Nat Geo_Human Extinction
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00:00Our world is teeming with life, from the simplest creatures to humankind itself.
00:10Now imagine a world bathed in deadly gases, seared by heat waves, or crippled by lethal
00:16falls in temperature.
00:18A world smashed by asteroids, blasted by superheated shock waves.
00:23A killer planet, capable of wiping out 95% of all the creatures that live on it.
00:29It sounds like a science fiction nightmare, but it's hard scientific fact.
00:35That planet is ours.
00:37Planet Earth.
00:39In the past, mass extinctions have wiped out more species than exist today.
00:45And one day, it's going to happen again.
00:5999% of all the animals and plants that ever lived on planet Earth are now extinct.
01:11Some were out-competed by other species, but many died in mysterious wipeouts that devastated
01:17life across the globe.
01:20What disasters might we face in the future?
01:23We investigate mass extinctions, and ask, if another wipeout strikes, what are our chances
01:30of survival?
01:35Organisms may be just doing great, and suddenly some chance event comes along and wipes out
01:40a large component of life.
01:42What we see is the nearest the Earth has come to losing all life.
01:46Something like this happens again, we may well be one of those species that will disappear.
01:50I think for humans this would be essentially Armageddon, it would be the end of our species.
01:55Scientists call these wipeouts mass extinction events.
01:59On five separate occasions, a catastrophe has wiped out many thousands of species in
02:04a single stroke.
02:06These events happened millions of years ago, so why should we be worried today?
02:12The answer is simple, it could happen again.
02:16In fact, it could already be happening, and this time, we could be the cause.
02:21Undoubtedly, we are going through a sixth mass extinction at the moment.
02:26We are creating an equivalent to a natural catastrophe, but it's entirely man-made.
02:32Could we survive a wipeout?
02:37Scientists are piecing together past extinction events.
02:40What happens in a wipeout?
02:43What does it take to survive?
02:46Clues from the past may be our best hope for the future.
02:51Our first stop is 65 million years ago.
02:55This is the time of some of the most extraordinary creatures that ever lived, the dinosaurs.
03:02After ruling the planet for more than 160 million years, they suddenly died.
03:08Why?
03:10Geodetective Phil Curry is leading the search to find out.
03:16Provincial park in the Canadian Badlands, this is dinosaur country.
03:21Curry comes here to hunt for clues about the death of the most famous dinosaur of them
03:26all.
03:28These bones have a very distinctive shape, and a very distinctive texture, and that tells
03:33us that this is Tyrannosaurus rex.
03:36The ground is littered with fossils.
03:38We have the top of the leg bone.
03:40It really isn't that much bigger than my leg bone.
03:44It's a small Tyrannosaurus rex.
03:46A full adult can be up to eight times my length, and weigh as much as six tons.
03:54T. rex was the ultimate predator, a killing machine that dominated its environment for
03:5920 million years.
04:02Then suddenly, it died out.
04:05What happened?
04:09Fossil evidence shows that dinosaurs disappeared everywhere almost instantly.
04:15Whatever killed them, it must have been big.
04:21I'm one of the people who believes that something catastrophic, something really big happened.
04:29Here's a scientist who's hot on the trail is astronomer Dan Durda.
04:37Durda is studying an unusual layer of clay in Trinidad, Colorado.
04:44In the rocks below the layer, there are dinosaur fossils.
04:48Above it, none.
04:50Something wiped them out.
04:52In this clay layer are some materials that one wouldn't ordinarily find.
04:57There is a metal iridium, which is very rare in the crust of the earth.
05:03The layer contains more than 200 times the normal level of iridium.
05:08A clue to how it got there comes from quartz crystals in the clay.
05:12They are fractured and shocked.
05:16Those tiny grains show deformations, changes to the crystal structure, which is the evidence
05:21of the intense pressures and temperatures.
05:25What has the power to do that to quartz?
05:28We can only do it one way.
05:34With a nuclear explosion.
05:36But the dinosaurs died 65 million years before nuclear technology.
05:44What natural phenomenon could pack enough punch to shock quartz and produce huge quantities
05:50of iridium?
05:52The likely answer, an asteroid.
05:58Space 65 million years ago, a lump of rock the size of a mountain, nine and a half kilometers
06:07wide, and weighing about a trillion tons, is on a collision course with earth.
06:18So picture, if you will, Mount Everest slamming out of the sky on top of you at 40,000 miles
06:23per hour.
06:25The asteroid smashes into the coast of what is now the Yucatan Peninsula in Mexico, with
06:30a force two million times more powerful than the biggest nuclear bomb ever detonated.
06:39The impact creates a crater 160 kilometers across.
06:43It sets in motion a deadly chain of events.
06:47A miles an hour blast of wind propagated out from the impact site and wiped flat forests
06:53for several hundred miles in every direction.
06:56The impact triggers colossal earthquakes.
07:01A plume of vaporized rock soars into the stratosphere.
07:05Some of it blasts hundreds of kilometers away before raining back down on earth.
07:13A superheated blast wave roars out at twice the speed of sound.
07:19It incinerates everything in its path.
07:22The blast wave, the fireball, and the debris raining back from the impact crater itself
07:29would have blown T-Rex flat, burned him to a crisp, and then buried him.
07:37Temperatures rocket upward.
07:39The debris heating the atmosphere to pizza oven temperatures would have wiped out animals
07:44and plants even on the other side of the planet.
07:48Fragments from the impact hurtle around the planet.
07:52Some make it to Trinidad, Colorado.
07:55This thin white layer of clay is the little bits of Mexico that came to visit Colorado
08:0165 million years ago.
08:0465 million years ago, an asteroid smashes into the coast of Mexico with a force 2 million
08:10times more powerful than the largest nuclear bomb ever detonated.
08:14This is a global disaster.
08:16Its effects reach right around the world.
08:19Dust from the impact is thrown into the sky, blocking the sun.
08:23The temperature suddenly drops.
08:25Plants and animals begin to die in the cold.
08:29Suddenly big predators like T-Rex have nothing to eat.
08:33The food chain collapses.
08:35Even at the furthest point from the impact, we were looking at a mass starvation setting
08:41in.
08:42The asteroid strike changes the face of planet Earth forever.
08:46For T-Rex, this is the end of the road.
08:50Every last dinosaur is wiped out.
08:57What would have happened if dinosaurs like T-Rex had survived?
09:03I think the dinosaurs would have continued to evolve, change and improve.
09:08They probably would have out-competed us.
09:10Perhaps we would be hunted by dinosaurs even.
09:14Are we in danger today?
09:17Meteor crater in Arizona proves impacts do happen.
09:21Or Tunguska in Siberia.
09:23A hundred years ago, a strike flattened at least 2,000 square kilometers of forest.
09:29Scientists put the chances of a major impact in the next century at one in 5,000.
09:35Those sorts of impacts happen very rarely in Earth's history.
09:38But that doesn't mean we're completely safe.
09:40An event of that scale could happen tomorrow.
09:47Could humanity survive?
09:50Generally, species that survived the dinosaur wipeout were the most adaptable.
09:55Scavengers who could eat whatever was available.
10:01And some birds, the living descendants of dinosaurs.
10:06Birds could fly and cover huge distances for the food that they needed to survive.
10:12And of course, they were still able to find mates.
10:16They survived.
10:18But could we?
10:19We're adaptable.
10:22But are we adaptable enough?
10:25Human adaptability is actually very good.
10:28We can go into outer space or we can go to the bottom of the sea.
10:31The problem is, if the world that we love to live in is totally altered, if all of our
10:37food sources perish, we might survive, but we might not like what we survive in.
10:42As a species, humanity may find new ways to survive.
10:47But only the lucky few.
10:49As our crops fail, many will starve.
10:52But unfortunately, I think, with an impact of that magnitude, for humans this would be
10:58essentially Armageddon.
10:59It would be the end of our species.
11:02If it happens, the outlook for our species is not good.
11:07Wipeout.
11:10Even a smaller asteroid would be disastrous.
11:14An impact from a rock 800 metres across could destroy a city or set off a tsunami.
11:22It doesn't have to be this way.
11:25We are unique.
11:26We can fight back.
11:31Scientists at NASA track large asteroids that might one day hit the Earth.
11:36So far, we are safe.
11:38But what will they do if they find one?
11:40There are several possibilities.
11:43Blow them up.
11:45Dock with them and nudge them out of the way.
11:49Use solar power to vaporise their surface and to make them change course.
11:54These ideas are untested.
11:56But scientists are confident.
11:59Moving an asteroid almost 10 kilometres wide is a challenge.
12:05And if we have enough time, it may not be impossible.
12:12250 million years ago, a previous extinction event wiped out 95% of all species on Earth.
12:20It didn't come from outer space.
12:23It might have come from deep inside the Earth.
12:28An unstoppable process that could threaten our very existence.
12:35On five separate occasions in our planet's ancient past, a catastrophe has wiped out
12:40millions of species in a single stroke.
12:44The asteroid strike that wiped out over half the species on the planet, including the dinosaurs,
12:50happened 65 million years ago.
12:54But the mother of all extinctions happened 250 million years ago.
12:59A single event which killed 95% of all species.
13:05Clues to this catastrophe lie in the Karoo Basin, South Africa.
13:10Today, it is barren and inhospitable.
13:14300 million years ago, during the Permian era, this was a lush floodplain teeming with life.
13:22Kings of the Karoo were the Gorgonopsians.
13:27A mammal-like reptile that lived long before dinosaurs.
13:31They were killers, successful, ruthless, and quite literally, cold-blooded.
13:38Then 250 million years ago, they all vanished.
13:42What happened?
13:43Why did such successful predators die out?
13:48Gorgonopsian expert Roger Smith explains.
13:51The top predators of the late Permian were undoubtedly the Gorgonopsians.
13:55And clearly, they're armed for the kill.
13:57These long, saber-like teeth brought the animal down.
14:01They were the saber-toothed cats of the late Permian.
14:04Today, the Karoo is a Gorgonopsian graveyard, a paleontologist's paradise.
14:12The rocks here hold vital evidence as to what wiped them out.
14:17Layers of rocks create a timeline going back into Earth's ancient past.
14:23As Smith tracks through the layers, he comes to a tipping point at the end of the geological
14:28period called the Permian.
14:32Below this layer are fossils of Gorgonopsian.
14:35Above, there are no fossils at all.
14:40These blue-green mudrocks of the Permian represent lush, wet floodplains, lots of life, lots
14:46of flourishing vegetation, and many types of mammal-like reptile.
14:51This is the point of extinction.
14:54Here, over 95% of life on Earth went extinct.
14:58The Permian extinction wiped out nearly all the species on the planet.
15:03Above it, a dead zone, almost completely barren of life.
15:08This is one of the greatest murder mysteries of all time.
15:13Destruction on an unimaginable scale.
15:15Just one in 20 species survived.
15:20What could cause such incredible devastation?
15:25It perplexed scientists for over a hundred years.
15:28Then, in the 1990s, they made a breakthrough, not in Africa, but thousands of kilometres
15:34away in Siberia.
15:36Beneath these frozen wastes, scientists discovered the Siberian Traps, thousands and thousands
15:42of square kilometres of lava.
15:44No conventional volcano can produce this quantity of lava.
15:48Instead, it wells out from cracks or fissures in the Earth's crust.
15:53It's called a flood basalt.
15:55Enormous pulses of heat rising from the Earth's core cause the rips.
15:59Geologists call them mantle plumes.
16:03As the hot plume rises through the mantle, it melts vast amounts of rock, which build
16:08up below the Earth's crust like a giant mushroom.
16:12The pressure is immense.
16:14It forces the crust to split open.
16:16Then, the lava flow begins.
16:20Disaster scientist Bill McGuire believes flood basalts are one of the most powerful forces
16:24on the planet.
16:26The Siberian flood basalts were the biggest outpouring of volcanic material for the last
16:31half a billion years.
16:33We've seen absolutely nothing like it in the whole history of humankind and our planet.
16:39The Siberian Traps continue to erupt for nearly a million years, producing up to a million
16:44cubic miles of lava a mile deep.
16:48One of the largest volcanic explosions humans have ever witnessed was Krakatoa in 1883.
16:54It lasted less than a day and threw out just six cubic miles of lava.
16:59Yet, over 36,000 people died.
17:03So you need 150,000 Krakatoas to meet the size of the Siberian Traps, which is extraordinary.
17:11Was this the smoking gun?
17:14Did lava trigger the Permian extinction?
17:17The Siberian Traps were pumping out lava at the right time.
17:22Now scientists are beginning to piece together how a flood basalt in Siberia could become
17:27a global killer.
17:32Siberia, 251 million years ago.
17:36The ground shakes.
17:38The Earth's crust rips apart as the largest catastrophe ever to hit the planet begins.
17:44It would be hell on Earth because you would have these huge fractures slashing the crust
17:49open.
17:50Then you would have very fluid lava fountaining out, pouring off in all directions at more
17:55than 1,000 degrees temperature.
17:57So any life around there would be burnt.
18:00Any animal too close dies instantly.
18:04But animals died all around the world.
18:07It turns the Permian flood basalt from a local disaster into a global wipeout.
18:14Smaller, more recent flood basalts may hold the answer.
18:221783, Laki, Iceland.
18:26The ground rips open, spewing out lava.
18:31Lava pumps out for almost a year.
18:33It covers 700 square kilometres.
18:37Everything in the immediate vicinity dies, but like the Siberian event, its effects reach
18:42much further.
18:45Soon much of northern Europe is suffering.
18:50At the National Ice Core Laboratories in Denver, Colorado, Jim White studies ice cores drilled
18:56from deep within the snow and ice to study what happened at Laki.
19:02The ice absorbs and preserves chemicals from the atmosphere.
19:08Over thousands of years as snow falls, layers of ice build up to create a unique record
19:13of the environment through history.
19:16White uses these layers of ancient snow to study atmospheric conditions from the past.
19:24By testing the electrical conductivity of the ice layer from the time of the Laki eruption,
19:29White discovers a massive surge in atmospheric sulphur.
19:33He concludes that the eruption released huge quantities of sulphur dioxide into the atmosphere.
19:42So you see the original eruption here on Laki, the big one, is quite sizeable.
19:47Pulse of sulphate here is maybe ten times higher than the background levels you see
19:53before that.
19:55A poisonous haze spreads across northern Europe.
19:59Sulphur dioxide reacts with water vapour in the atmosphere to create sulphuric acid.
20:06Acid rain falls, killing the plants and eroding the soil.
20:09You can imagine if you're a plant you really don't want to be bathed in sulphuric acid,
20:14but that is indeed what happens.
20:16That's just the beginning.
20:19Sulphur high in the stratosphere blocks out sunlight.
20:24The droplets act like tiny mirrors reflecting sunlight back into space.
20:31Across Europe, temperatures plummet.
20:37The Laki flood basalt releases 122 million tonnes of sulphur into the atmosphere.
20:44Crops fail, famine begins to bite, 50% of the cattle die, 25% of Iceland's population
20:52is wiped out.
20:57Laki helps scientists explain how flood basalts could cause mass death, toxic gases.
21:04But is gas alone enough to wipe out nearly all the species on Earth?
21:10White thinks it could be, because the gas coming from the Siberian eruptions was in
21:15the trillions of tonnes.
21:17What we saw on that screen would be dwarfed by what you would see.
21:22Compared to the Siberian event 250 million years earlier, Laki was a mere hiccup.
21:29The Laki eruption occurred in south-east Iceland, and it actually covered a relatively small
21:33area, about a third the size of greater London, and that was a devastating event.
21:38If we look at the Siberian flood basalts, they covered a huge area that amassed the
21:44whole of the central part of Russia, something like the size of the United States.
21:51That means that the volume of the Siberian flood basalts was something like a quarter
21:55of a million times bigger than the volume of the Laki basalts.
21:59That's why they wiped out 95% of all species.
22:02Not only were they bigger, they went on for nearly a million years longer.
22:08After the sulphur gas, a new crisis hit, another gas and another climate change.
22:14Lush fertile land turned to desert, the oceans turned to poison.
22:19It was a disaster almost nothing could survive.
22:27250 million years ago, 95% of the species living on Earth vanished.
22:33It is one of the great murder mysteries of all time.
22:37We know that flood basalts in Siberia released sulphur dioxide into the atmosphere, creating
22:42deadly acid rain.
22:46Oceans dropped suddenly, causing a savage freeze.
22:50Plants and animals perished.
22:52But that was just the beginning.
22:54Next, the Siberian traps released an even more deadly killer, a killer that triggered
23:00a heat wave.
23:05The molten rock released huge amounts of another gas into the atmosphere, carbon dioxide.
23:12As much as six times more carbon dioxide was pumped into the atmosphere as we have
23:17today, according to paleontologist Peter Ward.
23:21Carbon dioxide goes up in the atmosphere, doesn't go into space, blankets the planet
23:26and it acts very much like a greenhouse, that's why we call this a greenhouse gas.
23:30It effectively traps heat, sunshine comes through the blanket one way but can't get
23:35back out again, it's trapped inside.
23:38The blanket of heat warmed the entire planet, there's no place to hide, no place to go.
23:44As the Siberian traps continue to erupt, carbon dioxide builds up, and up, the heat wave begins.
24:00In the Karoo Basin, temperatures suddenly leap 15 degrees, the result, catastrophe.
24:07If it had happened over a million years or so, animals and plants would have adapted,
24:10but the reason it was so disastrous is it could have happened over a very short space
24:15of time, perhaps 10,000 years at some estimates.
24:21The rock layers reveal the exact time when the sudden heat increase turned this lush
24:25plain into a desert.
24:29The bluish grey mud rocks of the Permian represent wet flood plains, a time when the water tables
24:36were very high, there's lush vegetation.
24:40As the Earth heats up, more of the planet's water evaporates, rivers dry out, iron-bearing
24:47rocks normally underwater turn rust red as they react with oxygen in the air.
24:53In short, it's the first time we see drought in the Karoo Basin.
25:00Carbon withers and dies, plant-eating animals begin to starve and die out.
25:08As the plant-eaters vanish, predators like the Gorgonopsian lose their prey.
25:17They too start to die.
25:20The wipeout has begun.
25:23The land is dying, but life is not safe even in the oceans.
25:29Marine life too is under pressure.
25:35Geologist Paul Wignall discovered vital clues about the marine wipeout when he travelled
25:39to Greenland in the 1990s.
25:44He found an extraordinary record of mass extinction.
25:49Wignall discovered that the Permian marine extinction wasn't a single event, it happened
25:54in stages.
25:56In early rocks he found plenty of fossils.
25:59Clearly there were many species alive at this point.
26:03Then over tens of thousands of years, the number of species dropped off until suddenly
26:08almost none remained.
26:11We really lose virtually everything that was in the oceans at the time.
26:14We've wiped out about 95% of the species and the oceans are pretty empty now.
26:19What killed them?
26:21The rocks here tell a story.
26:23The creatures may have suffocated from lack of oxygen.
26:27What we found in the sediments is lots of crystals of a mineral like this, which is
26:32pyrite, which is also known as fool's gold.
26:34It cannot form in the presence of oxygen, it just hates oxygen.
26:39If pyrite crystals could form in the sea, then there wasn't much oxygen.
26:45Normally oxygen dissolves into seawater at the surface.
26:49Currents that circulate between the equator and the poles then carry the oxygen with them.
26:55Because there's more sunlight at the equator, the sea here is warmer.
27:00As this warm water moves towards the poles, it cools and sinks, carrying that dissolved
27:05oxygen down into deep water.
27:08The oxygen-rich water then runs back to the equator along the ocean floor and the cycle
27:14continues.
27:16Two hundred and fifty million years ago, in the aftermath of the massive volcanic explosions
27:20in Siberia, that mechanism fails.
27:24The same heat that has dried out the floodplains now warms the oceans, even at the poles.
27:31I'm sure the oceans around the equator may have reached temperatures approaching a hundred
27:35degrees Fahrenheit, real warm sort of bathwater temperatures near the poles and so on.
27:39You're looking at temperatures more akin to California today.
27:43With no cold water at the poles, the ocean's circulation system shuts down.
27:49As the oxygen no longer mixes down into the water, sea creatures start to die off.
27:56First the land, then the seas.
27:59The Siberian flood basalts affect everything.
28:03They leave the oceans almost entirely dead.
28:06But scientists aren't convinced that heat alone could wipe out as much as ninety-five
28:10percent of life on land.
28:13For that, they need another explanation, another smoking gun.
28:20They now believe that at the end of the Permian, conditions are just right for one last killer,
28:26the most deadly of them all.
28:30It's a killer that's still with us today.
28:33All you need is a swamp.
28:36Eww, that smell is a horrible rotten egg smell.
28:40It comes from the bacteria in the sediment.
28:43They produce hydrogen sulfide gas, which is a very toxic gas.
28:47They do so only in conditions of low or no oxygen, such as the swamp.
28:51In the low oxygen conditions of the Permian period, these bacteria do more than just smell.
28:57In larger quantities, the gases they produce kill.
29:01If the Earth had low oxygen everywhere, then the small amount of hydrogen sulfide would
29:06be multiplied immensely, to the point that it would be dangerous for life on the planet.
29:12We know sulfur-producing bacteria flourished during the Permian extinction.
29:17Their fossils are there in the rocks, in quantity.
29:21The presence of the fossils suggests that by the end of the Permian period, the air
29:26itself was deadly.
29:28And you have a new type of kill mechanism, and this is the evidence that, to me, has,
29:33I think, solved the whole mystery.
29:35The sulfur gases may have delivered the final blow, even to the hardy Gorgonopsians.
29:41So if our poor Gorgonopsian is in a case with low water, low oxygen, plus it's harder to
29:48find food, and now hydrogen sulfide is coming out, well, you've got a weakened animal, much
29:53more susceptible to dying off.
29:55And then finally comes a year where one poor last female is looking for the male that isn't
29:59there, or vice versa, and then they're extinct.
30:05It's happened once.
30:12Could it happen again, this time, to us?
30:19Flood basalt strike on average every 20 million years.
30:23Really big ones, like the Siberian traps, are rarer still.
30:27But they do happen.
30:29At some point in the future, it will happen again.
30:36There's nothing we could do about it.
30:37We'd have to stand back and be awed and amazed, and it would be rather depressing as well.
30:43If the worst does happen, humanity may not survive.
30:47The Siberian flood basalt has constituted the greatest mass extinction ever seen on
30:51our planet, and if something like this happens again, we may well be one of those species
30:55that will disappear.
30:56In terms of sustaining a complex civilization, that would be the last thing on people's
31:00minds.
31:01The simple fact of existing would be trouble.
31:03Just how would we survive as a species?
31:07Very few Permian creatures survived beyond the extinction line, and those that did were
31:12the most adaptable.
31:14One, the lystrosaurus, evolved a long, sturdy snout, handy for chopping through the few
31:20hardy drought-resistant plants that were still around.
31:25Another species made it through by living underground.
31:29Could we survive in the same way?
31:31Most scientists are unsure.
31:34Surviving underground for hundreds of thousands or millions of years is just not going to
31:37happen.
31:38We don't have any technology that would be able to cope with this sort of thing at all.
31:42So when it comes to the crunch, it's going to be those humans that are better adapted
31:46to living underground, low oxygen conditions.
31:49They will be the ones that survive.
31:50It will be survival of the fittest again.
31:53But even the fittest may not be fit enough.
31:56The planet above those few hardy cave-dwellers would be wrecked.
32:01Hardly any vegetation, almost no life at all.
32:08If the Earth is hit again by a Siberian-scale flood basalt, our chances of survival are
32:14slim.
32:17Not all mass extinctions are caused by asteroids or volcanic lava.
32:23Some may be triggered by a sudden blinding flash of light from exploding stars.
32:29A burst so powerful that it may have caused the second largest wipe-out in Earth's history.
32:35And could happen again.
32:38We are the top species on Earth, but that's no guarantee of survival.
32:43Look back 440 million years.
32:47The dominant species is the trilobite, a crab-like creature that has already ruled the
32:53seas for 100 million years.
32:58Now they are dying out, along with 70% of all the species alive.
33:04It's the second largest wipe-out in Earth's history, the late Ordovician.
33:10Unlocking the secrets of their demise is paleontologist Bruce Lieberman.
33:15If an alien from outer space was visiting, it would probably characterize life on Earth
33:20as dominated by trilobites.
33:23Most of them were probably scavengers.
33:25There may have been a few forms that were predatory on things like worms.
33:31Scientists originally thought that a global ice age had caused the wipe-out.
33:35There was evidence of ancient ice deposits in the Sahara Desert.
33:40But Lieberman isn't convinced it could have killed so many species.
33:45We have had other glaciations in the geological past that do not produce mass extinctions
33:51on the scale that we see at the end of the Ordovician.
33:53So that's why we can't treat it as a smoking gun.
33:57By chance, Lieberman was invited to a discussion held by astrobiologist Adrian Mallott, who
34:02had an idea he wanted to explore.
34:06Mallott studies gamma-ray bursts, the most powerful explosions in the universe.
34:14NASA's Swift satellite, launched in 2004, detects on average a gamma-ray burst a day.
34:22Scientists don't know what causes all gamma-ray bursts, but they do know that the worst and
34:27most deadly for Earth came from giant exploding stars called supernovae.
34:33A small fraction of those would be on our side of the galaxy and would have the beams
34:38pointed at us.
34:39If that happens, we have a recipe for disaster.
34:42The effects are so severe that you naturally think of mass extinctions.
34:48Mallott was sure that one must have hit the Earth in the past, but he didn't know when.
34:53He asked Lieberman if there was a mass extinction that might fit with his ideas.
34:59When Adrian Mallott described what would happen if a gamma-ray burst was sufficiently near
35:03our planet, I literally had an aha moment.
35:06Professor Lieberman said that sounds like the Ordovician extinction, and we thought
35:11we had a connection.
35:13The two scientists needed to reconstruct what would have happened if a gamma-ray burst had
35:17hit in the time of the trilobites.
35:20Space, 440 million years ago, a supernova explodes.
35:27It's within 10,000 light-years of Earth, but that's close enough to be deadly.
35:32A massive burst of gamma-rays heads towards our planet.
35:36First to suffer is the atmosphere.
35:39The gamma-rays change the chemistry of the air itself.
35:43The huge blast of energy rips apart nitrogen and oxygen molecules, enveloping the Earth
35:48in smog, including the deadly gas nitrogen dioxide.
35:53The very dangerous genie in this bottle is nitrogen dioxide, normally present in the
35:59atmosphere in very small amounts.
36:02If I were to open this bottle, we'd all be choking and gasping.
36:05In the aftermath of a gamma-ray burst, the whole atmosphere of the Earth would contain
36:11large amounts of this.
36:13This smog blocks out sunlight and possibly even triggers an ice age.
36:19But there's another, more devastating effect.
36:22The initial gamma-ray burst and the growing cloud of nitrous oxides destroy the ozone
36:27layer, the very thing that shields the Earth from the sun's lethal ultraviolet radiation.
36:35Many plants and animals would die.
36:37It might even be sufficiently severe to cause a mass extinction.
36:42The sun itself becomes an agent of death.
36:45Five times the normal amount of UV radiation hits the Earth.
36:49It damages the DNA of almost every plant or animal it hits.
36:54The smog acts like a sieve.
36:57It allows UV light, which has a short wavelength, through.
37:02But it blocks the longer wavelength light that provides us with heat.
37:06In this demonstration, represented by the sugar.
37:10Unlike the flower, it can't get through.
37:14Without the heat from the sun, temperatures sink to a deadly low.
37:20A single gamma-ray burst, just a few seconds long, could trigger an ice age and damage
37:26the DNA of almost every living thing.
37:31That was the breakthrough Lieberman had been looking for.
37:34That in effect is the one-two punch that may have caused this mass extinction.
37:39UV radiation can only penetrate a certain distance through water.
37:43The deeper an animal lived in the ocean, the more likely it would be to survive.
37:48And that's what Lieberman found.
37:51The young of some trilobite species stuck close to their families on the ocean bed.
37:57These guys are the lucky ones.
37:58They didn't know that this mass extinction was coming, but they were shielded from the
38:02ultraviolet radiation.
38:05Babies of other species swam close to the surface.
38:09Which makes them far more susceptible to the effects of a gamma-ray burst.
38:12In particular, the increase in the ultraviolet radiation would mean that the baby or juvenile
38:17form would be far more likely to be zapped.
38:21Lieberman had finally found a likely suspect for the trilobite extinction.
38:26To find something that could potentially be responsible was extremely exciting.
38:31The mystery of what could have killed 70% of all life in the oceans was solved.
38:43So what are the chances that we'll follow the trilobites into oblivion?
38:49We think there isn't a star close enough to cause damage.
38:52At the moment.
38:54But if a burst hit, we'd be in trouble.
38:58Gamma-rays travel at the speed of light.
39:01The fastest thing in the universe.
39:04There would be no warning.
39:06The first we'd know would be a blinding flash of light.
39:11If you're looking at the sky, you might well be blinded by such an event.
39:17The pulse would be over in seconds.
39:20Within hours, a cloud of smog would engulf the planet.
39:24The immediate danger would be sunburn and skin cancers.
39:29Then the DNA damage would begin.
39:32Adrian Mallott has calculated that it could be years before it was safe to go back outside.
39:38Even after four years, the level of DNA damage is still higher than it was before the burst
39:44went off.
39:45And it'll take ten years to get essentially back to normal.
39:49Animals might survive by staying indoors, but plants and wildlife would be devastated.
39:55Billions of people would have starved to death because many plants and animals would die.
40:00I think the human species would survive, but the amount of population that could be supported
40:06would be perhaps five to ten percent of the population on the planet today.
40:11So it would be a full-scale worldwide disaster.
40:14A gamma-ray burst also delivers an electromagnetic pulse.
40:18It would fry anything that relies on electronics.
40:22Our entire infrastructure would collapse.
40:25Many of us would be living in the 19th century again.
40:29This disaster wouldn't last as long as a flood basalt.
40:32It wouldn't be Armageddon like an asteroid impact.
40:36A gamma-ray burst would affect us maybe for a decade.
40:39Billions would die.
40:41But this is one wipeout we might just survive.
40:45Gamma-ray bursts near Earth are rare.
40:48Mass extinctions are rarer still.
40:51So is it likely that we're safe?
40:53Not quite.
40:55Scientists say we're in the middle of a mass extinction right now.
40:59Could our own date with destiny be just around the corner?
41:04In the last 440 million years, there have been five major mass extinctions on planet
41:10Earth and several smaller ones.
41:13Together, they have wiped out vast numbers of creatures.
41:19What's worrying is scientists have noticed a pattern.
41:23Not in the causes of extinctions, but in timing.
41:28They hit around every 62 million years.
41:34Astrobiologist Adrian Mallott believes the trigger may not be on Earth.
41:39It could be in space.
41:42While obsessing about this for a long time, it's a tantalizing hint because a long cycle
41:49like 62 million years sounds like something associated with a galaxy.
41:56Mallott has joined forces with fellow astrophysicist Misha Medvedev.
42:02They suggest a new prime suspect for triggering extinctions.
42:06The way we move through space.
42:09Our galaxy travels through space at 200 kilometres per second.
42:14When it hits a gas cloud, it creates a massive shockwave.
42:21Just like a plane breaking the sound barrier.
42:26It's exactly the same with the galaxy.
42:27When the galaxy moves through gas in the outer space, it creates a shockwave around it.
42:34These shockwaves accelerate atoms of charged gas, creating high-intensity cosmic rays capable
42:40of creating cancers and mutations.
42:44This intense, lethal radiation then rains down on our galaxy, and eventually, Earth.
42:51Normally, our galaxy and our planet are protected from cosmic rays by the galaxy's magnetic
42:56field.
42:58But our solar system moves up within the galaxy.
43:01When we're deep inside, we're protected.
43:04When we move up from the thick central disk, we're exposed.
43:09Imagine a diver deep in the ocean.
43:12It's pitch black.
43:13No light gets through.
43:15As she rises to the surface, they're exposed to more and more light.
43:20And when the solar system and the planets go close to the surface of a galaxy, it's
43:24like being close to the surface of an ocean.
43:27They are more exposed.
43:30Medvedev has calculated that this increased exposure happens every 62 to 64 million years.
43:38That's when extinctions happen.
43:41Only one event doesn't quite fit the pattern.
43:44The asteroid strike that wiped out the dinosaurs.
43:49If the galaxy theory is right, does that mean the flood basalt and gamma-ray burst theories
43:54are wrong?
43:56Not according to Mallotte.
43:57He says the cosmic rays periodically weaken life on Earth.
44:02When the other catastrophes hit, organisms are less able to survive.
44:07It's like being shot when you have the flu.
44:10You're much less likely to survive.
44:13According to the theory, Earth will get cosmic flu again in 10 million years.
44:18But will humans be around to experience it, or will we have destroyed our world before
44:24then?
44:28In the Permian extinction, volcanic eruptions started a chain reaction.
44:35Carbon dioxide swamped the atmosphere.
44:38Temperatures rose by 15 degrees.
44:40It killed 95% of all the species on Earth.
44:45This is beginning to sound familiar.
44:49In the last century, we have more than doubled the amount of carbon dioxide in the atmosphere.
44:55And it's still rising.
44:57The next wipeout could be entirely our fault.
45:02It is a warning from history, the fact that the sort of rate of increase in the levels
45:05that are being attained has happened before.
45:07And it was bad before, and if we're not careful, it'll be bad again.
45:10Well, I'm very worried that we are heading down a very nasty path.
45:14If there is a chance that we could head to another big mass extension because of what
45:19we're doing to the climate, then we have to do everything in our power to stop it.
45:26At present, no one knows if man-made climate change will cause a global wipeout.
45:34But we certainly have the power to make life on Earth as tough as it was during the Permian
45:38extinction.
45:41Our world could become uninhabitable.
45:44Our survival a question of luck.
45:49Beyond even that threat lies another.
45:52If we survive asteroid hits, gamma ray bursts and the effects of our own success, in the
45:57long run the sun will make life on Earth impossible.
46:00It will expand and melt the surface of our planet.
46:04Humans will only survive if we change our address.
46:08After that, billions of years from now, the universe could rip apart and that would be
46:13the end of everything.
46:14No life, nothing.
46:17One day, it will all be over.
46:20The question is, how long have we got?