×ÖÄ» Imagine a world frozen solid.
×ÖÄ» Encased in ice from pole to pole.
×ÖÄ» No life roams these frozen plains.
×ÖÄ» Only glaciers move across them.
×ÖÄ» This is not some distant planet starved of sunlight.
×ÖÄ» According to a radical theory,
×ÖÄ» this was earth, 650 million years ago.
×ÖÄ» No one knows what triggered this calamitous freeze.
×ÖÄ» But if it happened once,
×ÖÄ» could it happen again?
×ÖÄ» Imagine the most profound ice age you can think of.
×ÖÄ» Temperatures plummet to below zero even at the equator.
×ÖÄ» Before long, ice encloses our entire planet.
×ÖÄ» Research suggests that this may have happened not once,
×ÖÄ» But three times in our planet's past.
×ÖÄ» Welcome to snowball earth.
×ÖÄ» A snowball earth is certainly
×ÖÄ» the most extreme climate change
×ÖÄ» we can conceive the earth has ever experienced!
×ÖÄ» What would happen if the world froze over today?
×ÖÄ» Massive glaciers creep from the poles,
×ÖÄ» crushing everything in their path.
×ÖÄ» First of all the Canadians are gonna have to
×ÖÄ» dig much deeper holes when they go ice fishing.
×ÖÄ» Next the governors of Washington, Idaho, Montana,
×ÖÄ» North Dakota, Minnesota, Michigan and Maine
×ÖÄ» will probably declare federal emergencies.
×ÖÄ» Cities grind to a halt.
×ÖÄ» Power lines collapse and water supplies freeze.
×ÖÄ» Cars won't start.
×ÖÄ» Roads are cut off by ice.
×ÖÄ» Buildings strain and crumble under deep drifts of snow.
×ÖÄ» Everything is entombed.
×ÖÄ» Our frail bodies struggle to survive.
×ÖÄ» I think human society would collapse
×ÖÄ» if a snowball earth happened again.
×ÖÄ» The oceans freeze over.
×ÖÄ» Fish die and marine ecosystems collapse.
×ÖÄ» Our food supply becomes exhausted.
×ÖÄ» Any surviving humans are trapped in an icy hell.
×ÖÄ» We'd have a mass die of humans simply because
×ÖÄ» we could not grow enough food under these conditions.
×ÖÄ» Incredible though it may seem,
×ÖÄ» the earth may have frozen over completely.
×ÖÄ» Scientists are finding clues in unexpected places.
×ÖÄ» The story begins with a mystery.
×ÖÄ» In the arid flinder's ranges in Southern Australia,
×ÖÄ» layers of ancient rock hold unusual stones.
×ÖÄ» These stones are foreign to the area.
×ÖÄ» They are known as "dropstones".
×ÖÄ» The dropstones are embedded in an ancient sedimentary seafloor
×ÖÄ» that formed around 650 million years ago.
×ÖÄ» Tim Raub,
×ÖÄ» a geologist at the Californian Institute of Technology,
×ÖÄ» hunts for these dropstones at the flinder's range.
×ÖÄ» This is an example of a dropstone,
×ÖÄ» a piece of ancient rock
×ÖÄ» that was dropped into the ancient mud on the sea bottom.
×ÖÄ» These dropstones are visibly different
×ÖÄ» from the sedimentary bedrock around them.
×ÖÄ» Something powerful must have carried them here - but what?
×ÖÄ» Dan Schrag,
×ÖÄ» professor of earth and planetary sciences at Harvard University,
×ÖÄ» believes the answer is glaciers.
×ÖÄ» We actually have ice cubes here
×ÖÄ» that we've made with pieces of sand and gravel in them,
×ÖÄ» so these ice cubes are like the glaciers
×ÖÄ» that are taking rocks from the land
×ÖÄ» and then when we put them in the water,
×ÖÄ» they'll actually melt
×ÖÄ» and the sand and gravel will fall to the bottom,
×ÖÄ» just like the dropstones on the ocean floor.
×ÖÄ» These dropstones are evidence that at some point,
×ÖÄ» this area was covered in a thick layer of ice.
×ÖÄ» But it's difficult to explain
×ÖÄ» how ice could have covered the sun-baked mountains
×ÖÄ» of Southern Australia.
×ÖÄ» Unless the continent itself was in a very different place.
×ÖÄ» The continents haven't always been where they are now.
×ÖÄ» They move very slowly across the earth
×ÖÄ» due to the movement of tectonic plates.
×ÖÄ» 650 million years ago,
×ÖÄ» around the time the flinders dropstones landed here,
×ÖÄ» the continents were positioned very differently.
×ÖÄ» Since the dropstones formed under ice,
×ÖÄ» perhaps Australia sat nearer to the poles
×ÖÄ» where temperatures were lower.
×ÖÄ» To locate the position of a continent in the distant past,
×ÖÄ» scientists study the magnetic identity of rocks.
×ÖÄ» Certain sedimentary rocks contain magnetic particles.
×ÖÄ» When the rocks form, these iron particles
×ÖÄ» line up with the earth's magnetic field.
×ÖÄ» Near the poles, they align at right angles
×ÖÄ» to the sedimentary layers.
×ÖÄ» And around the equator,
×ÖÄ» they align parallel to the layers.
×ÖÄ» Even if the rock moves,
×ÖÄ» the distinctive configuration of these magnetic particles
×ÖÄ» remains the same and provides a kind of
×ÖÄ» 'geological fingerprint' of the rock.
×ÖÄ» Professor Joe Kirschvink at Caltech, Pasadena,
×ÖÄ» is fascinated by magnetism.
×ÖÄ» Magnetism is magic.
×ÖÄ» I've even named my eldest child after a magnet.
×ÖÄ» So I enjoy magnetism.
×ÖÄ» Kirschvink and his team analysed
×ÖÄ» the 650-million-year-old rock surrounding the dropstones.
×ÖÄ» The results were unexpected.
×ÖÄ» That part of Australia was within 5 degrees of the equator.
×ÖÄ» And it just was the most weird place
×ÖÄ» to expect a glacier.
×ÖÄ» But Kirschvink had to be sure that
×ÖÄ» this ancient rock still held its original magnetic signature.
×ÖÄ» If the rock heated up after it formed,
×ÖÄ» its native magnetism would have changed.
×ÖÄ» Back at flinders range,
×ÖÄ» Kirschvink's team uncovered
×ÖÄ» clearly striped sedintary rock that provided proof.
×ÖÄ» Here. Tens of thousands of layers of mud
×ÖÄ» grew one at a time upward and upward in a glacial lake
×ÖÄ» at the edge of a 635-million-year-old ice sheet.
×ÖÄ» When this rock formed,
×ÖÄ» layers of sediment built up in distinctive horizontal lines.
×ÖÄ» By comparing the direction of the layers of sediment
×ÖÄ» with the direction of the lines of magnetism,
×ÖÄ» Kirschvink could determine
×ÖÄ» whether the rock's magnetic signature had changed.
×ÖÄ» We can explain the process of magnetism
×ÖÄ» with a simple eraser.
×ÖÄ» If the rocks formed on the equator
×ÖÄ» and gained a magnetism very early
×ÖÄ» we would expect the magnetic direction to be parallel
×ÖÄ» to the bedding very precisely like this.
×ÖÄ» And then since the rocks are soft
×ÖÄ» if they get squeezed or deformed geologically
×ÖÄ» you would expect them to rotate like this
×ÖÄ» and the magnetism would shift with the bedding.
×ÖÄ» On the other hand
×ÖÄ» if the rock had been squeezed and deformed first
×ÖÄ» and then gained the line of magnetism like this,
×ÖÄ» we would see them warped relative to the bedding today.
×ÖÄ» If the rock still had its original magnetic orientation,
×ÖÄ» the lines of magnetism
×ÖÄ» would follow the lines of sediment.
×ÖÄ» But if its orientation had changed,
×ÖÄ» the magnetism would cut through the sedimentary layers.
×ÖÄ» Raub drills into the formation
×ÖÄ» to retrieve solid cores of the rhythmic rocks.
×ÖÄ» Back in the lab,
×ÖÄ» the team analyses samples of rock
×ÖÄ» using a custom-built,
×ÖÄ» hypersensitive magnetometer.
×ÖÄ» This magnetometer measured the orientation
×ÖÄ» of the magnetic particles in the rock.
×ÖÄ» It confirms that the particles
×ÖÄ» were aligned with the rock layers.
×ÖÄ» Kirschvink was astonished.
×ÖÄ» Quite frankly when I saw the original study
×ÖÄ» I didn't believe it.
×ÖÄ» What in fact we see is that they're like this
×ÖÄ» and as the rock was warped the magnetism warped.
×ÖÄ» That proves conclusively
×ÖÄ» the magnetism formed before the rock was solidified.
×ÖÄ» This discovery seems to provide irrefutable evidence that
×ÖÄ» the glacial rocks formed close to the equator.
×ÖÄ» Paul Hoffman, professor of geology at Harvard University,
×ÖÄ» was intrigued.
×ÖÄ» So if there were glaciers close to the equator
×ÖÄ» where the temperature was warmest,
×ÖÄ» one has to assume that there was also ice
×ÖÄ» in higher latitudes where the temperatures were colder.
×ÖÄ» 650 million years ago,
×ÖÄ» Australia sat near the equator.
×ÖÄ» And yet evidence suggests
×ÖÄ» that its arid rocks lay buried beneath ice.
×ÖÄ» And if there was ice at the equator,
×ÖÄ» there would have been ice all over the globe.
×ÖÄ» These dropstones became the catalyst for a radical theory:
×ÖÄ» snowball earth.
×ÖÄ» This is a theory about the most extreme ice age imaginable.
×ÖÄ» And if it happened before,
×ÖÄ» it could happen again.
×ÖÄ» Our world has been through many periods
×ÖÄ» of extreme climate change.
×ÖÄ» But some scientists have proposed
×ÖÄ» that 650 million years ago,
×ÖÄ» the earth may have frozen over completely.
×ÖÄ» This radical hypothesis raises one crucial question:
×ÖÄ» how could it have happened?
×ÖÄ» Kirschvink proposes that the giant freeze
×ÖÄ» was triggered by the same mechanism
×ÖÄ» that lies behind today's global warming:
×ÖÄ» the greenhouse effect.
×ÖÄ» Except 650 million years ago,
×ÖÄ» it worked in reverse.
×ÖÄ» Greenhouse gases in our atmosphere
×ÖÄ» act like an insulating blanket,
×ÖÄ» trapping the sun's energy
×ÖÄ» to keep earth's temperatures mild.
×ÖÄ» When the snowball began, scientists suspect that
×ÖÄ» levels of these vital gases fell dramatically.
×ÖÄ» The earth's atmosphere might have been filled with
×ÖÄ» methane and carbon dioxide
×ÖÄ» and the earth became addicted
×ÖÄ» to this high level carbon dioxide and methane
×ÖÄ» in order to keep its temperature.
×ÖÄ» Then if some unknown event crashed
×ÖÄ» the carbon dioxide out of the atmosphere
×ÖÄ» earth might have run away into a snowballed earth.
×ÖÄ» If enough carbon dioxide
×ÖÄ» had been pulled out of the atmosphere,
×ÖÄ» an anti-greenhouse effect would have
×ÖÄ» plunged the earth into a deep freeze.
×ÖÄ» But this theory is only possible
×ÖÄ» if there's a mechanism
×ÖÄ» that could pull all the co2 out of the air.
×ÖÄ» In fact, carbon dioxide is removed all the time
×ÖÄ» through a process called silicate weathering.
×ÖÄ» Weathering is a geological term
×ÖÄ» for the decomposition of rocks
×ÖÄ» under the influence of the weather.
×ÖÄ» It's a process that converts solid rock into soil.
×ÖÄ» Now its importance for the climate
×ÖÄ» is that weathering consumes carbon dioxide,
×ÖÄ» which is a major greenhouse gas in the atmosphere.
×ÖÄ» Carbon dioxide reacts with water vapour in the atmosphere
×ÖÄ» to form carbonic acid rain.
×ÖÄ» This acidic rain weathers limestone on land
×ÖÄ» and carries away the eroded sediment to the sea.
×ÖÄ» Here it builds up,
×ÖÄ» eventually forming limestone on the seafloor.
×ÖÄ» So the presence of the limestone
×ÖÄ» is the evidence that weathering is occurring.
×ÖÄ» The more carbon dioxide gets locked away in limestone,
×ÖÄ» the cooler the earth becomes.
×ÖÄ» But if this is happening all the time,
×ÖÄ» something must have amplified the process
×ÖÄ» for the whole world to have frozen over.
×ÖÄ» Again, clues lie in the position of the continents.
×ÖÄ» The movement of tectonic plates continually
×ÖÄ» shifts the continents around the globe.
×ÖÄ» Today, a lot of the continents
×ÖÄ» are located in higher latitudes.
×ÖÄ» If the earth cools,
×ÖÄ» the continents near the poles ice over.
×ÖÄ» The ice prevents the rocks beneath
×ÖÄ» from being weathered by carbonic acid rain.
×ÖÄ» This means that the weathering cycle is kept in balance.
×ÖÄ» With less carbon dioxide locked away
×ÖÄ» in limestone rock and more in the atmosphere,
×ÖÄ» the planet warms.
×ÖÄ» Scientists have worked out
×ÖÄ» that 650 million years ago,
×ÖÄ» the continents were clustered around the equator,
×ÖÄ» where it's warm and wet.
×ÖÄ» Consequently, the global rate of weathering
×ÖÄ» would have been higher.
×ÖÄ» During the snowball earth we suspect that
×ÖÄ» there were many more continents at the equator
×ÖÄ» and this would actually lead to a situation
×ÖÄ» that would make the planet vulnerable
×ÖÄ» to a global glaciations.
×ÖÄ» As the earth cooled,
×ÖÄ» ice formed at the poles.
×ÖÄ» But because there were no continents in the higer latitudes,
×ÖÄ» ice accumulated on sea rather than on land.
×ÖÄ» There was nothing to regulate the weathering.
×ÖÄ» The cooling continued unchecked.
×ÖÄ» And the world got colder and colder.
×ÖÄ» This runaway scenario is certainly plausible.
×ÖÄ» But although extreme weathering could be behind an ice age,
×ÖÄ» it doesn't explain how the earth
×ÖÄ» could have iced over completely.
×ÖÄ» Geologist Galen Halverson from Adelaide University
×ÖÄ» studies triggers of the snowball.
×ÖÄ» One of the hardest parts about
×ÖÄ» understanding the snowball is really how it began.
×ÖÄ» We think that it must have began like a normal glaciation.
×ÖÄ» Normally, living things help to regulate the earth's climate
×ÖÄ» by maintaining the delicate balance of carbon dioxide.
×ÖÄ» Plants consume carbon dioxide.
×ÖÄ» Animals produce it.
×ÖÄ» These consumers and producers
×ÖÄ» help to regulate the climate,
×ÖÄ» ensuring that the earth is neither too hot nor too cold.
×ÖÄ» If there's more carbon dioxide in the atmosphere,
×ÖÄ» it gets warmer.
×ÖÄ» This causes plants to grow faster
×ÖÄ» which in turn draws out more co2 from the air.
×ÖÄ» So that is a feedback against it getting too warm.
×ÖÄ» And in reverse if it starts getting too cold
×ÖÄ» then the biosphere responds
×ÖÄ» by not drying down as much co2
×ÖÄ» and that allows co2 to go back up
×ÖÄ» so there's a sort of buffer
×ÖÄ» in the form of life on earth
×ÖÄ» that prevents temperatures from getting too cold
×ÖÄ» or from getting too hot.
×ÖÄ» But rewind 650 million years
×ÖÄ» and witness a very different story.
×ÖÄ» Life on land was in its early stages of evolution.
×ÖÄ» Most living things were in the oceans.
×ÖÄ» This half-formed biosphere could not prevent the earth
×ÖÄ» from swinging to wild extremes of temperature.
×ÖÄ» Colonies of single-celled organisms called cyanobacteria
×ÖÄ» were the first to photosynthesize.
×ÖÄ» They pulled carbon dioxide from the surrounding water
×ÖÄ» to precipitate calcium carbonate
×ÖÄ» and they used this to make dome-like structures of rock.
×ÖÄ» Geologist Jim Gehling is an expert on these early life forms.
×ÖÄ» Temperatures were dropping at this time,
×ÖÄ» 650 million years ago,
×ÖÄ» for the very good reason that carbon dioxide,
×ÖÄ» the principle greenhouse gas,
×ÖÄ» was being taken out of the atmosphere
×ÖÄ» and locked away on the seafloor.
×ÖÄ» This trapped carbon dioxide can still be seen today
×ÖÄ» in stromatolite reefs.
×ÖÄ» This is actually solid carbon dioxide,
×ÖÄ» and I can release that carbon dioxide
×ÖÄ» after all those hundreds of millions of years,
×ÖÄ» when I put this weak acid on the rock.
×ÖÄ» And there you see it erupt
×ÖÄ» as white bubbles of carbon dioxide
×ÖÄ» spring out of the rock,
×ÖÄ» go back into the atmosphere,
×ÖÄ» to take up their role all over again as greenhouse gases.
×ÖÄ» The more carbon dioxide the cyanobacteria locked away,
×ÖÄ» the cooler earth became.
×ÖÄ» Our planet was hurtling towards a deep freeze
×ÖÄ» that would last for millions of years.
×ÖÄ» 650 million years ago
×ÖÄ» the earth's temperature plummeted.
×ÖÄ» Huge sheets of ice spread across the planet.
×ÖÄ» Eventually, they engulfed most of the land and sea.
×ÖÄ» This was no regular ice age.
×ÖÄ» Ice ages are nothing out of the ordinary.
×ÖÄ» We experience them every hundred thousand years or so.
×ÖÄ» They are mainly caused by
×ÖÄ» small changes in the earth's orbit,
×ÖÄ» which alter its position relative to the sun.
×ÖÄ» When our planet receives less of the sun's energy
×ÖÄ» over a number of years,
×ÖÄ» ice forms high in the northern hemisphere and advances down.
×ÖÄ» At the height of the last ice age 21,000 years ago,
×ÖÄ» these icy sheets reached down as far as New York.
×ÖÄ» But the tropics stayed warm.
×ÖÄ» Only about 30 percent of our planet froze.
×ÖÄ» But 650 million years ago,
×ÖÄ» some scientists believe the ice kept coming,
×ÖÄ» until eventually, most of the globe was covered.
×ÖÄ» What could have caused this runaway cooling?
×ÖÄ» The answer must lie in
×ÖÄ» how our planet keeps or loses the sun's energy.
×ÖÄ» The earth's surface is made up of land,
×ÖÄ» open ocean and ice.
×ÖÄ» These all reflect sunlight to a different degree.
×ÖÄ» The amount of light a surface reflects
×ÖÄ» is known as its albedo.
×ÖÄ» Deep, dark ocean has a low albedo,
×ÖÄ» since it absorbs a lot of sunlight.
×ÖÄ» White snow and ice have a very high albedo,
×ÖÄ» since they reflect more of the sun's energy back into space.
×ÖÄ» Tim Raub demonstrates how dark and light colours
×ÖÄ» heat up under the sun.
×ÖÄ» The temperature of a white car sitting in the sun
×ÖÄ» is only 120 degrees.
×ÖÄ» The temperature of a black car sitting in the sun
×ÖÄ» is 144 degrees.
×ÖÄ» The black car absorbed light;
×ÖÄ» the white car reflected it.
×ÖÄ» White reflects more sunlight,
×ÖÄ» keeping whatever it's covering cooler.
×ÖÄ» The more ice that covers the earth,
×ÖÄ» the more of the sun's energy reflects back into space
×ÖÄ» and the colder the planet becomes.
×ÖÄ» This cooling leads to more ice forming,
×ÖÄ» which further increases the surface albedo.
×ÖÄ» This is known as ice-albedo feedback.
×ÖÄ» And something may have accelerated
×ÖÄ» this cycle of cooling even more.
×ÖÄ» New research suggests that
×ÖÄ» when sea water freezes at very low temperatures,
×ÖÄ» its composition changes
×ÖÄ» and it can become more reflective.
×ÖÄ» At the University of Washington's Polar Science Center,
×ÖÄ» Bonnie Light studies the relationship between ice and light.
×ÖÄ» In the cold room,
×ÖÄ» she freezes buckets of saltwater
×ÖÄ» at different temperatures.
×ÖÄ» One of the buckets is exposed to
×ÖÄ» snowball earth temperatures of minus 10 degrees.
×ÖÄ» The surface of this ice becomes much whiter.
×ÖÄ» Okay, so this bucket contains sea water
×ÖÄ» that was frozen in a cold room
×ÖÄ» at 5 degrees above zero Fahrenheit,
×ÖÄ» And we consider this a good laboratory analogue
×ÖÄ» for modern day sea ice on planet earth.
×ÖÄ» And you can see that the surface of it
×ÖÄ» is kind of grey coloured, dull appearance.
×ÖÄ» And this bucket also has salt water in it,
×ÖÄ» although this salt water was frozen
×ÖÄ» at about 10 degrees below zero Fahrenheit,
×ÖÄ» and we consider the ice in this bucket
×ÖÄ» to be a good analogue for sea ice on snowball earth,
×ÖÄ» because it's very cold.
×ÖÄ» At snowball earth temperatures,
×ÖÄ» the composition of the sea ice changes,
×ÖÄ» turning its surface from grey to bright white.
×ÖÄ» And you can see that these crystals, in this case,
×ÖÄ» have started to collect at the surface of this cold ice.
×ÖÄ» And so I can kinda scratch away these crystals,
×ÖÄ» and you see that it's basically just a deposit
×ÖÄ» on the surface of this cold ice.
×ÖÄ» At minus ten degrees,
×ÖÄ» salt molecules in the ice begin form tiny crystals.
×ÖÄ» If you can isolate them under the microscope,
×ÖÄ» um... they look like little gemstones.
×ÖÄ» They're really quite beautiful.
×ÖÄ» These bright crystals have a dramatic effect on albedo.
×ÖÄ» The white surface scatters more of the sun's energy
×ÖÄ» and reflects it back to the atmosphere,
×ÖÄ» causing temperatures to drop even further.
×ÖÄ» In the 1960s,
×ÖÄ» Soviet climate modeller Mikhail Budyko
×ÖÄ» demonstrated how runaway ice-albedo
×ÖÄ» could cause a cooling earth to freeze completely.
×ÖÄ» Like many of his contemporaries,
×ÖÄ» Budyko was worried about the affects of a nuclear winter.
×ÖÄ» Nuclear explosions create clouds of smoke and dust,
×ÖÄ» which block the sun's energy
×ÖÄ» and lower the earth's temperature.
×ÖÄ» Budyko developed a mathematical model
×ÖÄ» to work out the odds of a global ice age.
×ÖÄ» He showed that if the earth's climate cooled enough
×ÖÄ» for ice to form in the lower latitudes
×ÖÄ» then the ice albedo would rise at a faster rate.
×ÖÄ» And once the glaciers reached a certain latitude,
×ÖÄ» the earth would tip into a snowball-like state.
×ÖÄ» Somehow or the other the ice line, the glaciers,
×ÖÄ» reached some critical latitude around 30 degrees
×ÖÄ» and once they hit that latitude very, very rapidly,
×ÖÄ» the earth would have been entombed in ice.
×ÖÄ» Budyko's ground-breaking work
×ÖÄ» showed how this snowballing effect
×ÖÄ» could create a totally frozen planet
×ÖÄ» within just a few decades.
×ÖÄ» Budyko's work was extremely important
×ÖÄ» in demonstrating that it was possible
×ÖÄ» that earth might slip into a climate catastrophe
×ÖÄ» which was covered completely with ice.
×ÖÄ» It had not been recognized as a possibility before that.
×ÖÄ» If the earth became entirely covered in ice,
×ÖÄ» nearly all the heat it received from the sun
×ÖÄ» would be reflected back into space.
×ÖÄ» The planet would become locked in ice.
×ÖÄ» So if runaway freezing trapped the earth in ice,
×ÖÄ» how did it ever escape?
×ÖÄ» Some scientists suggest that 650 million years ago,
×ÖÄ» our entire planet was locked in an extreme ice age.
×ÖÄ» It was a unique event,
×ÖÄ» the result of a set of improbable triggers.
×ÖÄ» Or so they thought.
×ÖÄ» Then, in 1997, evidence emerged that
×ÖÄ» it could have happened more than once.
×ÖÄ» It came from glacial deposits in South Africa
×ÖÄ» that were over 2 billion years old.
×ÖÄ» Paleomagnetic evidence suggests that
×ÖÄ» these formed near to the equator.
×ÖÄ» Geologist Paul Hoffman believes
×ÖÄ» this is could be proof
×ÖÄ» that the earth froze over completely
×ÖÄ» at least twice in its history.
×ÖÄ» The best evidence for a snowball earth
×ÖÄ» 2.22 billion years ago
×ÖÄ» is paleomagnetic evidence from South Africa.
×ÖÄ» There are glacial tills there, boulder clays,
×ÖÄ» that formed very close to the equator.
×ÖÄ» This discovery raised the possibility
×ÖÄ» that the snowball wasn't a single event,
×ÖÄ» but a recurring condition.
×ÖÄ» Scientists raced to find the trigger
×ÖÄ» for this earlier deep freeze.
×ÖÄ» The cooling appeared to coincide
×ÖÄ» with the great oxidation event.
×ÖÄ» Deep in the ocean,
×ÖÄ» some of the oldest organisms on earth
×ÖÄ» evolved a mechanism called photosynthesis:
×ÖÄ» the ability to convert carbon dioxide
×ÖÄ» into oxygen using the sun's energy.
×ÖÄ» As these photosynthesizing microbes thrived,
×ÖÄ» they had a profound affect on the earth's climate.
×ÖÄ» Two billion years ago,
×ÖÄ» our sun was weaker.
×ÖÄ» Our planet relied on greenhouse gases
×ÖÄ» like carbon dioxide and methane
×ÖÄ» to maintain its temperate climate.
×ÖÄ» But photosynthesis uses up carbon dioxide.
×ÖÄ» It also produces oxygen,
×ÖÄ» which destroys methane.
×ÖÄ» The result: a double-whammy.
×ÖÄ» Levels of both these vital greenhouse gases plummeted.
×ÖÄ» You have a precarious situation.
×ÖÄ» You have a planet that's greenhouse needs methane
×ÖÄ» and you suddenly evolve a bug that destroys the methane.
×ÖÄ» Maybe the first bacteria that photosynthesised
×ÖÄ» crashed the carbon dioxide and methane from the atmosphere
×ÖÄ» and triggered the first snowball earth.
×ÖÄ» As oxygen production soared,
×ÖÄ» methane levels tumbled.
×ÖÄ» With the loss of that greenhouse gas,
×ÖÄ» the earth spiralled into a deep freeze.
×ÖÄ» It's kind of a frightening conclusion
×ÖÄ» that a single celled mutant bug
×ÖÄ» destroyed an entire planetary ecosystem.
×ÖÄ» Destroyed it.
×ÖÄ» Turned it into a snowball for maybe
×ÖÄ» 35 to 100 million years.
×ÖÄ» Some scientists now think that
×ÖÄ» our planet has been encased in ice
×ÖÄ» on at least three separate occasions.
×ÖÄ» And each time,
×ÖÄ» under different circumstances.
×ÖÄ» But these snowball earth scenarios present a huge problem:
×ÖÄ» if our planet was trapped in ice,
×ÖÄ» how did it ever escape?
×ÖÄ» Something must have caused a global meltdown.
×ÖÄ» Evidence of the thaw is right here.
×ÖÄ» On top of the dropstones
×ÖÄ» sit layers of a peculiar reddish-grey rock.
×ÖÄ» These layers stretch for miles
×ÖÄ» in the flinders range, death valley
×ÖÄ» and almost everywhere where the glacial rocks appear.
×ÖÄ» These are carbonate rocks,
×ÖÄ» which form in warm, shallow seas.
×ÖÄ» But here,
×ÖÄ» they appear right on top of the glacial deposits.
×ÖÄ» So here's a sample that actually
×ÖÄ» shows the end of the snowball event.
×ÖÄ» So that's an ice-rafted stone
×ÖÄ» that dropped out of an iceberg
×ÖÄ» and these are little grains
×ÖÄ» that also fell out of an iceberg,
×ÖÄ» and immediately on top of that
×ÖÄ» without any break in sedimentation is cap carbonate,
×ÖÄ» it's precipitating directly out of the ocean
×ÖÄ» and accumulating on the seafloor.
×ÖÄ» And exactly the same relationship is seen all over the world.
×ÖÄ» Their presence is evidence
×ÖÄ» of a sudden and drastic temperature increase.
×ÖÄ» Only one thing could have triggered this:
×ÖÄ» a huge injection of greenhouse gases
×ÖÄ» such as carbon dioxide.
×ÖÄ» In order to melt snowball earth,
×ÖÄ» concentrations of carbon dioxide
×ÖÄ» had to be around 350 times higher than they are today.
×ÖÄ» But if the whole planet was encased in a thick layer of ice:
×ÖÄ» where did the carbon dioxide come from?
×ÖÄ» Caltech professor Joe Kirschvink had a revelation.
×ÖÄ» I woke up in the middle of the night and said:
×ÖÄ» "Ah, that's gotta be it!"
×ÖÄ» It just dawned on me that these sub aerial volcanoes
×ÖÄ» like in the Aleutians, in Yellowstone...
×ÖÄ» These volcanoes sit and they spew co2 into the air.
×ÖÄ» Even beneath the ice,
×ÖÄ» volcanoes would have continued
×ÖÄ» to release immense volumes of carbon dioxide.
×ÖÄ» With the rest of the planet still ice-locked,
×ÖÄ» there would have been no mechanism
×ÖÄ» to remove it from the atmosphere.
×ÖÄ» Over millions of years,
×ÖÄ» levels of this greenhouse gas would have built up,
×ÖÄ» eventually producing enough warming
×ÖÄ» to off-set the ice albedo.
×ÖÄ» The ice melted,
×ÖÄ» sending global temperatures spiralling upwards.
×ÖÄ» As this melted ice water evaporated,
×ÖÄ» it swelled the atmosphere with moisture,
×ÖÄ» creating torrential rainfall.
×ÖÄ» Carbon dioxide in the atmosphere acidified the rain.
×ÖÄ» Acidic downpours rapidly eroded limestone,
×ÖÄ» washing sediment down into the oceans.
×ÖÄ» Here, layers of carbonate sediment
×ÖÄ» accumulated on the seafloor
×ÖÄ» to become carbonate rock.
×ÖÄ» Dan Schrag demonstrates what could have happened.
×ÖÄ» Instead of carbonic acid from carbon dioxide,
×ÖÄ» we're gonna use a little stronger acid
×ÖÄ» to make it go a little faster.
×ÖÄ» This is nitric acid
×ÖÄ» and when we add it to the limestone,
×ÖÄ» it will dissolve away the limestone
×ÖÄ» and ultimately create a very clear solution,
×ÖÄ» so you can see it bubbling away.
×ÖÄ» And when this is added to the ocean,
×ÖÄ» it reacts and creates the cap carbonates
×ÖÄ» that blanket at the top of the glacial deposits.
×ÖÄ» But still not everybody is convinced that
×ÖÄ» the planet was ever entirely covered in ice.
×ÖÄ» Some scientists argue that
×ÖÄ» the chances of life surviving a full-on snowball
×ÖÄ» are virtually nil.
×ÖÄ» Above the ice,
×ÖÄ» the freezing conditions were too hostile;
×ÖÄ» while below it,
×ÖÄ» life was cut off from sunlight.
×ÖÄ» Yet there is evidence that photosynthesizing organisms
×ÖÄ» did survive the freeze:
×ÖÄ» single-celled cyanobacteria existed right the way through.
×ÖÄ» Cyanobacteria can survive extremes of temperatures
×ÖÄ» many degrees above and below zero.
×ÖÄ» But if the oceans froze over,
×ÖÄ» a thick layer of ice would have sealed them away
×ÖÄ» from the sunlight.
×ÖÄ» With their energy source blocked,
×ÖÄ» they would have died out.
×ÖÄ» If you'd been diving on a tropical reef before snowball,
×ÖÄ» this is what you would've seen,
×ÖÄ» slime world.
×ÖÄ» Mounds of slime on the seafloor.
×ÖÄ» And if you touched them,
×ÖÄ» you'd find out that they're actually rock.
×ÖÄ» A surface covered with slimy material,
×ÖÄ» which is actually cyanobacteria,
×ÖÄ» the earliest, the most primitive form of life,
×ÖÄ» that's been around for 3 billion years up until this point,
×ÖÄ» and is still around today.
×ÖÄ» Critics argue that nothing could have survived
×ÖÄ» the ferocious conditions of a total global freeze.
×ÖÄ» They maintain snowball earth was nothing
×ÖÄ» more than a severe ice age
×ÖÄ» with extensive glaciers
×ÖÄ» and a belt of open water around the equator.
×ÖÄ» Phillip Allan, a geologist at imperial College London,
×ÖÄ» believes that the evidence for a frozen planet is flawed.
×ÖÄ» I am not in agreement with the snowball earth hypothesis.
×ÖÄ» The major hole in the theory as far as I am concerned
×ÖÄ» is the sedimentary rocks deposited
×ÖÄ» during the glacial period do not support
×ÖÄ» the idea of a long term global freeze.
×ÖÄ» Phillip Allan's counter evidence
×ÖÄ» includes the ripple-like patterns in rocks
×ÖÄ» formed 650 million years ago.
×ÖÄ» When you look at these sedimentary rocks
×ÖÄ» it indicates open water.
×ÖÄ» The wind was able to blow over the water surface
×ÖÄ» and form ripples,
×ÖÄ» wave ripples on the seabed.
×ÖÄ» So they are really conclusive proof of an open water surface
×ÖÄ» rather than an ice covered one.
×ÖÄ» Waves in an open sea left imprints
×ÖÄ» in the sediment of the seafloor.
×ÖÄ» When the water disappeared
×ÖÄ» and the sediment turned into rock,
×ÖÄ» the ripples remained.
×ÖÄ» Allan and others propose an alternative to the snowball earth.
×ÖÄ» It's known as the slushball.
×ÖÄ» In their version of events,
×ÖÄ» the earth never froze completely.
×ÖÄ» Some patches of open land and sea remained.
×ÖÄ» But proponents of the snowball theory don't suggest
×ÖÄ» that the frozen planet was without any open water.
×ÖÄ» Schrag believes that life could have survived in ice holes
×ÖÄ» around sources of geothermal energy.
×ÖÄ» Even in a completely ice covered planet
×ÖÄ» there's still plenty of places
×ÖÄ» where there are hot springs,
×ÖÄ» geo-thermal heat from volcanoes;
×ÖÄ» probably thousands and thousands of such little hole's
×ÖÄ» and in many of those little hole's
×ÖÄ» you could sustain life
×ÖÄ» throughout the snowball earth period.
×ÖÄ» The reality is we may never know
×ÖÄ» if the world was ever completely frozen over.
×ÖÄ» Sometimes people think of the snowball earth like a cartoon,
×ÖÄ» an over-simplified planet
×ÖÄ» but the real world is much more complicated than that.
×ÖÄ» Whether slushball or snowball,
×ÖÄ» everyone agrees there was a profound global freeze.
×ÖÄ» And when our world emerged,
×ÖÄ» 635 million years ago,
×ÖÄ» something unexpected appeared:
×ÖÄ» Life,
×ÖÄ» far more complex than anything that had gone before.
×ÖÄ» After the big freeze,
×ÖÄ» life on earth exploded.
×ÖÄ» Our planet, 650 million years ago.
×ÖÄ» Temperatures plummeted,
×ÖÄ» turning our planet into a hostile place.
×ÖÄ» Yet somehow, life survived.
×ÖÄ» And as soon as the frozen earth began to thaw,
×ÖÄ» it(ÓÐÒ»¸ö´ÊÐèÒªÌýÒë),
×ÖÄ» and there was an unprecedented explosion
×ÖÄ» of life on earth.
×ÖÄ» Before the deep freeze,
×ÖÄ» life was primitive.
×ÖÄ» Single-celled organisms and algae
×ÖÄ» were mostly confined to the oceans.
×ÖÄ» Shortly after it ended,
×ÖÄ» the first multicellular animals appeared.
×ÖÄ» The big question is,
×ÖÄ» did the global cooling trigger this great leap in evolution?
×ÖÄ» The best evidence is the coincidence in timing,
×ÖÄ» the fact that the first fossils
×ÖÄ» of multicellular animals
×ÖÄ» occur less than 3 million years
×ÖÄ» after the last snowball termination.
×ÖÄ» Shuhai Xiao is a geologist at Virginia Tech.
×ÖÄ» He unearthed some of the oldest
×ÖÄ» multicellular animal fossils in the world,
×ÖÄ» in a mine in southern China.
×ÖÄ» The fossils are really small.
×ÖÄ» They're about half millimetre in diameter.
×ÖÄ» So you can barely see them with naked eyes.
×ÖÄ» Under a microscope,
×ÖÄ» he discovers that the fossils
×ÖÄ» are very primitive animal embryos.
×ÖÄ» Some fossils show clusters of
×ÖÄ» 2, 4, 8 and more cells.
×ÖÄ» It was really exciting when we found those.
×ÖÄ» Those are some of the earliest animal fossils
×ÖÄ» and they have found in rocks
×ÖÄ» that were deposited
×ÖÄ» almost immediately after the snowball earth ended.
×ÖÄ» Fossils of these animal embryos date back to
×ÖÄ» just 3 million years after the global freeze ended.
×ÖÄ» That might seem a long time in human terms,
×ÖÄ» but geologically, it's the blink of an eye.
×ÖÄ» Over the next 80 million years,
×ÖÄ» animal life exploded.
×ÖÄ» In the flinders ranges in southern Australia,
×ÖÄ» geologists have found some of the oldest fossil evidence
×ÖÄ» of complex, multicellular life.
×ÖÄ» These creatures bear little resemblance to life today,
×ÖÄ» but they offer clues to the genesis of early life.
×ÖÄ» 650 million years ago,
×ÖÄ» this area was beneath a shallow sea.
×ÖÄ» Bizarre, soft-bodied organisms similar to
×ÖÄ» jellyfish or flatworms lived on the seafloor.
×ÖÄ» Some of the organisms got trapped in
×ÖÄ» fine silt of tidal flows where they fossilized.
×ÖÄ» Jim Gehling has been investigating these early animals.
×ÖÄ» Probably our ancestors lie in these rocks.
×ÖÄ» This thing here, Dickensonia,
×ÖÄ» can best be described as some kind of a pancake
×ÖÄ» or mat-like worm creature.
×ÖÄ» It doesn't have a proper head,
×ÖÄ» has what appears to be a gut,
×ÖÄ» very fine segments.
×ÖÄ» But this doesn't explain how the severe planetary freeze
×ÖÄ» could have triggered a leap in evolution.
×ÖÄ» What we do know is that there is a striking coincidence
×ÖÄ» between the rise of oxygen and the end of the snowball.
×ÖÄ» Oxygen levels rise following these global ice ages
×ÖÄ» because of a chemical reaction between ice and sunlight.
×ÖÄ» 650 million years ago,
×ÖÄ» there was no ozone to stop the sun's ultraviolet light
×ÖÄ» hitting the earth's icy surface.
×ÖÄ» This UV light reacted with water vapour
×ÖÄ» to form hydrogen peroxide.
×ÖÄ» The hydrogen peroxide remained in the ice
×ÖÄ» for millions of years.
×ÖÄ» But when the ice melted,
×ÖÄ» the hydrogen escaped,
×ÖÄ» and the peroxide converted into water and oxygen.
×ÖÄ» Oxygen reached the same level as we have today.
×ÖÄ» At roughly the same time,
×ÖÄ» the first complex animals appeared.
×ÖÄ» More oxygen enabled life to become multicellular.
×ÖÄ» A single cell can take up gases and oxygen from water.
×ÖÄ» Oxygen fuses across the cell membrane into the cell.
×ÖÄ» However, when several cells link together,
×ÖÄ» it's harder for oxygen to diffuse from one cell to the next.
×ÖÄ» The pressure of oxygen needs to be higher.
×ÖÄ» You need to have a high enough pressure of oxygen
×ÖÄ» to have a circulatory system
×ÖÄ» bring the oxygen all around the body.
×ÖÄ» After the last snowball earth,
×ÖÄ» oxygen seems finally to have been high enough
×ÖÄ» to support hundreds, thousands,
×ÖÄ» millions of small early animals on the seafloor.
×ÖÄ» Oxygen turned out to be the lynchpin of evolution.
×ÖÄ» The ability to use oxygen
×ÖÄ» enabled life to become dramatically more diverse.
×ÖÄ» Oxygen gives us 20 times more energy
×ÖÄ» or more than 20 times more energy
×ÖÄ» than any other thing that we can breathe
×ÖÄ» or that life can breathe.
×ÖÄ» You can have bigger animals.
×ÖÄ» You can have more active organisms.
×ÖÄ» You can have plants that grow
×ÖÄ» and transport liquids from roots to the leaves.
×ÖÄ» And, you know, you can have birds energetic enough to fly.
×ÖÄ» Without oxygen, we would all be slime.
×ÖÄ» A frozen earth may have given early life a push forward,
×ÖÄ» but if the world were to freeze over again,
×ÖÄ» we'd struggle to survive.
×ÖÄ» We know it's happened several times before.
×ÖÄ» The question is: could it happen again?
×ÖÄ» One factor in our favour is the sun.
×ÖÄ» Over the last 4.8 billion years,
×ÖÄ» it has been getting more and more active
×ÖÄ» as nuclear fusion generates more and more heat.
×ÖÄ» The sun today is about 6% warmer
×ÖÄ» than 650 million years ago.
×ÖÄ» So in the short term,
×ÖÄ» another deep freeze is unlikely.
×ÖÄ» The sun is warmer today;
×ÖÄ» that probably helps us out
×ÖÄ» but until scientists really understand
×ÖÄ» the cause of snowball earths,
×ÖÄ» I don't think any of us know
×ÖÄ» whether we're in danger or not of a recurrence.
×ÖÄ» But there is one possibility
×ÖÄ» that could result in another global freeze:
×ÖÄ» a shift in the position of the continents.
×ÖÄ» We know their arrangement influences the climate.
×ÖÄ» If the majority of the continents were clustered
×ÖÄ» on or near the equator,
×ÖÄ» the earth would be more likely to turn into a snowball.
×ÖÄ» Cooling temperatures would cause
×ÖÄ» ice to form in the open ocean of the poles,
×ÖÄ» while weathering would continue on the continents.
×ÖÄ» With more and more carbon locked away,
×ÖÄ» temperatures would plummet.
×ÖÄ» If all the continents were near the equator,
×ÖÄ» then the earth could get colder and colder
×ÖÄ» and sea ice could form and get to lower and lower latitudes
×ÖÄ» closer and closer to the equator.
×ÖÄ» Today our world seems safe.
×ÖÄ» Most of the continents are near the poles.
×ÖÄ» But plate movement never stops.
×ÖÄ» At some point,
×ÖÄ» millions of years in the future,
×ÖÄ» the continents may group around the equator once again.
×ÖÄ» I think it's possible some day,
×ÖÄ» if the continents were in a very different position,
×ÖÄ» maybe hundreds of millions years from now,
×ÖÄ» that we could indeed have another snowball earth.
×ÖÄ» It's unlikely that mankind will be around to
×ÖÄ» witness another snowball earth.
×ÖÄ» But the earth will still be here.
×ÖÄ» And just like it has in the past,
×ÖÄ» our planet will survive
×ÖÄ» and enter a new phase in its extraordinary history.