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  • Thanks to Brilliant for supporting this episode of SciShow.

  • Go to Brilliant.org/SciShow to learn more.

  • [ intro ]

  • Antarctica is famously frigid.

  • Unlike the North Pole,

  • which is covered in floating sea ice, Antarctica is a true continent.

  • It's twice the size of Australia, with mountains, valleys, and other landforms

  • -- but it's almost completely covered in ice sheets

  • that average two kilometers thick.

  • But it wasn't always that way.

  • In fact, Antarctica's current icy state wasn't exactly inevitable.

  • It was sort of in the wrong place at the wrong time sort of thing.

  • Scientists have spent many years and many studies examining the questions of

  • how, why, and when Antarctica froze over.

  • For much of the continent's history,

  • it was surprisingly lush.

  • And lots of organisms lived there...

  • before it was cool.

  • If you go back about 100 million years, for example,

  • Antarctica was home to vast coniferous forests populated with all sorts of animals,

  • including dinosaurs.

  • This was possible because Antarctica was farther north back then,

  • but also because the world was a warmer place.

  • It was a “greenhouse world,”

  • meaning there were no permanent polar ice caps.

  • It wasn't until much later that our planet transitioned to a more familiar condition:

  • an icehouse - a world with perpetually frozen north and south poles.

  • Geologists generally agree that Antarctica's ice sheets began to form around 34 million

  • years ago,

  • during the transition between the Eocene and Oligocene Epochs.

  • As for why the continent went cold

  • well, it's a bit complex.

  • Antarctica was going through a lot of changes at the time.

  • While the early Jurassic was overall warmer and wetter everywhere,

  • Antarctica was still cooler than the other continents,

  • and was already starting to suffer from the occasional cold and dark winter.

  • And although Antarctica was located further north than it is today,

  • tectonic forces were gradually sliding the continent southward.

  • But on top of that, Antarctica got lonely.

  • No, seriously.

  • It was around this time that Antarctica lost its last lingering connections

  • to South America and Australia.

  • This gave rise to one of the South Pole's most famous features:

  • the antarctic circumpolar current.

  • This is an ocean current that surrounds Antarctica.

  • Other ocean currents,

  • like the Gulf Stream, carry warm and cold water to new places.

  • Many researchers believe the Antarctic current keeps cold water flowing around Antarctica,

  • keeping warmer water out and making sure everything stays chilly.

  • But that's still an active area of study, and not everyone believes that's the case.

  • Either way, it wasn't just Antarctica --

  • the whole world was cooling down.

  • Some scientists have referred to this time period asThe Big Chill.”

  • And perhaps unsurprisingly,

  • scientists have pinned this major climate shift on fluctuations in carbon dioxide.

  • Ancient fossils and sediments capture chemical evidence that atmospheric and ocean compositions

  • were changing at this time in ways that suggest CO2 levels around the world were dropping.

  • See, those shifting continents created a bunch of mountains.

  • Around the time Antarctica started freezing,

  • India collided with Asia and began the uplift of the Himalayas.

  • This created a lot of exposed rock, which is subject to chemical weathering --

  • and chemical weathering takes up carbon from the atmosphere.

  • The way it works is this:

  • CO2 reacts with rainwater to make carbonic acid,

  • which dissolves some of those rocks.

  • The dissolved rock gets washed downriver into the ocean, where marine organisms like plankton

  • use those dissolved ingredients --

  • CO2 included --

  • to make their shells, locking away even more CO2.

  • In fact, plankton are a seriously potent force

  • for climate change, for multiple reasons.

  • These tiny floating organisms leave behind a lot of fossil evidence,

  • and geologists have observed a rise in fossilized plankton

  • remains at the same time Antarctica was splitting off from the other continents.

  • This leads some scientists to suspect that as the Antarctic Circumpolar Current grew

  • stronger,

  • there was an increase in upwelling -

  • the movement of cold, nutrient-rich water from the deep ocean up to the surface.

  • Upwelling acts as a fertilizer for shallow waters,

  • providing fuel for blooms of ocean plankton.

  • And the more they grow,

  • the more carbon dioxide they incorporate in their skeletons.

  • When they die and sink to the ocean floor,

  • those shells become incorporated into limestone,

  • locking away a lot of carbon with them.

  • And some experts say these oceanic changes weren't just in the south.

  • As the continents shifted, ocean currents around the globe were changing.

  • One 2015 study examined these changes

  • by plugging these ancient conditions into the same sort of computer models

  • that scientists use to study climate and oceans today.

  • What they found is that changes in the flow of the Antarctic Current,

  • like the opening of the Drake Passage between Antarctica and South America,

  • had effects on oceans around the globe.

  • That would have led to an overall slowdown of the currents

  • that bring deep-water carbon up to the surface, keeping more of that carbon stuck underwater.

  • Those changes, and others,

  • helped global CO2 decline over the past 40 million years or so.

  • Global temperatures dropped with it,

  • and things started getting chilly at the South Pole.

  • Glaciers would have started forming,

  • first in Antarctica's high mountains, then spreading to fill valleys and lowlands - the

  • start of ice sheets.

  • And here's the thing about ice sheets -

  • once they start, they tend to get carried away.

  • Ice has a very high albedo --

  • that is to say, it's very reflective.

  • Sunlight bounces off the ice and takes solar heat away with it.

  • The more ice cover there is, the more sunlight gets reflected, and the less heat sticks around.

  • And as the ice grows thicker,

  • it builds higher, where the air is thinner and colder,

  • so more ice forms.

  • These are forms of positive feedback:

  • the ice is a self-perpetuating system.

  • More ice leads to more ice.

  • As the millennia passed,

  • global temperatures continued to fall and Antarctic ice continued to grow.

  • The Antarctic ice sheets that we know and admire today

  • had formed by around 14 million years ago.

  • Antarctica had finally frozen over.

  • But just because our southern continent is in a deep freeze,

  • doesn't mean your brain has to be.

  • There are always more opportunities to learn new stuff.

  • And that's why today's episode is brought to you by Brilliant.org.

  • Brilliant has over 60 interactive courses about science, engineering,

  • computer science and math.

  • They're there to help you achieve your science and math learning goals,

  • one small commitment at a time.

  • For example,

  • they have a brand new course on search engines that'll tell you how Google and the like

  • can answer questions so fast

  • even though they have billions of websites to trawl through.

  • It teaches computer science,

  • and it demystifies a technology many of us use every single day!

  • You can check out the search engines course and many more over on Brilliant.

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  • will get 20% off an annual Premium subscription.

  • And by giving them a look, you're also helping to support SciShow.

  • Thanks!

  • [ outro ]

Thanks to Brilliant for supporting this episode of SciShow.

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B1 中級

南極はいかにして凍りついたか (How Antarctica Froze Over)

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    林宜悉 に公開 2021 年 01 月 14 日
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