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The Ice Ages — When the World Froze Over · Talking Science

0:00
Imagine you could step into a time machine and
0:02
travel back exactly 20 ,000 years. Which, you
0:05
know, in geological time is basically nothing.
0:08
Right. It's just a fraction of a blink. I mean,
0:10
modern humans are already walking the Earth.
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We're painting those incredible scenes in caves.
0:15
We have music, complex tools. But the world itself.
0:19
It would look entirely alien to us today. Exactly.
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Because, well, you look up, and sitting directly
0:25
over what is now Canada and the northern U .S.
0:28
is just this solid, towering sheet of ice. Yeah,
0:32
and we weren't just talking about a bad winter
0:33
frost here. No, we're talking about an ice sheet
0:35
up to three kilometers thick. Just three kilometers
0:39
of crushing weight. And it wasn't just North
0:42
America either. Right. Northern Europe was buried
0:45
too. Yeah. If you tried to stand where London
0:46
or Berlin are today, you'd be trapped under thousands
0:49
of feet of permanent blue ice. And, you know,
0:51
to build ice sheets that massive, all that water
0:53
had to come from somewhere. It came directly
0:55
from the oceans. Right. It evaporated, fell as
0:58
snow, and just never melted. Exactly. So because
1:01
so much of the Earth's water was literally locked
1:04
up on land, global sea levels were drastically
1:07
different. They were roughly 120 meters lower
1:11
than today. Which means the maps we rely on just
1:14
completely wouldn't apply. I mean, you could
1:17
have walked on dry land all the way from Asia
1:19
to America across the Beringland Bridge. You
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could stroll from the Australian mainland right
1:23
over to Tasmania without getting your feet wet.
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It's wild. And this was the absolute peak of
1:28
the last frozen era. So the mission. of our deep
1:31
dive today is to explore this reality, because
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this frozen world didn't just happen once in
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some freak accident. No, it's happened over and
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over again on this strikingly regular beat. Yeah,
1:41
so today we're going to unpack the astronomical
1:43
clockwork that controls that rhythm. And this
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journey is just so essential. I mean, what's
1:48
at stake here is far more than just trivia about
1:50
woolly mammoths. Oh, for sure. Understanding
1:52
this deep past is basically the ultimate key
1:54
to understanding the climate machinery of our
1:56
present. It shows us how the entire planetary
1:59
system operates when you start, you know, aggressively
2:02
pushing its buttons. Okay, but before we get
2:05
into how a planet freezes, we need to clear up
2:08
a massive popular misconception. Oh, definitely.
2:10
The terminology trips everyone up. Yeah, because
2:13
when I hear Ice Age, my brain instantly... Goes
2:17
to the movie version. I picture the whole globe
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covered in frost, eternal blizzards, just a total
2:23
deep freeze. Right. But scientifically speaking,
2:26
that pop culture image is actually entirely misleading.
2:29
OK, let's unpack this. What is the actual definition
2:32
then? So using the strict scientific definition,
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an ice age is simply any period in Earth's history
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where there is permanent ice at the poles. That's
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it. Wait. Permanent ice at the poles. But Greenland
2:44
has a permanent ice sheet. and antarctica is
2:46
totally covered in ice yeah are you telling me
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that scientifically i'm sitting in an ice age
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right now you are firmly inside an ice age right
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now we have been for a couple of million years
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actually that is wow that completely flips the
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whole cultural narrative on its head it really
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does what you're picturing with the mammoths
3:01
and the three kilometer thick ice burying new
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york is more precisely called a glacial period
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okay a glacial period right it's a specific intensely
3:11
cold phase that happens within a larger ice age.
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It's when those polar ice sheets aggressively
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advance and crush the continents. OK, so if the
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freezing mammoth heavy part is a glacial period,
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what do we call the phase we're in today? Because
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I definitely don't see any glaciers advancing
3:29
down my street. We are in what climatologists
3:32
call an interglacial period. Basically, these
3:35
are the relatively brief, warmer gaps between
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the deep freezes. The ice retreats, sea levels
3:40
rise. Exactly. The planet gets a bit of a breather.
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And our current interglacial gap began about
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11 ,500 years ago. That timeline is genuinely
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staggering to think about. It really puts things
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in perspective. It does, because all of human
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civilization, I mean, the invention of agriculture,
3:55
the first cities in Mesopotamia writing, the
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Roman Empire, smartphones, all of it fits entirely
4:00
inside this one brief, mild interglacial window.
4:03
It just highlights how delicate our global civilization
4:06
really is. For millions of years, the Earth has
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been trapped in this rhythm. Cycling back and
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forth. Right. Brutal glacial phases lasting tens
4:15
of thousands of years. And then these very brief
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warm gaps. We just happen to build our entire
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society during one of the warm gaps. Which introduces
4:23
a massive mystery, honestly. If human history
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is just a warm gap in a deep freeze. What actually
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flips the switch? That is the million dollar
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question. I mean, we're talking about moving
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oceans of water onto land and freezing them.
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What kind of force can reliably turn an ice age
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on and off? Well, the answer comes from the sky.
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And the guy who first figured this out was a
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Serbian mathematician named Milutin Milankovic.
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Okay. His story is pretty incredible. During
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the First World War, he was actually interned
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as a prisoner of war. Wow, really? Yeah. And
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while he was a prisoner and for years afterward,
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he just dedicated himself to calculating the
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Earth's orbital variations by hand with paper
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and a pencil. Hold on. You're telling me a mathematician
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sitting in a World War I internment camp was
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doing hand calculations to figure out the climate
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from hundreds of thousands of years ago. That's
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exactly what he did. How could he possibly know?
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what the Earth's orbit was doing before human
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beings even existed. Well, celestial mechanics
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are incredibly predictable if you know the physics.
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His core insight was that the Earth's journey
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around the sun isn't a static, unchanging loop.
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Right. It's not a perfect track. Exactly. It
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gets tugged on. Massive planets like Jupiter
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and Saturn exert their own gravity on the Earth.
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So over tens of thousands of years, that gravity
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slowly warps our orbit. And as our orbit changes,
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it slightly shifts. exactly how and where sunlight
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hits the planet. Yes, and he broke this... shifting
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down into three distinct orbital cycles. Let's
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walk through them because this is really the
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clockwork of the ice ages. Yeah. Cycle number
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one is the shape of Earth's orbit, right? Correct.
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It is in a perfect circle. Over about 100 ,000
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years, the gravity of those other planets slowly
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stretches our orbit out. So it becomes slightly
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more elliptical, like a stretched oval. Right.
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And then it slowly relaxes back into a more circular
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shape. That's the first cycle. Got it. And the
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second cycle is the tilt of the Earth's axis.
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Yeah. The Earth isn't perfectly straight. up
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and down. It's tilted, which is why we have seasons.
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But that tilt isn't permanently fixed. It changes
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over time. It does. Over roughly 41 ,000 years,
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the Earth slowly nods back and forth, changing
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its tilt from about 22 degrees to 24 .5 degrees.
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So a steeper tilt would just mean more extreme
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seasons, right? Exactly. The summer hemisphere
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points more directly at the sun, and the winter
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points further away. Okay, and the third cycle
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is the rubble. Yes, the wobble. If you've ever
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watched a toy top spinning, you know it doesn't
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stay perfectly upright. As it spins, the top
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of the axis slowly wobbles around in a circle.
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The Earth does the exact same thing. It takes
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about 23 ,000 years to complete one full wobble.
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And what this wobble does is change which season
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we're experiencing when the Earth is at its closest
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point to the Sun in that slightly oval orbit.
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Right. And when you put all three of these together,
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you have to remember one crucial fact. None of
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these changes actually alter the total amount
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of solar energy hitting the Earth. Wait, really?
7:21
The total amount stays the same? Pretty much,
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yeah. The sun isn't getting hotter. We aren't
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moving drastically closer to it overall. What
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changes is purely the distribution. Okay. It
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alters the timing and the specific latitude of
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where that sunlight lands. So to visualize this,
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I'm picturing these three cycles. the shape,
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the tilt, the wobble, as three different metronomes
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sitting on a desk. That's a great way to look
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at it. Right. So one metronome is ticking every
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100 ,000 years, one every 41 ,000, and the third
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every 23 ,000. Totally out of sync most of the
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time. Exactly. They're a chaotic mess. But eventually,
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if you wait long enough, there comes a specific
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moment where all three metronomes finally click
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on the exact same beat. And that alignment is
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exactly what Milankovitch was looking for. He
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calculated when those celestial metronomes align
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to create one very specific condition. Which
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is? Exceptionally cool summers in the Northern
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Hemisphere. Okay, wait. That feels entirely backwards.
8:18
Yeah. If I want to build a giant glacier that
8:20
buries Canada, my brain instantly thinks I need
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the most brutal, freezing, apocalyptic winters
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possible. Right. It's totally counterintuitive.
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Yeah. So why is a cool summer the trigger for
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an ice age? It's all about geography. To understand
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it, just look at a globe. The vast majority of
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the landmass where an ice sheet can actually
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grow is mostly in the northern hemisphere. Right.
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You've got Canada, Siberia, northern Europe.
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Exactly. Antarctica is massive, sure, but it's
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isolated at the bottom of the world, surrounded
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by ocean. To change the global climate, you need
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ice spreading across northern continents. Okay.
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We have the real estate in the north. But that
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still doesn't explain why the freezing winters
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don't matter as much. Think about the winters
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in northern Canada right now. It is already dangerously
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cold. It's more than cold enough to snow. That's
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true. Making the winter even colder -like, dropping
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it from minus 30 to minus 40, doesn't fundamentally
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change anything. The snow is already guaranteed.
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So the critical vulnerability for a glacier isn't
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the winter at all. Nope. It's the summer. Oh.
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The snow has to survive July. Precisely. If the
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summer is just a little bit too cool to melt
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away all of the previous winter's snowfall, a
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layer of snow survives into the autumn. And then
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the next winter arrives and just dumps a fresh
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layer on top of it. Exactly. The pile never resets
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to zero. Year after year, it accumulates. And
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the sheer weight of those layers compacts the
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bottom into dense, solid ice. So warm summers
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mean the slate gets wiped clean every year. But
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cool summers mean the snow compounds into an
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ice age. Yes. And Milankovitch predicted that
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whenever his three metronomes aligned to deprive
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the north of summer heat, the ice would build
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up. It's an incredibly elegant mathematical theory.
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It really is. I mean, it's just math on a piece
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of paper, right? He's doing this a century ago.
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A guy with a pencil claims he knows the global
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temperature from 200 ,000 years ago. The scientific
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community definitely didn't just take his word
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for it. Right. I mean, we needed physical proof,
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but... Time machines don't exist. How did anyone
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actually verify this? Well, it took arguably
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the greatest piece of scientific detective work
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of the 20th century. We needed a physical record
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of the Earth's atmosphere and we found it hiding.
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In the ice itself. Wait, you mean we just went
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out and dug up ancient ice? We drilled for it,
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yeah. In places like deep Antarctica and Greenland,
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it has been snowing for hundreds of thousands
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of years and it never melts. It just packs down.
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Right. So scientists go into these freezing landscapes
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and drill hollow tubes deep into the ice sheet.
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When they pull it up, they extract this long
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cylinder called an ice core. Because the layers
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build up year after year. Pulling up an ice core
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is literally pulling up a physical column of
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frozen time. Exactly. It's like tree rings. The
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deeper you drill, the older the ice. Okay, but
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a block of ice is just frozen water. Yeah. How
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does frozen water act as a thermometer for 100
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,000 years ago? This is where the chemistry comes
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in. Water is made of oxygen and hydrogen. But
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not all oxygen atoms are identical. Oh, like
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isotopes? Yes. Most oxygen is a lighter version,
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but a small percentage is slightly heavier. So
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you have heavy water and light water. Precisely.
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And think about evaporation. It takes heat from
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the sun to evaporate water out of the ocean so
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it can eventually fall as snow. Okay, I felt.
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Because those heavier molecules weigh more, it
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takes slightly more heat energy to lift them
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out of the ocean. Ah, I see where this is going.
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If the global climate is really cold, There isn't
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enough heat energy to evaporate the heavy water
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molecules. They just get left behind in the ocean.
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You nailed it. The mechanism is foolproof. Scientists
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melt a slice of ancient ice, measure the ratio
12:02
of heavy to light water, and if there's very
12:05
little heavy water, it proves that specific year
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was intensely cold. That is just brilliant. But
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there is something else trapped in that ice,
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isn't there? Yes, there is. Because as the snow
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gets crushed into solid ice, it traps air. Right.
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Microscopic bubbles of air get completely sealed
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in the ice. And those bubbles aren't a guess
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or a proxy. They are direct, pristine, physical
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samples of the Earth's atmosphere from the exact
12:28
moment that snow fell. They're like tiny, frozen
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time capsules of ancient breath. That's a beautiful
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way to put it. We extract the air from those
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bubbles and measure precisely how much carbon
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dioxide was in the atmosphere 800 ,000 years
12:42
ago. So we extract the ice cores, measure the
12:44
temperature using the heavy water, and measure
12:47
the CO2 using the bubbles. We build this massive
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timeline. And when we lay... that physical record
12:53
over Milankovitch's hand -drawn predictions.
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It matches flawlessly. Wow. The physical evidence
12:59
proved that the ice sheets advanced and retreated
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in exact rhythmic time with the orbital wobbles
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he calculated. That is an incredible triumph.
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But, you know, digging into this reveals a glaring
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logical problem. Optic energy problem. Yeah.
13:12
We established that the three metronomes don't
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change the total amount of sunlight. They just
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shift it around slightly. It's just a tiny celestial
13:18
nudge. Right. And a tiny nudge in summer sunlight
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simply... cannot possess enough raw energy to
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bury a continent under three kilometers of solid
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ice. The math doesn't add up. And physics completely
13:30
agrees with you. The orbital cycles alone are
13:33
profoundly weak. They might drop the temperature
13:35
by a fraction of a degree. So how does a fraction
13:38
of a degree cause an ice age? They require a
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mechanism to amplify that tiny nudge into a planetary
13:45
catastrophe. They require climate feedbacks.
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Feedbacks. We hear this term constantly in modern
13:51
climate science. Basically, think of it like
13:53
a microphone and a speaker. Okay. If you whisper
13:56
into a microphone, the speaker amplifies it.
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If you put the mic too close, it catches its
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own amplified whisper, feeds it back in, and
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suddenly it escalates into a deafening screech.
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So how does the Earth create a feedback loop
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like that? The first major amplifier is reflectivity,
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which scientists call albedo. It's just a measure
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of how much sunlight a surface bounces back into
14:16
space. Like wearing a black shirt versus a white
14:18
shirt on a hot day. Exactly. Dark things like
14:21
forests or the ocean have low albedo. They absorb
14:24
heat. White things like snow and ice have high
14:27
albedo. They act like a giant mirror. Okay, so
14:30
the celestial metronomes line up, provide a tiny
14:33
nudge of cooling. A little extra snow survives
14:36
the summer in Canada. And because that new snow
14:38
is bright white, it reflects the sun's energy
14:41
back out into space. That patch of land... isn't
14:45
absorbing heat anymore. Which makes the surrounding
14:47
air slightly colder. Yes. Which means the next
14:50
year, even more snow survives. The white patch
14:53
grows larger. The mirror gets bigger. Wow. It
14:56
reflects even more heat away, aggressively dropping
14:59
the temperature, creating even more ice. The
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whisper turns into a screech. That is the albedo
15:05
feedback. But there is a second, far more powerful
15:08
feedback loop, and it involves the oceans. Okay,
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how does the ocean fit in? As the planet cools
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from the ice mirror, the oceans also start to
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chill. And here's a fundamental law of chemistry.
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Cold water can hold much more dissolved gas than
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warm water can. Oh, it's the same reason a warm
15:24
soda goes flat immediately if you leave it in
15:26
the sun. But a cold soda kept in the fridge stays
15:28
fizzy. A perfect analogy. As the global oceans
15:31
cool, they act like a massive sponge, aggressively
15:34
pulling carbon dioxide out of the atmosphere
15:36
and dissolving it into the cold water. But carbon
15:39
dioxide is a greenhouse gas. It traps heat in
15:42
the atmosphere. Right. So as the ocean absorbs
15:44
the CO2 out of the air, the Earth's thermal blanket
15:48
gets thinner. The atmosphere loses its ability
15:51
to hold onto heat. The planet plunges into an
15:54
even deeper freeze, making the ocean even colder,
15:56
which lets it suck even more CO2 out of the sky.
15:59
You've got it. So the Earth's orbit is just the
16:02
person lightly tapping the first domino. But
16:05
the reflective ice and the CO2 are the actual
16:08
heavy machinery doing all the physical work of
16:11
freezing the planet. Exactly. And the ice cores
16:13
confirm this mechanism with terrifying clarity.
16:16
Across 800 ,000 years, CO2 and global temperature
16:20
move in absolute unbroken lockstep. Wow. When
16:24
CO2 drops, the temperature plummets. When CO2
16:27
rises, the temperature spikes. Okay, so if the
16:30
sky controls this whole natural rhythm, it inevitably
16:33
leads to a massive question about today. I know
16:36
exactly where you're going with this. Because,
16:37
I mean, a really common argument is, look, the
16:39
climate has always changed naturally. You just
16:41
explain how ice ages come and go based on orbits.
16:44
So modern warming is just another natural swing,
16:46
right? It is a totally understandable assumption,
16:48
honestly, if you only look at half the picture.
16:50
But when you examine the full ice core record,
16:52
it completely demolishes that argument. It does.
16:55
Completely. The history of the ice ages doesn't
16:58
reassure us that modern warming is natural. It
17:01
explicitly warns us that it isn't. Let's break
17:04
down exactly why the ice cores destroy that argument,
17:07
because this is crucial. Point number one has
17:09
to be the sheer speed. Speed is the great biological
17:12
filter. When we look at natural cycles driven
17:14
by Milankovitch's clockwork, they are agonizingly
17:17
slow. The transition from a deep freeze to a
17:20
warm period takes roughly 10 ,000 years. Right,
17:23
so a forest can't pack up and move. But over
17:26
10 ,000 years, an ecosystem can slowly migrate
17:28
further north. Animals have hundreds of generations
17:32
to evolve and adapt. But the warming we are subjecting
17:35
the planet to right now is happening over a century
17:37
or two. We are taking a 10 ,000 year warming
17:40
phase and compressing it into a single human
17:43
lifetime. It's dizzyingly fast. By biological
17:46
standards, it's violently destructive. Ecosystems
17:50
just cannot migrate or evolve fast enough to
17:52
keep up. Yeah. And point number two is the actual
17:55
trend we're supposed to be on right now. Because
17:58
if we check the three celestial metronomes today,
18:00
the shape, tilt, and wobble, what are they telling
18:03
the Earth to do? Well, if humans had never discovered
18:06
fossil fuels, the natural orbital cycles are
18:09
currently nudging us gently toward a cooling
18:11
phase. Wait, we should be cooling right now.
18:14
Yes. Over the next several thousand years, we
18:17
should be sliding back toward the next deep freeze.
18:20
The natural trajectory is cooling, but instead
18:23
the planet is experiencing explosive warming.
18:27
So we are literally swimming directly against
18:29
the astronomical current. We are fighting the
18:31
sky and we are winning. Which brings us to the
18:36
third and most critical point, the control knob.
18:39
The deep past proves beyond any doubt that carbon
18:42
dioxide is the primary amplifier. It's the master
18:45
control knob for the global thermostat. And right
18:48
now, by burning fossil fuels, we've injected
18:50
massive amounts of it back into the atmosphere.
18:52
Exactly. The concentration of CO2 has shot higher
18:54
and faster than anything seen in the entire 800
18:58
,000 -year ice core record. So we took the microphone,
19:01
hooked it up to the biggest amp we could find,
19:03
and cranked the volume dial to 11. Pretty much.
19:06
The deep past isn't a comforting reassurance
19:09
that everything is fine. It is the ultimate warning
19:12
label on the climate machine. It proves that
19:14
CO2 dictates the temperature. And we're turning
19:17
that knob harder and faster than nature ever
19:20
did. Because the fundamental physics haven't
19:22
changed. The greenhouse effect works exactly
19:25
the same today as it did 200 ,000 years ago.
19:28
The only variable that has changed is who is
19:31
operating the machinery. That is wow. Here's
19:35
where it gets really profound. The next time
19:37
you look at a map of the world, I really want
19:39
you to see the ghosts of the ice. Yeah, those
19:41
invisible connections. Alright. When you notice
19:44
that narrow stretch of water between Britain
19:46
and France or the shallow sea between Alaska
19:48
and Russia, remember that those maps are temporary.
19:52
Human beings, people just like us, walked across
19:55
those gaps on dry land. They live their lives
19:58
on the edge of a world dictated by the slow ticking
20:01
of a celestial clock. It's an incredible piece
20:04
of planetary machinery. It is. And it leaves
20:06
us with a profound, almost existential reality
20:09
to mull over. I mean, every empire we've built,
20:13
every technological leap only exists because
20:15
the Earth's orbit happened to grant us this brief,
20:18
fragile window of warmth. So what does it mean
20:21
for the future of our species now that we have
20:23
taken the climate control knob out of the hands
20:26
of the cosmos and put it directly into our own?
20:29
A heavy but totally essential question to leave
20:32
lingering in the air. So take a look outside
20:34
at this rare, beautiful, interglacial world we
20:36
inherited, and just remember, the massive force
20:39
is operating out of sight. Thanks for diving
20:41
deep with us today, and we'll catch you on the
20:43
next one.