:59:59.999 McKay: The transparency of this ice is very high 99:59:59.999 --> 99:59:59.999 and it's due to the fact that it's freezing very slowly 99:59:59.999 --> 99:59:59.999 underneath this thick ice cover. 99:59:59.999 --> 99:59:59.999 Now let's imagine this snowball Earth where 99:59:59.999 --> 99:59:59.999 there's also a thick ice cover, 99:59:59.999 --> 99:59:59.999 the ice on the bottom is freezing very slowly as well, 99:59:59.999 --> 99:59:59.999 it should have the same optimal properties 99:59:59.999 --> 99:59:59.999 as this clear, clean ice. 99:59:59.999 --> 99:59:59.999 Narrator: By freezing slowly, water rejects impurities 99:59:59.999 --> 99:59:59.999 such as salt or dirt which make it cloudy. 99:59:59.999 --> 99:59:59.999 So the ice becomes clean and transparent. 99:59:59.999 --> 99:59:59.999 Because of this transparency, 99:59:59.999 --> 99:59:59.999 life giving sunlight would've been reaching down 99:59:59.999 --> 99:59:59.999 into the snowball seas. 99:59:59.999 --> 99:59:59.999 McKay realized that even at its height, 99:59:59.999 --> 99:59:59.999 there would've been havens around the snowball's 99:59:59.999 --> 99:59:59.999 equator where the ice was thin enough 99:59:59.999 --> 99:59:59.999 for photosynthetic life to cling on below but there was more. 99:59:59.999 --> 99:59:59.999 Although disciples of the theory accept 99:59:59.999 --> 99:59:59.999 the snowball would've been death to most things, 99:59:59.999 --> 99:59:59.999 they argue whatever life did survive 99:59:59.999 --> 99:59:59.999 would've emerged into a new world almost 99:59:59.999 --> 99:59:59.999 devoid of competition. 99:59:59.999 --> 99:59:59.999 The perfect conditions for an explosion of evolutionary change. 99:59:59.999 --> 99:59:59.999 [Music] 99:59:59.999 --> 99:59:59.999 These fossils show the new species which appeared 99:59:59.999 --> 99:59:59.999 soon after the time of the snowball. 99:59:59.999 --> 99:59:59.999 All display a massive increase in size and complexity 99:59:59.999 --> 99:59:59.999 from the simple organisms that lived before. 99:59:59.999 --> 99:59:59.999 These are the first multi-cellular lifeforms. 99:59:59.999 --> 99:59:59.999 Even those with some questions about the snowball theory 99:59:59.999 --> 99:59:59.999 recognize a new climate after a mass extinction 99:59:59.999 --> 99:59:59.999 could be a spur for evolution. 99:59:59.999 --> 99:59:59.999 [Music] 99:59:59.999 --> 99:59:59.999 But if the snowball can change the course of life on the planet, 99:59:59.999 --> 99:59:59.999 there remains one final question. 99:59:59.999 --> 99:59:59.999 No one knows why the snowball happened 99:59:59.999 --> 99:59:59.999 or if it could happen again. 99:59:59.999 --> 99:59:59.999 The sun is hotter than it was 600 million years ago 99:59:59.999 --> 99:59:59.999 and is getting hotter all the time. 99:59:59.999 --> 99:59:59.999 Climate trends would have to change dramatically. 99:59:59.999 --> 99:59:59.999 If it ever did happen again, 99:59:59.999 --> 99:59:59.999 it almost certainly wouldn't be for millions of years 99:59:59.999 --> 99:59:59.999 but if it ever did, that would mean death to almost everything 99:59:59.999 --> 99:59:59.999 including us. 99:59:59.999 --> 99:59:59.999 [Music]