曾被科学家们认为是荒谬的一种假说-- There is a theory that scientists once dismissed as absurd -
很久以前地球曾一度极度严寒 that long ago the Earth became so cold
冰盖遮覆了每一寸土地 every inch of it was entombed in ice.
整个地球被冻成一个巨型雪球 The whole planet became one vast snowball
这种状态持续了一千万年 and it lasted for ten million years.
地平线 雪球地球
没有比冰川时期更可怕的自然力量了 Earth has no more terrible force than an ice age.
在地球四十亿年的历史中 Only five times in its four billion year existence
遭受了五次这种灾难 has our planet suffered this kind of catastrophe.
没人能确定冰川时期何时开始和出现原因 No one is certain what started them or why they happened,
但大多数科学家都认同冰川时期的 but there is one thing about ice ages on which most scientists agree.
传统观点： The conventional view
任何一次冰川时期中，无论寒流推进得多么凶猛 is that no matter how ferocious the advance of the cold in any ice age
世界上总有一些地区很稳当地温暖 there will always be parts of the world that are safe and warm.
冰冻从两极缓慢蔓延 The freeze spreads out slowly from the poles,
但从未蔓延至地球最热的地区--热带地区 but never reaches the hottest part of the planet, the tropics.
长久以来都认为一项基本的自然法则是 It has long been assumed that this is a basic law of nature.
热带地区永远不会被冻结 The tropics can never freeze.
但这项基本的自然法则 But then this basic law of nature
无法解释六亿年前发生在纳米比亚的事情 can't explain what happened 600 million years ago in Namibia.
保罗·霍夫曼是那些挑战传统观点的地质学家中的一员 Paul Hoffman is one of those defiant geologists who disputes the conventional view.
他花了数年时间想让同事们相信地球也曾是个雪球 He has spent years trying to convince his colleagues about the snowball Earth.
作为一名科学家，我完全相信它是对的 As a scientist I was completely convinced it was right,
但我也知道 but I knew
要靠双手实现奋斗目标 I was going to have a struggle on my hands,
保罗·霍夫曼教授 哈佛大学
因为假说全新 because any idea as radical as, as this
又很难被认同，肯定会有人提出质疑 is going to be difficult to get across and there are going to be sceptics for sure.
霍夫曼和同事丹·施拉格多次到纳米比亚展开研究 The place Hoffman and his colleague, Dan Schrag, come back to study again and again is Namibia.
雪球地球的证据并不在很明显的地方 It is not an obvious place in which to seek evidence of a catastrophic ice age.
这片土地高温从不间断 It is a land of relentless heat,
几亿年来都是热带地区 one of those locations that has been in the tropics for hundreds of millions of years.
传统的自然观点说冰河时代永远不会蔓延到这里 The conventional laws of nature say an ice age could never have penetrated here,
但沉积地层中的巨砾 but lurking among the sun-baked rocks
用传统自然观点无法解释 is something that the conventional laws of nature cannot explain away.
六亿年前，这些山丘位于海底 600 million years ago these hills were at the bottom of the ocean
这些神秘的巨砾截然分布在古老的海床上 and lying on the old seabed are mysterious boulders.
地质学家把这些巨砾称为坠石 Geologists call them drop stones
坠石只能是冰川所带来 and drop stones could only have been brought here by one thing: a glacier.
要在实验室还原坠石产生过程，需要先准备冰层 To recreate drop stones in the lab first you need ice.
要等一阵才结冰 This is going to take a while.
冰层需要浮在沙质海底上的浅水中 The ice needs to float on shallow water above a sandy sea floor.
这是为了模拟 This is to simulate a glacier
冰川从陆地滑动到海面时的过程 as it flows off the land onto the surface of the sea.
冰川滑动时修整了地貌 Because it has carved through the landscape
大量的岩石碎片脱落被冰川裹挟而动 a glacier always collects a huge amount of rock debris.
冰层中被各种巨石和鹅卵石填充 The ice would then get filled up with all sorts of boulders and cobbles
丹·施拉格教授 哈佛大学
我们就照样子把一些砾石 and in this case we're just using some gravel
平铺在水箱冰层之上 which we can form on top of this layer of ice in our tank here.
重现出一块填满砾石的冰川，接着融化冰层 Having recreated a boulder-filled glacier, you need to melt the ice.
随着冰层融化，砾石从冰层中脱落下来 As the ice melts the boulders are released.
当大部分冰融化的时候 When most of the ice has melted is a point
可以看到进入海洋的冰川中所有砾石 you can see that all the rocks the glacier's carried out to the ocean
都沉到海底 are now dropped to the sea floor.
沉积层除了冰川带来的砾石 In addition to all of this,
同时沉降的 in the real world of boulders,
有大量的沙子和很多很细的颗粒沉积物 there'd also be a lot of sand and a lot of much finer grain sediment
从而砾石被很细的颗粒沉积物所包裹 and so these rocks would get filled in with very fine grain sediment as well.
经过几千年的地质变化 In a few thousand years
细颗粒沉积物粘合在一起包裹着巨砾 the fine grain sediment will cement itself together to form rocks,
形成岩层 with the boulders trapped inside
这就是坠石产生的过程 and that's how drop stones are made.
产生这一沉积层就是冰川作用 All you need is a glacier.
沉积层中存在大量坠石的唯一原因是 The message from the drop stones suggests just one thing:
热带地区的纳米比亚 Namibia in the tropics,
一个在学界人人都认为已经热了几亿年的地方 a place that everyone says has been hot for hundreds of millions of years,
一定曾经被冰覆盖过 must once have been covered in ice.
这里最有说服力的证据是 The most compelling evidence here
在极细微的层状沉积物中 is the juxtaposition of a large boulder like this
包裹着像这样的大砾石 enclosed within very fine and delicately layered sediment
因此，除了用雪球地球假说解释之外 and so it's difficult, if not impossible, to imagine any way to account for this
如果用其它观点解释，很难解释得通 other than a very extensive glaciation.
但在五十年前 But for 50 years,
首次提出热带地区也被冰层覆盖的假说 since the idea that an ice age must have smothered the tropics was first proposed,
学术界认为这个假说太过激进 it has been deemed so radical
大多数科学家都认为它很荒谬 that most scientists have dismissed it as absurd.
这个假说起源于二十世纪四十年代 The theory has its roots in the 1940s
当一位名叫布莱恩·哈兰德的年轻地质学家 when a young geologist called Brian Harland
深入北极腹地进行了一系列勇敢的实地考察 made a series of intrepid field trips deep into the Arctic.
气候条件非常恶劣 Conditions were very different then -
苏格兰式人工推进 manhauling(manhandling?) in the old Scots tradition.
天气很冷，但已经习惯了 It's cold but you get used to it,
什么事情都可以习惯 you can get used to anything really.
在北极地区，哈兰德有足够的机会 In the Arctic Harland had ample opportunity
去了解所有有关冰川时期的印记--坠石 to learn all there was to learn about that telltale sign of glaciation - drop stones.
冰川沉积物非常明显 Glacial deposits were very dramatic,
沉积层纹路非常独特，识别容易 currents very distinctive, easy to identify
当我看到沉积层时，我的经验告诉我 and when I saw them I knew from my own experience
布莱恩·哈兰德 剑桥大学
这是冰河期的 that this is glacial.
判断它不太费力 I didn't have to think very hard about it.
当布莱恩·哈兰德从北极回来后 But it was when he returned from the Arctic,
凭借着新发现 armed with this new-found knowledge,
他在学术上取得了突破 that Harland made his scientific breakthrough.
他以专家的眼光 With his now expert eye,
分析了来自世界各地的坠石 he analysed rocks from all over the world.
他总结出来的结论让他自己都很吃惊 What he discovered amazed him.
他分析的每个地方的坠石 Almost everywhere he looked,
几乎都是六亿年前产生的 in rocks formed about 600 million years ago,
这些坠石是经典的证据 he found drop stones:
其表明，即使在最热的大陆上也曾覆盖过厚厚的冰层 classic evidence that there had once been thick ice even on the hottest continents.
坠石分布很广泛 The drop stones were so widespread
于是，哈兰德向看似绝不可能的结论考虑 that Harland began to contemplate the unthinkable.
我们已获得了全球冻结成冰的证据 The fact is we've got evidence of a global glaciation.
他清楚这没有意义 He knew it made no sense.
他知道在最后一个冰川时期的高峰时期 He knew that at the peak of the last Ice Age,
刚在一万年前结束 which ended just 10,000 years ago,
冰层蔓延到纽约这个纬度 ice spread down to where New York is today,
纽约纬度：北纬40°43’0”
就截止了 but no further,
摆在哈兰德面前的证据 but the evidence was staring Harland in the face,
仿佛无视自然规律 evidence which seemed to flout the laws of nature.
六亿年前的坠石表明 600 million years ago the drop stones said
冰盖已遍布各大洲 ice had spread over every continent,
完全覆盖热带地区直到赤道 right across the tropics to the Equator itself.
也就是说一定有个难以想象的狂暴冰河期 It meant there must have been an ice age of unimaginable ferocity,
真是无法想象啊 in fact too unimaginable.
哈兰德提出雪球地球假说时 When Harland presented his theory of global glaciation
同事们觉得很荒谬 colleagues dismissed it as laughable.
他们（质疑者）依据很多理由认为哈兰德错了 The sceptics had a whole host of reasons why Harland had to be wrong,
这些理由有十足的说服力 reasons that were utterly convincing,
这些理由是基于关于世界运转的最基本的规则 as they were based on the most basic rules about how the world works.
首先，已知环绕着地球的 For a start, it's given that the tropics,
热带地区 that wide belt around the Earth,
一直直接受到太阳强烈的照射 has always received the full glare of the sun.
由于热带地区总是沐浴在阳光下 As they have always basked in sunlight,
所以始终保持温暖 so they must always have remained warm.
即使在上一个冰川时期 Even during the last Ice Age
地质证据也表明热带地区的气候非常炎热 geological evidence suggests the tropics were pleasantly hot
他们还有另一个理由认为雪球地球假说必定是错的 and they had another reason why the snowball theory must be wrong.
对于热带地区的坠石，有一个更为简单的解释 There was a far simpler explanation for drop stones in the tropics,
这种解释与科学认识完全一致 one that was absolutely consistent with scientific knowledge
还不涉及到像热带地区被冰封那样荒谬的想法 and which didn't involve a ludicrous idea like the tropics freezing over.
这个解释就是大陆漂移假说 Continents drift.
大陆以大约与指甲生长相同的速度 They scrape across the surface of the Earth
缓慢地持续水平运动 at about the same pace that your fingernails grow.
大约经过四亿年 This means that in about 400 million years
大陆可以从地球的一端漂移到另一端 continents can shift from one end of the globe to the other.
显然，质疑者是说 Obviously, the doubters said,
六亿年前，所有大陆都已经漂移到严寒的极地地区 600 million years ago all the continents had drifted into icy, polar regions.
一旦到达那里，冰川就会自然形成 Once there, glaciers would have formed
在这个纪元，坠石如同普通石头一样常见 and drop stones would be common in all rocks of this era,
这些坠石在极地地区形成后 and after those drop stones had been formed at the poles
大陆和坠石 the land, and the drop stones,
又经大陆漂移作用漂移回热带地区 would have been carried back down to the tropics by continental drift.
这种说法解释了坠石为何分布在世界各地 For years this remained the accepted explanation
多年以来，这是受认可的说法 for why drop stones appeared everywhere.
使雪球地球假说可信的唯一方法是 The only way to make the theory of a global glaciation credible
把不可能变成可能 was for someone to make the impossible possible.
他们需要找到一种方法让热带地区冻结 They would need to find a way for the tropics to freeze over,
这就是接下来发生的事情 and this is just what happened next.
（俄罗斯倒计时）准备，5，4，3，2，1，0！ (RUSSIAN COUNTDOWN) OK, 5, 4, 3, 2, 1, 0!
那是二十世纪六十年代，冷战的高峰期 It was the 1960s, the height of the Cold War.
全世界一门心思推断爆发核战幸存的几率 The world was obsessed with calculating the odds of surviving a nuclear Holocaust.
众所周知，一连串的大规模核爆炸会产生 It was known that a series of massive nuclear explosions
满天尘埃、烟云和烟灰 would create clouds of dust, smoke and soot.
遮住阳光 Sunlight would be blocked out.
假设地球将进入一个人造冰川时期的 Hypothetically the Earth would enter a nuclear winter,
核冬天 a man-made ice age.
苏联时期，搞清楚人造冰川时期造成的严重程度的任务 In the Soviet Union finding out how severe this man-made ice age could be
落在一位气候学家身上 became the task for one climatologist.
米哈伊尔·布迪科就是那位气候学家 Mikhail Budyko was that man.
米哈伊尔·布迪科 列宁格勒地球物理天文台的地球物理学家
很久以前，大概二十五年或三十年以前 Long ago, probably 25 or 30 years,
米哈伊尔·布迪科教授 圣彼得堡国立水文研究所
我编撰了一些 I compiled a number of studies
可用于描述冰川时期起源的研究 which could be used to describe origin of ice ages.
布迪科的研究结论 What Budyko was to uncover
与传统观点截然相反 would fly in the face of conventional wisdom.
他的研究结论指出 He would show how those predictions
热带地区无法冻结的那些预言非常自恋和无事实根据 that the tropics couldn't freeze over were complacent and unfounded.
布迪科清楚陆地和海洋的颜色较深 Budyko knew that because the land and oceans are dark
吸收了太阳光的大部分热量 they absorb most of the heat coming from the Sun's rays
是地球温暖的原因 and that is how our planet is warmed up;
但是冰层是白色的 but sheets of ice are white.
像镜子一样把阳光反射掉 They reflect sunlight like a mirror,
因此，被冰雪覆盖的地球吸收的太阳热量要少得多 so an ice-covered Earth absorbs far less solar heat.
在冰川时期，冰川不断蔓延 During an ice age as the freeze spreads
随着地球变得越来越白 the Earth grows whiter,
越来越多的热量被反射掉，吸收的也随之减少 more heat is reflected away so less and less heat is absorbed
地球也越来越冷 and so the Earth grows ever colder.
这意味着 It means that potentially
地球可能陷入一个无法停止的冰冻恶性循环中 the Earth could be caught in a vicious circle of unstoppable freezing.
布迪科将这个光被反射出去的假设情景 Budyko converted this hypothetical scenario
用数学公式来表达 into a mathematical formula
这个公式推断出了一个可怕的预言 and that formula produced a terrifying prediction:
地球变成雪球有一个理论上的纬度突破值 the Earth's climate has a theoretical breaking point.
只要冰原在极地附近保持不蔓延，地球就是安全的 As long as the ice sheets remain close to the poles the Earth is safe,
但如果冰原蔓延开来 but if the freeze continues,
譬如核大战后的核冬天 such as might happen in a nuclear winter,
冰原可能蔓延到达德克萨斯市这个纬度 they could advance down to about where Texas is today.
德克萨斯市的纬度：北纬29°23’0”
一旦冰原蔓延到这条纬度线 Once the freeze had reached that point
地球大部分区域将被白色冰原所覆盖 so much of the Earth would be covered in white ice
使地球保持温暖的太阳能 that over half the solar heat that normally warms the planet
有一半以上会被反射回太空 would be reflected back into space.
没有足够的热量来保持地球温暖 At that point there wouldn't be enough heat left to warm up the Earth.
一旦冰原越过纬度30°，理论上，冰冻蔓延失控 Once this happens there could, in theory, be a runaway freeze,
且不可逆转 a freeze that nothing can stop.
气温骤降 Temperatures plummet,
冰盖遍布所有大陆、海洋 ice sheets spread across all the continents, the oceans,
覆盖了热带地区 and eventually even the tropics.
如果这种情况发生，整个地球将被冰层包裹 If this was ever to happen the entire planet would be trapped in ice.
成为一个宇宙大雪球 There would be a snowball Earth.
布迪科的推断中最令人困扰的是 What was most disturbing about Budyko's calculations
被冰层包裹的地球反射的太阳能热量如此之大 was that an Earth encased in ice would reflect so much solar heat
到了永远无法升温到足以解冻的地步 it could never warm up enough to thaw, ever.
二十年前我的看法是 It was my opinion twenty years ago
那时的地球冰冻系统长时间保持稳定 that such a system will be stable for very long time and
可能造成地球所有生命都将消失 possibly all life will disappear.
这是布迪科的悖论 This was Budyko's paradox.
他证明了在理论上一次蔓延越过纬度30°的 不可逆冰冻可能冻结热带地区 He had shown that a runaway freeze could ice over the tropics in theory,
但他也表明了在现实中永远不会发生 but he had also shown that it could never have happened in practice,
因为，如果发生的话，当今地球仍将是一个雪球 because, if it had, the world would still be a snowball today
也就是说人类也应该不存在 and that would mean we would never have existed.
布迪科的悖论大意是：有产生雪球地球推论，却无融冰推论
十五年来，没有人能解决布迪科的悖论 For 15 years nobody could solve the paradox of the runaway freeze.
雪球地球推论仍是不符合大逻辑的 The snowball Earth theory remained a logical impossibility.
为了使这个推论起死回生，相信它的人得要求证两方面 To resurrect the theory believers would have to do two things.
首先，必须证明冰原已覆盖了地球热带地区 First, they had to prove that ice really had reached the hottest parts of the planet, the tropics,
并且热带地区的坠石不是通过大陆漂移而来 and that the drop stones hadn't arrived there merely through continental drift.
其次，必须提出一个理论来解释地球如何解开冰封 Second, they had to come up with a theory that offered an escape from the runaway freeze.
其时一位新科学家加入这研究中来 That was when a new scientist entered the picture.
七十年代末，约瑟夫·科什文克 是地质学领域冉冉升起的明星之一 In the late 70s Joe Kirschvink was one of geology's rising stars
那时，他认为地球雪球的推论太疯狂不靠谱了 and back then he thought the idea of a snowball Earth was mad.
让人乐意接受 The willingness to accept
地球曾被冰层包裹这样一个疯狂的推论 a nutty idea like freezing the whole planet
约瑟夫·科什文克教授 美国古地磁学家 美国加州理工学院
真的需要有令人信服的强而有力证据的支持 really needs to be supported by incredibly strong evidence
总的来说，要是在七十年代末问我接受与否 and overall if you had asked me to place a wager back in the late 70s
我站在保守这边（持怀疑态度） I would have been on the conservative side on that one.
科什文克之所以持保守主义 Kirschvink's conservatism came about
是因为他相信那个对现今赤道坠石来由的 because he believed the simple, conventional explanation
既简单又传统的解释 for why there were drop stones at the Equator today.
他认为，六亿年前 600 million years ago, he thought,
当坠石形成时 all the continents must have been up near the icy poles
所有大陆都一定是靠近极地 when the drop stones were formed.
然而，只有一种技术可以证明他们的推测 There was, however, only one technique for testing their theory
就是事实真相 and it just so happened
恰恰科什文克整个研究生涯都奉献给了这种技术 that Kirschvink had dedicated his entire professional career to that technique.
技术的一部分是关于磁体， 而科什文克又是一个热爱研究磁体的人 It involved magnets and Kirschvink is a man in love with magnets.
我用磁铁把信用卡消磁了好多次 I have wiped out my credit cards so many times by playing with strong magnets
我不想想起它 I, I don't like to remember it.
但很少有人知道每块石头都是一块弱磁体 Few people realise it, but every rock is a weak magnet.
每一块都含有磁性矿物质 Each one contains magnetic minerals
这些矿物的磁场方向往往取决于 and the direction these minerals pull in
在赤道附近定形的石头的磁场方向是水平方向 极地附近的则是会垂直于地面
它沉积的过程中环境磁场的方向 is fixed for ever the moment the rock is first formed
并在形成矿石后 and what gives these minerals their magnetic direction
依然保持这个磁场方向不变 is the magnetic field round the Earth's core.
在两极形成的岩石 This means that rocks formed at the poles
磁场方向始终具有上下指向（垂直于地面） will always have a magnetic direction that points up and down.
在赤道上形成的岩石将始终具有 Rocks formed at the Equator will always have a magnetic direction
从一侧指向另一侧的磁场方向（水平方向） that points from side to side.
由于岩石中的磁性矿物 The rocks have the ability to preserve magnetic direction
使得岩石具有保持磁场方向的能力 because of magnetic minerals in them.
这些磁性矿物颗粒就像一个个小指南针 自动按方向排列 They act like little compass needles and will line themselves up.
无论岩石随着大陆漂移数百万年到达那里 By finding out a rock's magnetic direction
通过找出岩石的磁场方向 Kirschvink is able to identify where in the world it was originally formed,
科什文克都能够识别出岩石 no matter where continental drift might have taken it
最初形成于地球的哪个地方 in the millions of years that followed.
在二十世纪八十年代， 科什文克听说有一个澳大利亚团队进行过测试 In the 1980s Kirschvink heard of a team of Australians
他们声称这些测试给出了热带曾被冰川覆盖的证据 who had run tests which, they claimed, showed evidence of glaciers in the tropics.
抱着怀疑的态度，科什文克决定用更先进的设备 Ever the sceptic, Kirschvink decided to duplicate their tests
重复他们的测试 on his own, more sophisticated equipment.
也就是复测的坠石要来自地球最热的热带地区 It meant examining drop stones from some of the hottest parts of the Earth.
贴纸：南非，霍塔泽尔，Ongeluk火山岩
要解答的问题是：冰层形成时，这些坠石就已经在赤道了 The question was: were they already at the Equator when they were formed by the ice,
上贴纸：中国南方 陈杰（音）凯文 下贴纸：（左）吉尔吉斯斯坦，Weberling，99年； （右）中国，陈杰（音）凯文，99年
还是在极地形成后，通过大陆漂移被带到热带 or had they been created at the poles and then carried to the tropics by continental drift?
为了解答这个问题，科什文克祭出他的心头好 To answer the question Kirschvink turned to his pride and joy:
一台定制的、高度灵敏的磁力计 a custom built, hypersensitive magnetometer.
这台仪器棒极了 It's, it's an incredible instrument.
将样品放入磁力计内腔 You lower the sample into the actual magnetometer chamber.
样品的弱磁场产生弱电流 The weak magnetic field of the sample generates an electric current
磁力计可以测量非常弱的磁体的磁矩 and so it allows us to measure the magnetic moment of an extraordinarily weak magnet,
十亿分之一，比如我认为的普通手持磁铁矿石品位分析仪的水平 one billionth, say, the level of an ordinary hand magnet I think.
磁力计联接上一台计算机 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,
科什文克 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.
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.
科什文克 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.
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.
To get out of the deep freeze
科什文克 what Kirschvink needed was a power that would stay hot,
even when the whole planet had frozen over,
布迪科 something that Budyko hadn't thought of,
something that could burn for ever,
something like hell.
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.
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.
科什文克 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,
科什文克 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,
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.
科什文克 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.
And that seemed to be a natural and possible escape,
certainly enough to break the snowball, the ice condition.
约瑟夫·科什文克 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.
The realisation that we may have found the way out of the snowball was wonderful.
科什文克 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.
保罗·霍夫曼 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.
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.
霍夫曼。。。。科什文克 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
纳米比亚 and he knew just where to start looking: Namibia.
霍夫曼。。。。纳米比亚 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.
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.
What really puzzled geologists was that caps made from calcium carbonate
are usually formed in warm water,
纳米比亚 but in Namibia they had suddenly appeared on top of an ice-cold glacier.
What this indicates is that you go from glacial conditions
to an ocean that is warm in a flash.
To most geologists this instant change from cold to hot rock defied their understanding,
霍夫曼 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?
霍夫曼 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,
丹·施拉格 so he turned to a geochemist, Dan Schrag.
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.
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.
The most elemental powers of nature would be unleashed upon the Earth.
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.
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 constituent parts,
one of which was calcium.
This would then fuse with the carbon in the acid rain.
The result: mountains of calcium carbonate
纳米比亚 exactly as they could see right above the drop stones in Namibia.
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.
霍夫曼和施拉格 Hoffman and Schrag had found the final evidence.
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.
霍夫曼和施拉格 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.
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.
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.
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.
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.
Light penetration through ice is not great
if the ice is more than a couple of tens of metres thick.
霍夫曼和施拉格 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.
You cut off photosynthesis
and within a few years, let alone a few million years, mass extinction.
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.
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?
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,
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.
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?
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.
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.
And McKay had a hunch as to why this photosynthetic life could survive.
It was something about the nature of the ice itself.
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.
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.
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.
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