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Welcome to Stuff You Should Know, a production of iHeartRadio.
Hey, and welcome to the podcast.
I'm Josh and there's Chuck and Jerry's here too.
And this is Stuff You Should Know.
And what we've got on our hands today, Chuck, is a hot potato of a real deal geological mystery.
Hot potato?
Mm-hmm.
That's how Emily's family says potato.
I think I would only say it when it's preceded by hot.
I would only say it like, like I wouldn't say, I'm going to have a baked potato.
Like you'd be out of your mind to say it like that.
Nor would you say a hot potato.
Like, what, a square?
Yeah, that is pretty square, isn't it?
Get the stick out of your butt, fella.
Loosen up.
Loosen up.
Get the cardigan out from around your shoulders.
Yeah, geez.
So that was obscure, but still.
So like I said, this is a mystery today.
And just to give a brief brush stroke overview with the widest brush, real lot of space in between bristles, just barely any paint on it.
That kind of overview.
Oh, wow.
What we're talking about is something called the Younger Dryas, which is a pretty terrible name for this if you want to be catchy, right?
Yeah.
I mean, I think a lot of people would probably say WTF.
Right.
Well, let's just call it the YD if we're going to put initials on things.
How about that?
Or abbreviate things.
So the YD, the Younger Dryas, is this surprising, shocking period in Earth's history, fairly recent history, where we came out of the last ice age.
Everything was going smoothly.
And then, bam!
We got hit by another Ice Age out of nowhere.
That lasts for over a thousand years.
And then, bam!
It goes away just as fast as it came along.
And paleo geologists, paleo climatologists all the paleos are perplexed as to what caused it and then why it stopped so suddenly too.
That's why I said it's a real deal geological mystery.
Yeah.
And a time as we'll see where the earth and its inhabitants and animals and nature was all going like.
All right, thank God we can.
We can finally get down to business and start being an earth like a legit earth.
Right.
And then, you know, the YDs come along and say, not so fast.
Right.
And you call it a legit Earth because when it finally ended, and actually that period in between the last ice age and the YD, those are like really habitable for human beings.
Like we love those kind of conditions, so much so that, after the YD ended, it became the age of humans.
Yeah.
Like, this is the age where we began our civilizations.
We started farming.
We began to flourish as a species and basically take over the planet.
And what's interesting is this is the most recent ice age.
There have been seven in Earth's history.
And just as an aside, this was the Wisconsinian Ice Age that we're talking about.
The last one was 250 million years before, right?
So it was pretty significant that we just— geologically speaking came out of an ice age, because there's not that many.
And then, not coincidentally, when the second to last ice age ended, that opened the door for the dinosaurs to come along and take over the Earth.
So big things happen when an ice age changes.
So for it to switch back to an ice age all of a sudden and then switch back to nice and temperate for us humans it is just very weird.
It was actually, I've seen it described as an extreme weather millennial event.
Yeah, and it kind of made me wonder, Had this not happened?
Because we were sort of headed toward, you know, legit Earth, like I said before, you know, I mean it would obviously wouldn't change the year.
But let's say as far along this goes, we would be like the year 3000 something.
Right.
Like.
Would we be just that much further along as a planet, or would we be nowhere, because in a thousand years we will have already destroyed ourselves?
The second one.
Okay.
So it's a good thing this happened or we wouldn't be talking about it right now.
Exactly.
That's exactly right.
So, yeah, that's a really anthropic way of looking at it.
Well.
Speaking of, I bought a copy of If Anyone Builds It, Everyone Dies, the Eliezer Yudkowsky Nate Soros book that just came out.
Okay.
I did not know about it.
You say that as if it was just on the tip of my tongue, which I appreciate.
No, you do.
We remember I basically mischaracterized it in the Zizians episode and had.
Oh, yeah.
Okay.
That book, right?
About building AI.
And if anyone builds it, then we're all going to die.
Like just the existential threat of AI.
It is really good.
You could read it in if you had a day that you could dedicate to reading it.
You could read it in a day.
It's really popularly written.
Lots of really cool anecdotes.
It's just very good.
So I strongly recommend that book.
Great.
Okay.
So let's kind of just back it up a little bit and go to the previous Ice Age, the Wisconsinian Ice Age, and talk about what Earth looked like like that.
Yeah, so it was very icy.
There were huge ice sheets covering a lot of Earth, like most of North America, Northern Europe and Asia.
And as we'll see, a lot of the...
The YD has affected the northern hemisphere much more than the southern, but also weird things happen in the southern hemisphere that don't quite jive.
That's why it's such a kind of a strange mystery.
Mm-hmm.
But lots of ice everywhere.
We had barren plains, very harsh conditions.
We had things like, you know, woolly mammoths, like ice-loving creatures dwelling the earth and, you know, human populations that were sparse and scattered and constantly kind of moving around trying to survive.
Right.
And so the last ice age started about 100000 years ago, and it took about 80000 years to reach its peak point.
So 20,000 years ago, it hit what's called the last glacial maximum, right?
So it took 80,000 years to get there and then 10,000 years to basically melt, right?
It melted a lot faster than it developed.
And just like right after it peaked, it just started warming up.
And it took about 9,000 years.
And all of the ice sheets, all the glaciers, all the stuff that was covering Earth and keeping it in an ice age just basically went away.
And Earth just blossomed. into a version that we like.
Yeah, and it became, you know, not terribly unlike what we're looking at now.
There was a jump in temperatures that was, you know, kind of close-ish to today.
And this was called the, I never know how to pronounce.
It's a umlaut.
Is it really?
Yeah.
That's all it is.
Yeah.
So all of our Nordic friends, the circle with the forward slash through or the O with the forward slash through, that's an umlaut right.
Yeah.
Okay, I mean, I guess.
We call it the null set because it looks like a zero with a slash to it.
So call it that.
I mean, we don't really call it that, but that's what it looks like.
Pronounce it like that.
What, with the umlaut?
No, the null set.
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Thank you.
So, Chuck, we've been talking, like, pretty big shots, pretty confident here, right?
Yeah, a couple of tough guys.
Exactly.
And the reason why we're doing that is because of ice core samples.
A lot from Greenland, also from Antarctica.
And sedimentation from high altitude European lakes.
That's why we're talking so tough right now.
Yeah.
And those are big deals.
You can find out a lot from an ice core.
You can find out like basically, what's what it accumulated there like stuff that the wind blew in there from other places?
You know, they've got these little little bubbles in that ice and you can trace gases even that were present back then and compare to like other parts of the world at that time and like, oh well, they have these gases here.
We have this sediment here.
We have a distinct lack of pollen for this period here, which means like probably a lot of plant life was killed off and stuff like that.
And then the lake stuff is super valuable too, right?
Yeah, because constantly sediment is accumulating at the bottom of a lake and very similar stuff's getting trapped down there, like air bubbles soot pollen, all that stuff.
And it just gets deposited year after year.
And you can actually date that stuff.
You can take a sample of a lake bottom and date that, and then see what was going on around the lake at that time too.
And lakes are advantageous because they're all over the world.
Ice shelves and glaciers are not all over the world.
You're limited from where you can take ice cores.
And then also lakes can last like thousands of years.
Whereas if you're reading tree rings, you're reading maybe decades, maybe a century or two worth a lake.
Yeah. it's going to give you way more than that.
So it's pretty clever the way that they can take this stuff, not only figure out when this sediment deposit or this ice deposit was put down, but also what like a lack of pollen or what type of pollen means for, like the world at large at the time.
I just think that's pretty cool that humans are able to do that.
And they don't seem to just be making this up.
You know a very niche nerdy Stuff You Should Know.
T-shirt could just be lake bottom greater than symbol tree ring.
That's super nerdy.
I think that would actually offset the be dumb and happy one that you got generated.
I think so.
But just you know, you walk around like Comic-Con and somebody will say I know exactly what you're doing, buddy.
And they'll just silently walk past you and give you the high five and the low five without stopping.
Right.
But it'll be a sort of a nerdy.
They'll miss the high five.
It won't be quite right.
They'll try and fist bump you when you put up your hand.
Right.
Or they lock fingers with you on the lo-fi and you guys keep trying to go in different directions.
That's got to be the worst.
I thought the fist bump and the hand together was the worst, but...
When you high five and someone holds on and locks fingers.
Not what you're after.
It's so weird.
Oh, goodness.
I don't know why I think of Oprah Winfrey.
Has she ever done that?
I think she likes to raise people's hands like they just won a boxing match or something.
All right, so we're settling into the YD.
I mentioned earlier that the Northern Hemisphere was the one that was really affected the most, especially around the North Atlantic.
The southern hemisphere, we'll talk about some weird abnormalities there as well.
But these ice sheets advanced across the Arctic Sea just like they did during the old LGM, the last glacial maximum.
Is it glacial or glacial?
Glacial.
Oh, I don't know.
Depends on whether you say potato or patata.
I guess so.
The Rockies here in what would later be the United States expanded.
The Alps expanded.
They found very weird things like penguin-like creatures in southern Italy.
Okay.
Like things were getting really out of whack again and everyone back then was like WTF.
Yeah, and even if you weren't living in an iced-over area, basically due south of that was tundra.
So Western Europe, which is nice and verdant and lush right now, was tundra at the time.
You've got grasses.
Maybe a fox or two if you're lucky and you're bored just looking around, because it's not the most pleasant landscape to look at.
I remember humans are running around at this time.
I think we shrunk tremendously as a population down to.
Maybe the highest estimate I saw was around 10 million people across the entire planet at this time.
That sounds nice, actually.
Kind of.
Talk about elbow room.
But all the plants and animals that had started to thrive in the middle in between the Ice Age and the YD.
They died back and all the animals and plants that had died back during that interstadial period.
They came roaring back and they were like we love the YD.
We're happy again.
Yeah, for sure.
I mentioned the pollen retreating being evidence of like, you know, the tree cover going away.
And they found evidence of that.
And I score once again.
This is in the Hulu cave in China and it showed, you know, a drastic reduction in tree pollen, which you know.
Basically, you can infer that like yeah, a lot of trees died out during this icy period.
Right.
And from that you can also infer that there was probably less rain, because trees help generate clouds and they also help keep the planet warm.
So it was obviously a lot colder and a lot drier.
That was the two characteristics of the YED.
Then like you said, the Southern Hemisphere experienced something radically different.
Remember, Northern Europe is covered in ice again.
It's tundra right below that.
And then if you go down to Antarctica, they basically hand you like a lei and a Hawaiian shirt and like a Mai Tai.
Yeah, it's so crazy.
When I was going through this stuff, that to me was like the big reveal.
I mean, I'm a big dummy when it comes to this stuff, so I just figured that would have been even colder and even worse.
But temperatures actually rose And the southern hemisphere and a lot of the southern hemisphere became warmer and wetter.
And the sea surface temperatures increased in the Caribbean and the tropical Pacific and the mountains of New Zealand.
It's all warmer conditions.
So it was like everything was kind of upside down all of a sudden.
Yeah, but you also mentioned earlier that it didn't happen.
There were weird pockets here and there, which also goes to underscore how bizarre the Younger Dryas was.
Like there's a basin around Venezuela.
And it actually had temperatures drop.
So this is the southern hemisphere, right?
It saw a temperature drop of three degrees Celsius, five and a half degrees Fahrenheit.
And every time I see something like that, I'm like, so?
Like, that's not that big of a deal.
It turns out it is a really big deal, even though to you, an average winter temperature of...
55 going down to 50 doesn't sound all that bad.
But climatologically speaking, any change in temperature, even by half degrees, has really huge weather consequences over say, the course of a year.
So in this case dropping by 3 degrees Celsius or 5 and 1 half degrees Fahrenheit, it might not have seen that much temperature-wise over an average winter.
But it also opened up the door for way more freak weather.
So they might have gone from like no blizzards whatsoever, maybe one blizzard a year, to three blizzards a year.
And that definitely impacts local conditions in the life there.
So it makes a really big difference, even when you see little changes to average temperatures.
Yeah, for sure.
There were likely some animal extinctions happening.
You mentioned, the amount of humans, which was probably at an all-time high at this point, started going back lower and lower.
Their available food was getting slimmer and slimmer.
We mentioned, they were dabbing their toe in the pond, of kind of settling down a little bit and not hunting and gathering and traveling everywhere.
And all of a sudden they had to pack it up and start moving around again to try and find food again.
Yeah, and the evidence that people, the human population, contracted meaning there was a lot of die-off shows up in archaeological sites like the Herrensberg culture in present-day Germany, Austria and Belgium.
During the YD, the archaeological sites dropped by half compared to what they had been just before the YD.
Stuff was like, well, we got to figure this out.
I mean, why did this happen?
And they have emerged with four main hypotheses.
The one that has the most traction we're going to start with is the melt water interrupting thermohaline circulation And that sounds very sort of like sciencey and nerdy and like guys.
I hope you explain this, but it's really very simple.
It is.
It's that a bunch of water disturbed the cycle of the warming cycle of the ocean very, very quickly.
Yeah, specifically in the North Atlantic, which has this thing that you mentioned, the thermohaline circulation, which is where warm water, warmed by surface air on top of the ocean falls down into the deep water.
And as it does it, it displaces the cold water which comes up to the top and gets evaporated, warmed itself, then it sinks and the cycle just keeps going on and on right.
This is how warmth is delivered to the northern hemisphere, right?
It moves northward, so the warmer seawater is always moving toward the north, so it's delivering warmth.
And at the same time, that cold water upwells in the southern hemisphere, so it delivers cold to the southern hemisphere, which is why, under normal conditions, with the thermohaline circulation, the Antarctica is cold and, comparatively speaking, the northern hemisphere is warmer.
That's right.
All right.
So park that in your, in your brain, and then understand that North America at the time had this huge 700 mile by 200 mile lake called Lake Agassiz, I guess.
Sure.
Agassiz, if you're an Andre Agassiz fan.
Yeah.
It developed as the Laurentide Ice Sheet and extended down to the Great Plains and blocked the Great Lakes.
And all the rivers that were flowing there backed up and it formed this big natural reservoir that was 700 by 200 miles big.
And this theory holds.
The meltwater theory holds that, as that last glacial maximum warmed things up, that ice sheet retreated, the blocked passage to those Great Lakes opened up and all of a sudden, billions and billions of gallons of freshwater make its way down to the North Atlantic.
They think a similar sort of thing happened in the Nordic region.
But you're like, all right, so that makes sense.
But was this water like super cold or something?
It was cold, but the main thing it did was desalinate that upper ocean water, right?
Yeah, and so fresh water is less dense than seawater.
And so a bunch of fresh water mixed in with seawater makes it less dense than it normally would be.
And it needs to be dense to fall down to the deep ocean, which makes the other deeper water come back up right.
Yeah.
So that means that the thermohaline circulation is interrupted.
And if you have an interruption in the thermohaline circulation, it just stops moving like that.
You would expect to see the northern hemisphere get cold because there's warmth is not being delivered there any longer.
You'd also expect to see Antarctica warm up, because that cold deep ocean water is not welling up around the very southern, southern hemisphere.
And that's exactly what you see with the Younger Dryas.
It's a really beautiful, elegant explanation that I think was hypothesized in 1982 and up until very, very recently.
You were a fringy nut if you believed anything besides that as the explanation for where the Younger Dryas came from.
Yeah.
I mean, it makes sense to me, but that's why I love science is people keep poking around and the fringe nut maybe one day is proven somewhat right.
And that, well, I still think that the melt water is probably the reason why, but...
One of the other hypotheses is the impact hypothesis.
And that is, like you said, was pretty controversial until more recently.
It has gained a little bit of traction because some things do kind of add up.
But the idea here is that a meteorite or a comet or something impacted Earth, the earth, maybe an airburst even.
And that released a thermal pulse that kind of set the world on fire, almost like all these massive wildfires across all the continents.
The air is filled with soot.
It's blocking sunlight.
And in fact there's so much soot, there's atmospheric dust, such that you reduce solar radiation, and all of a sudden you have what's called an impact winter or, you know, kind of like the idea of a nuclear winter.
Yeah, basically the same exact result, but just it was different things that got us there, right?
So there's evidence for this that most people point to as like, this is pretty good evidence.
There's something called a black mat evidence, which is a layer of carbon matter that seems to be a soot deposit that you find all over North America and in parts of Europe.
And all that suggests that there were wildfires going on on different continents at the same time, which would suggest like some sort of massive comet or meteorite bursting and setting off this thermal pulse.
So the fact that those things coincide on two different continents with the onset of the Younger Dryas has definitely made the impact hypothesis much more popular than it was before.
One thing detracting from it that probably makes some legitimate scientists wary of embracing it publicly right now is the writer and science contrarian Graham Hancock latched on to the impact hypothesis.
Because he has this theory I guess you could call it that that there was an ancient apocalypse around the time that the Younger Dryas happened, that wiped out massively advanced civilizations that we don't even know really existed and reset humanity, and then we had to rebuild from there.
There's not a lot of evidence, if any, for this stuff.
It's really, really fascinating.
But the evidence we do have, the scientific evidence we do have, doesn't set that up.
But anyway, he basically said see, this impact hypothesis supports my idea that there was an ancient apocalypse.
And scientists tend not to agree with Graham Hancock much.
Yeah.
And the other thing, there's a couple of more sort of smaller points that might support this.
One is there was a platinum spike in South Africa that preceded the YD and in some other places.
And platinum is, you know, a lot of time within meteorites.
So maybe some support there.
And also sometimes people kind of combine the first one and say maybe there was an impact combined with this meltwater thing.
So there was a low atmospheric explosion over North America, and that's what released all this meltwater all over the place.
Yeah.
Which makes sense.
It's basically like handing an olive branch from one hypothesizer to another, you know?
Let's work together.
Yeah.
Let's go get a drink and just settle it.
Also, as an aside, remember our Gobleki Tepe episode?
Oh, yeah.
It's been a while.
I don't remember exactly how to pronounce it.
We talked about how there was this guy who also kind of fringely concluded that some astronomical engravings at the site recorded the comet or the meteorite burst that happened at this time, which is kind of cool, but again there's not a lot of evidence to support it.
Moving on, there's a couple of other explanations that don't have nearly as much traction, but they do make sense.
One is a supernova explosion.
Supposedly a star went supernova in the Vela constellation at the right time.
That could have affected Earth by burning away its ozone layer, which would cool the upper stratosphere, which is the second most layer above Earth, which would prevent it from holding much water vapor.
Water vapor is a greenhouse gas and without a very strong greenhouse gas in the atmosphere, heat would just go out into space much more easily from Earth and it would get cooler as a result.
The problem with this one that I have.
Wouldn't that happen all over the planet if the ozone layer was burned away by a supernova?
I think so.
So I'm going to toss that one, okay?
Yeah, nailed it.
The last one is another sort of example where...
