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  • Thousands of years ago, a little white and yellow flower lived in the Arctic tundra,

  • blooming under the feet of Ice Age giants like mammoths and woolly rhinos.

  • This little flower belongs to the genus known as Dryas, and it’s still around today(!),

  • thriving in cold climates like the Rocky Mountains, Alaska, Iceland, and Sweden, although those

  • giant megafauna, sadly, are not.

  • But for a brief time, about 12,000 years ago, these flowers could be found all over the

  • Northern Hemisphere, from New England to central Europe.

  • So how did this little, cold-loving flower wind up in places that we associate today

  • with warmer climates, and for hundreds of years?

  • Well, at the time, those places weren’t warm.

  • After the northern glaciers reached their maximum extent around 20,000 years ago, the

  • planet started to warm up.

  • But thensomething happened.

  • And it sent the world into a cold snap that lasted around 1,200 years.

  • For example, in the Channel Islands of Great Britain, it may have gotten as cold as -20

  • celsius, whereas today it rarely drops below 9 degrees.

  • This cold spell was so intense that geologists have given it a name: The Younger Dryas, after

  • those little flowers that flourished in the newly cold climate.

  • And yes, in case you were wondering, there is also an Older Dryas, which was a shorter

  • cold event around 14,000 years ago that only lasted about 200 years.

  • The thing is, during the Younger Dryas, it didn’t just get cold.

  • This chunk of time also coincided with one of the largest mass extinctions in recent

  • history.

  • So for decades, scientists have been studying the cause of this event, and trying to figure

  • out if something like it could happen again.

  • And it turns out that what caused the Younger Dryas and the extinction of the megafauna

  • are both the subjects of ... heated debate.

  • In the late 19th and early 20th centuries, experts started finding fossils of Dryas flowers

  • in places where they didn’t seem to belong, like Denmark and the southern UK.

  • And they actually uncovered two layers of these fossils, representing two separate cold

  • spells -- the Older, and then the Younger, Dryas periods.

  • Now, you can’t find these flowers there today, because it’s too warm for them.

  • So, since the flowers could only have lived where it was cold, the fossils suggested that

  • this part of Europe had returned to a more tundra-like state

  • And in more recent decades, more evidence has been found to support this.

  • For example, in ice cores from Greenland, experts have found chemical signatures that

  • are consistent with a period of much colder temperatures.

  • And in the UK, scientists used fossils of insects and plants to estimate a temperature

  • range for the Channel Islands.

  • And they found that the warmest month was between 9 and 13°C, which is the same as

  • today’s winter temperatures.

  • But, while the evidence gave us a sense of what happened - that there was a sudden cold

  • event that allowed an arctic environment to expand south - it doesn’t explain how it

  • happened.

  • And figuring out the cause of the Younger Dryas has beenhard.

  • There are lots of different hypotheses, ranging from changes in ocean circulation to volcanic

  • eruptions and meteorite impacts.

  • One suspected culprit is the planet’s flow of water between oceans.

  • You can think of it like a conveyor belt.

  • Oceanographers call it the Atlantic Meridional Overturning Circulation.

  • This flow of ocean water starts near the Equator.

  • And when the warm water moves north, some of it evaporates, which makes it saltier.

  • It also cools down as it moves toward the Arctic, releasing its heat around Western

  • Europe.

  • Both of these processes make the water more dense, and density is what’s behind all

  • the movement of the conveyor belt.

  • If this circulation pattern slowed down, the Northern Hemisphere would get a lot cooler.

  • And that’s actually the premise of the movie The Day After Tomorrow, if that helps you,

  • where New York freezes over - albeit on a much shorter timescale.

  • But, what would make this system slow down?

  • Well, some scientists think that the Younger Dryas happened because a dam made of ice in

  • Arctic Canada that held back a huge glacial lake suddenly collapsed.

  • In this scenario, A LOT of glacial water ended up in the North Atlantic Ocean, and because

  • that water was fresh, not salty, it was less dense than the seawater.

  • That density difference messed up the whole system, because water sinking down to the

  • deep Atlantic is a crucial step in the ocean conveyor belt.

  • So, when that part broke down, it made the circulation slow down or even stop completely.

  • Then, the warm water got stuck near the Equator, so there was no heat transport to Europe.

  • And experts have found evidence of that meltwater -- in the form of a specific isotope of oxygen

  • -- in sediments north of Alaska dating back 13,000 years - which is around the same time as the

  • beginning of the Younger Dryas.

  • Now, we know that the sudden flow of meltwater made its way all the way to the Atlantic.

  • So the fact that it originally came from Alaska suggests that this wasn’t just a trickle:

  • it was a huge amount of water.

  • And what’s surprising is that this vast amount of meltwater was probably there in

  • the first place, because the ice sheet on land was melting, due to a warming climate.

  • So it’s possible that warming actually drove the Earth into a mini ice age.

  • Which sounds weird

  • Now, another hypothesis is that the Younger Dryas was caused by a big volcanic eruption

  • in Germany, which we know happened around the same time from radiocarbon-dating the

  • deposits that it left behind.

  • Now, the direct effects of such an eruption on the climate wouldve only lasted a couple

  • of years -- like, maybe some cooling caused by sulfur particles in the atmosphere reflecting

  • sunlight.

  • But it couldve had indirect impacts that lasted much longer, by causing changes in

  • ocean circulation or the expansion of sea-ice - or both - that in turn cooled the North

  • Atlantic.

  • So far, experts think it’s possible that an eruption triggered the Younger Dryas, but

  • theyre still running computer models to figure out how it all worked.

  • And maybe it happened alongside the meltwater event.

  • Then, in 2007, another possible cause gained traction: a major meteorite impact.

  • This event couldve unleashed that flood of meltwater.

  • In this scenario, sometimes called the Younger Dryas impact hypothesis, an asteroid at least

  • a kilometer wide hit the Earth around 12,900 years ago. Ow

  • But, evidence of this impact remained elusive until 2018, when geologists discovered a crater

  • buried under Greenland's Hiawatha glacier.

  • Early data show that it could be associated with the Younger Dryas.

  • And, in addition to the crater itself, researchers have found chemical signatures of an impact

  • all over the world.

  • For example, research from South Africa has found a spike in the element platinum in layers

  • dating to before the Younger Dryas.

  • Platinum is rare on Earth, but it’s more common in meteorites.

  • So the rapid increase in platinum could be the chemical footprint of some celestial impact.

  • But the case isn’t closed just yet.

  • To figure out if a giant impact really caused the Younger Dryas, we need to confirm how

  • old that crater in Greenland is.

  • Also much of the chemical evidence is still heavily debated.

  • One thing scientists are sure about is that there were major changes to ecosystems taking

  • place during the Younger Dryas.

  • At the same time as this cold spell, an extinction event wiped out many large vertebrates in

  • the Americas, Europe, Africa, and Asia.

  • By the end of this period, 36 genera of megafauna went extinct in North America, including ground

  • sloths, saber-tooth cats, and other iconic animals of the Ice Age.

  • Now, most of these extinctions happened at the same time as the cold event, but that

  • doesn’t mean the cold was the only factor to blame.

  • There are a few different ideas about how - and why - so many vertebrates disappeared.

  • Some researchers thought the most important factor was hunting by humans.

  • This model was developed in the 1960s and is called the overkill hypothesis.

  • But archaeologists weren’t convinced - why would people around the world start overhunting

  • megafauna all at the same time?

  • There is some limited evidence from one beach in Canada that people were hunting horses

  • and camels some 13,300 years ago.

  • In addition to the bones, archaeologists also found a nearby hunting trail with footprints

  • of mammoth, bison, and other game animals.

  • And there’s evidence that people hunted four other kinds of megafauna at different

  • sites in North America.

  • But the timing isn’t quite right: some genera of megafauna went extinct before humans even

  • arrived in North America.

  • So, even though there are some signs that point to hunting so long ago, the more popular

  • analysis of the extinctions says that they were caused by multiple factors, including

  • humans and climate.

  • And while a lot of research has been focused on the extinction of the megafauna, there’s

  • plenty of other evidence that shows that lots of big changes were taking place.

  • For example, fossils in Halls Cave, Texas show that the diversity of animals and plants

  • there dropped during the Younger Dryas.

  • And even though Texas is a long way from Greenland, cooling occurred simultaneously in both places.

  • The landscape in Texas changed from forest to grasslands, and some big herbivores - like

  • bison and two species of horse - disappeared for good.

  • But other animals just shifted their range.

  • Halls Cave shows that warm-adapted hares were present before and after the Younger Dryas,

  • but not during the cooling period itself.

  • And hares were part of the story on the other side of the Atlantic, too, where members of

  • the Natufian culture were living in the southern Levant.

  • These people hunted animals like hares, gazelles, deer, tortoises, and partridges.

  • And based on isotopes found in deep sea sediment cores, it took only 20 years for this region

  • to cool significantly, which had major impacts on both the people and the environment.

  • And in response to the colder temperatures, human society became more mobile.

  • People had to move around more because they couldn’t rely on the habitats that used

  • to sustain them.

  • Finally, after about 1,200 years of cold, the Younger Dryas period ended.

  • And because there’s no clear answer to what caused the Younger Dryas, it’s hard to know

  • exactly why it ended.

  • But based, again, on sediment cores, we know that the ocean conveyor belt started to strengthen

  • again about 400 years before the period ended.

  • And it’s still unclear why there was such a delay between changes in ocean circulation

  • and the climate of the Northern Hemisphere.

  • Despite this lag, it took less than a hundred years for this weird episode to finally end,

  • which is really fast by climate standards.

  • In fact, we know that temperatures in Greenland rose by up to 10 degrees C in just a few decades

  • or less!

  • Now, the events of the Younger Dryas might seem dramatic to us, or even unique, but this

  • was actually just the most recent event of its kind.

  • There were many climate changes before the Younger Dryas that were just as abrupt.

  • This one has gotten more attention because it happened at the same time as these other,

  • big, biological changes, like the extinction of megafauna and the movement of human populations.

  • As temperatures warmed, the Dryas flowers retreated back to their current habitats,

  • and much of the world went back to its previous state -- but many of the megafauna were gone

  • for good.

  • Ultimately, what were still learning about the Younger Dryas might help us better understand

  • the larger history of Earth’s climate and of living things.

  • To me, it’s nothing short of astonishing how much we can learn from a simple

  • little flower.

  • Ok. Wanna know more about what happens when an ice dam catastrophically fails on a glacial

  • lake?

  • Check out our episode How 7,000 Years of Epic Floods Changed the World.

  • Also thanks to this month’s super chill Eontologists: Sean Dennis, Jake Hart, Annie

  • & Eric Higgins, John Davison Ng, and Patrick Seifert!

  • Support us at patreon.com/eons for sweet benefits like submitting a joke for us to read!

  • Like this one from Lulu.

  • I love the La Brea tar pits.

  • There’s some beautiful spots there that can just hold you forever!

  • And as always thanks for joining me in the Konstantin Haase studio.

  • Subscribe at youtube.com/eons for more adventures in deep time.

Thousands of years ago, a little white and yellow flower lived in the Arctic tundra,

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当地球突然停止变暖时(When the Earth Suddenly Stopped Warming)

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    joey joey 發佈於 2021 年 05 月 03 日
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