前苏联气象分析员Mikhail Budyko Soviet climate modeller Mikhail Budyko
演示了runaway ice-albedo是怎么 demonstrated how runaway ice-albedo
让凉爽的地球完全冻结的 could cause a cooling earth to freeze completely.
正如他许多同时代的人一样 Like many of his contemporaries,
Budyko也很担忧核冬天的影响 Budyko was worried about the affects of a nuclear winter.
核冬天是指核爆炸产生的烟尘云团 Nuclear explosions create clouds of smoke and dust,
会阻碍太阳的能量 which block the sun's energy
并降低地球的温度 and lower the earth's temperature.
Budyko开发了一个数学建模 Budyko developed a mathematical model
来计算出全球进入冰期的几率 to work out the odds of a global ice age.
他的演示显示出如果地球的气候 He showed that if the earth's climate cooled enough
冷到了让低纬度结冰的话 for ice to form in the lower latitudes
那么ice albedo就会更快地上升 then the ice albedo would rise at a faster rate.
一旦冰川延伸到了一定的纬度 And once the glaciers reached a certain latitude,
整个地球就会变成一个大雪球状 the earth would tip into a snowball-like state.
冰原带 冰川以任何方式 Somehow or the other the ice line, the glaciers,
延伸到大约30度的临界纬度时 reached some critical latitude around 30 degrees
而且一旦它极其迅速地接触到30度纬度时 and once they hit that latitude very, very rapidly,
整个地球都会埋在雪里 the earth would have been entombed in ice.
Budyko开拓性的研究表明了 Budyko's ground-breaking work
这个滚雪球效应是怎么 showed how this snowballing effect
在短短几十年里让 could create a totally frozen planet
整个地球变成冰冻星球的 within just a few decades.
Budyko的研究极其重要 Budyko's work was extremely important
它展示了地球是很有可能 in demonstrating that it was possible
陷入气候灾难的 that earth might slip into a climate catastrophe
完全冰封的灾难 which was covered completely with ice.
在他之前还没有人意识到这种可能性 It had not been recognized as a possibility before that.
如果地球完全被冰封 If the earth became entirely covered in ice,
那么几乎它吸收的所有太阳的热量 nearly all the heat it received from the sun
都会被反射回太空 would be reflected back into space.
那么地球就被封锁在冰里了 The planet would become locked in ice.
所以如果 So if runaway freezing trapped the earth in ice,
那它是怎么逃走的呢 how did it ever escape?
有科学家提出 在6亿5千万前年 Some scientists suggest that 650 million years ago,
我们的地球曾被完全冰封于冰河时代 our entire planet was locked in an extreme ice age.
这是个非常特殊的事件 It was a unique event,
是由一系列的不可能发生的事引发的结果 the result of a set of improbable triggers.
他们大概是这么认为的 Or so they thought.
后来 在1997年有证据表明 Then, in 1997, evidence emerged that
冰河时期还不止发生过一次 it could have happened more than once.
这个证据就是南非的 It came from glacial deposits in South Africa
超过两亿年的冰积层 that were over 2 billion years old.
古地磁的证据表明 Paleomagnetic evidence suggests that
您看到的这些是在赤道附近形成的 these formed near to the equator.
地理学家Paul Hoffman认为 Geologist Paul Hoffman believes
这可以证明 this is could be proof
地球这一辈子至少 that the earth froze over completely
完全冻结过两次 at least twice in its history.
雪球地球理论的最佳证据就是 The best evidence for a snowball earth
22.2亿年前 2.22 billion years ago
字幕 is paleomagnetic evidence from South Africa.
字幕 There are glacial tills there, boulder clays,
字幕 that formed very close to the equator.
字幕 This discovery raised the possibility
字幕 that the snowball wasn't a single event,
字幕 but a recurring condition.
字幕 Scientists raced to find the trigger
字幕 for this earlier deep freeze.
字幕 The cooling appeared to coincide
字幕 with the great oxidation event.
字幕 Deep in the ocean,
字幕 some of the oldest organisms on earth
字幕 evolved a mechanism called photosynthesis:
字幕 the ability to convert carbon dioxide
字幕 into oxygen using the sun's energy.
字幕 As these photosynthesizing microbes thrived,
字幕 they had a profound affect on the earth's climate.
字幕 Two billion years ago,
字幕 our sun was weaker.
字幕 Our planet relied on greenhouse gases
字幕 like carbon dioxide and methane
字幕 to maintain its temperate climate.
字幕 But photosynthesis uses up carbon dioxide.
字幕 It also produces oxygen,
字幕 which destroys methane.
字幕 The result: a double-whammy.
字幕 Levels of both these vital greenhouse gases plummeted.
字幕 You have a precarious situation.
字幕 You have a planet that's greenhouse needs methane
字幕 and you suddenly evolve a bug that destroys the methane.
字幕 Maybe the first bacteria that photosynthesised
字幕 crashed the carbon dioxide and methane from the atmosphere
字幕 and triggered the first snowball earth.
字幕 As oxygen production soared,
字幕 methane levels tumbled.
字幕 With the loss of that greenhouse gas,
字幕 the earth spiralled into a deep freeze.
字幕 It's kind of a frightening conclusion
字幕 that a single celled mutant bug
字幕 destroyed an entire planetary ecosystem.
字幕 Destroyed it.
字幕 Turned it into a snowball for maybe
字幕 35 to 100 million years.
字幕 Some scientists now think that
字幕 our planet has been encased in ice
字幕 on at least three separate occasions.
字幕 And each time,
字幕 under different circumstances.
字幕 But these snowball earth scenarios present a huge problem:
字幕 if our planet was trapped in ice,
字幕 how did it ever escape?
字幕 Something must have caused a global meltdown.
字幕 Evidence of the thaw is right here.
字幕 On top of the dropstones
字幕 sit layers of a peculiar reddish-grey rock.
字幕 These layers stretch for miles
字幕 in the flinders range, death valley
字幕 and almost everywhere where the glacial rocks appear.
字幕 These are carbonate rocks,
字幕 which form in warm, shallow seas.
字幕 But here,
字幕 they appear right on top of the glacial deposits.
字幕 So here's a sample that actually
字幕 shows the end of the snowball event.
字幕 So that's an ice-rafted stone
字幕 that dropped out of an iceberg
字幕 and these are little grains
字幕 that also fell out of an iceberg,
字幕 and immediately on top of that
字幕 without any break in sedimentation is cap carbonate,
字幕 it's precipitating directly out of the ocean
字幕 and accumulating on the seafloor.
字幕 And exactly the same relationship is seen all over the world.
字幕 Their presence is evidence
字幕 of a sudden and drastic temperature increase.
字幕 Only one thing could have triggered this:
字幕 a huge injection of greenhouse gases
字幕 such as carbon dioxide.
字幕 In order to melt snowball earth,
字幕 concentrations of carbon dioxide
字幕 had to be around 350 times higher than they are today.
字幕 But if the whole planet was encased in a thick layer of ice:
字幕 where did the carbon dioxide come from?
字幕 Caltech professor Joe Kirschvink had a revelation.
字幕 I woke up in the middle of the night and said:
字幕 "Ah, that's gotta be it!"
字幕 It just dawned on me that these sub aerial volcanoes
字幕 like in the Aleutians, in Yellowstone...
字幕 These volcanoes sit and they spew co2 into the air.
字幕 Even beneath the ice,
字幕 volcanoes would have continued
字幕 to release immense volumes of carbon dioxide.
字幕 With the rest of the planet still ice-locked,
字幕 there would have been no mechanism
字幕 to remove it from the atmosphere.
字幕 Over millions of years,
字幕 levels of this greenhouse gas would have built up,
字幕 eventually producing enough warming
字幕 to off-set the ice albedo.
字幕 The ice melted,
字幕 sending global temperatures spiralling upwards.
字幕 As this melted ice water evaporated,
字幕 it swelled the atmosphere with moisture,
字幕 creating torrential rainfall.
字幕 Carbon dioxide in the atmosphere acidified the rain.
字幕 Acidic downpours rapidly eroded limestone,
字幕 washing sediment down into the oceans.
字幕 Here, layers of carbonate sediment
字幕 accumulated on the seafloor
字幕 to become carbonate rock.
字幕 Dan Schrag demonstrates what could have happened.
字幕 Instead of carbonic acid from carbon dioxide,
字幕 we're gonna use a little stronger acid
字幕 to make it go a little faster.
字幕 This is nitric acid
字幕 and when we add it to the limestone,
字幕 it will dissolve away the limestone
字幕 and ultimately create a very clear solution,
字幕 so you can see it bubbling away.
字幕 And when this is added to the ocean,
字幕 it reacts and creates the cap carbonates
字幕 that blanket at the top of the glacial deposits.
字幕 But still not everybody is convinced that
字幕 the planet was ever entirely covered in ice.
字幕 Some scientists argue that
字幕 the chances of life surviving a full-on snowball
字幕 are virtually nil.
字幕 Above the ice,
字幕 the freezing conditions were too hostile;
字幕 while below it,
字幕 life was cut off from sunlight.
字幕 Yet there is evidence that photosynthesizing organisms
字幕 did survive the freeze:
字幕 single-celled cyanobacteria existed right the way through.
字幕 Cyanobacteria can survive extremes of temperatures
字幕 many degrees above and below zero.
字幕 But if the oceans froze over,
字幕 a thick layer of ice would have sealed them away
字幕 from the sunlight.
字幕 With their energy source blocked,
字幕 they would have died out.
字幕 If you'd been diving on a tropical reef before snowball,
字幕 this is what you would've seen,
字幕 slime world.
字幕 Mounds of slime on the seafloor.
字幕 And if you touched them,
字幕 you'd find out that they're actually rock.
字幕 A surface covered with slimy material,
字幕 which is actually cyanobacteria,
字幕 the earliest, the most primitive form of life,
字幕 that's been around for 3 billion years up until this point,
字幕 and is still around today.
字幕 Critics argue that nothing could have survived
字幕 the ferocious conditions of a total global freeze.
字幕 They maintain snowball earth was nothing
字幕 more than a severe ice age
字幕 with extensive glaciers
字幕 and a belt of open water around the equator.
Phillip Allan Phillip Allan, a geologist at imperial College London,
字幕 believes that the evidence for a frozen planet is flawed.
字幕 I am not in agreement with the snowball earth hypothesis.