Name: 当地球突然停止变暖时（When the Earth Suddenly Stopped Warming）
Uploaded: 2021-05-03T08:53:11.000Z
Duration: 11 min 1 s
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Thousands of years ago, a little white and yellow flower lived in the Arctic tundra,

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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

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

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

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

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

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

And figuring out the cause of the Younger Dryas has been … hard.

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

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

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

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

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:

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.

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

Now, the direct effects of such an eruption on the climate would've only lasted a couple

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

But it could've 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

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

they're 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 could've 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

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

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

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