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Sea Level Rise – Why It Isn't the Same Everywhere · Talking Science

0:00
Picture a coastal street on a perfectly clear,
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sunny afternoon. You're just looking up at the
0:04
sky and there are absolutely no storm clouds
0:07
anywhere, no rain, nothing that looks even remotely
0:11
like flooding weather. Right. Just a beautiful
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day. Exactly. Just a beautiful day. Yet you look
0:15
down and seawater is quietly bubbling up out
0:19
of the stormwater drains. It spills over the
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curbs and slowly spreads across the road, pooling
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around the tires of parked cars. Yeah. It's a
0:28
striking image. And honestly, almost surreal
0:31
when you see it in person. It totally violates
0:33
our basic expectations, right? Like how weather
0:35
and the ocean are supposed to interact. Oh, absolutely.
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I mean, we are deeply conditioned to associate
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rising water with, you know. Violent storms,
0:45
hurricanes, high winds. Right. But when I first
0:48
started going through the source material for
0:50
today's deep dive, I realized this isn't some
0:52
rare freak event anymore in low -lying coastal
0:54
cities. It's become this recurring nuisance.
0:58
Yeah, people informally call it sunny day flooding
1:00
or sometimes a king tide event. Right. You expect
1:03
floods during a hurricane, but you don't expect
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them on a random Tuesday when you're just like
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walking the dog in sunglasses. No, you definitely
1:11
don't. And what really stood out to me in the
1:13
data was that this bizarre local phenomenon is
1:16
actually one of the clearest everyday signs of
1:19
a massive slow motion process playing out across
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the entire planet. Yeah. The overarching process
1:26
being sea level rise. Which, well, it sounds
1:29
like a very straightforward concept on the surface,
1:31
doesn't it? It does. But the underlying mechanisms
1:33
are actually full of surprises. It's vastly more
1:36
complex and, frankly, more fascinating than just,
1:40
you know, a simple line moving up on a graph.
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Okay, so let's unpack this because whenever I
1:44
hear the phrase sea level rise, I immediately
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picture a bathtub. And I think almost everyone
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does, right? Sure. It's the most common analogy.
1:51
Yeah, like you turn on the faucet, the water
1:54
slowly fills up the tub, and the surface just
1:56
rises perfectly and evenly from all sides. But
1:59
the research insists this intuitive mental image
2:02
is actually totally misleading. So I need you
2:05
to help me break this down. Why does the bathtub
2:08
analogy fail so spectacularly? Well, our brains
2:12
really rely on that analogy because it perfectly
2:14
aligns with our everyday experience of water
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in small containers. A bathtub, a sink, a glass.
2:19
Right. But to understand a planetary ocean, we
2:22
first have to define the baseline. Like, scientists
2:25
measure something called global mean sea level.
2:28
Wait, hold on. Global mean sea level? How do
2:31
you even calculate an average on an ocean that
2:33
is, you know, constantly churning? It's not easy.
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I mean, you've got massive waves daily, high
2:38
and low tides, storms pushing water all over
2:41
the place. You can't just take a ruler to the
2:43
beach. You certainly cannot. It requires continuous
2:46
measurement and a tremendous amount of mathematical
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filtering to sort of... smooth out all that daily
2:52
noise. So you have to mathematically subtract
2:54
the tides. Exactly. You subtract the familiar
2:58
rise and fall of the tides and the choppy unevenness
3:01
of local weather. And when you finally filter
3:04
out all that chaos, you are left with a planetary
3:06
baseline. Okay. And the data shows that this
3:09
global average baseline has been rising for well
3:12
over a century. Currently, the well -documented
3:16
rate is just a few millimeters a year. Okay,
3:18
a few millimeters. I mean, that is literally
3:20
the thickness of a couple of coins. I'm just
3:22
trying to figure out why anyone would panic over
3:23
something that small. But before we get into
3:26
the actual danger of that, I want to understand
3:28
what is physically pushing the water up in the
3:31
first place. Right. Because I always assumed
3:33
it was just melting ice caps, but the source
3:35
material points to the water itself as the first
3:38
major driver. Yeah, the very first cause we need
3:41
to look at is thermal expansion. And it's actually
3:44
a beautifully simple piece of physics. Okay.
3:46
Lay it on me. So the climate system is warming,
3:49
right? Yeah. And our oceans basically act as
3:53
a massive sponge for that extra heat. They absorb
3:56
an enormous share of it. Right. They're like
3:57
the planet's heat sink. Exactly. And as ocean
4:01
water absorbs that thermal energy, the individual
4:03
water molecules within it begin to move more
4:06
vigorously. They gain kinetic energy, bouncing
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off each other with a little more force. Which
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means they spread out. Yes. On average, they
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spread slightly further apart. Okay, so I was
4:17
trying to picture this on a human scale while
4:19
I was reading. It's almost like a crowded dance
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floor. Oh, I like that. Yeah. Like if a slow
4:23
song is playing. Everyone is just swaying in
4:26
place, packed in really tight. You can fit a
4:28
lot of people in a small space. But then the
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tempo speeds up. The tempo speeds up and people
4:33
start dancing more vigorously, jumping around,
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throwing their arms out. Right. And suddenly
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they naturally just need more space. The crowd
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expands and pushes outward, taking up way more
4:44
room on the floor, even though nobody knew walked
4:46
through the door. That captures the dynamic perfectly.
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Now, if you apply that thermal expansion to,
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say, a single glass of water on your kitchen
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counter, the effect is utterly negligible. You'd
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never see it. You would never notice the water
5:00
level rise. But we have to scale that microscopic
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jostling up to an ocean containing an almost
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unimaginable volume of water. spread across the
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entire globe. And miles deep, right? Yes, extending
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kilometers deep. So when you multiply that tiny
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per molecule expansion across miles of depth,
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it adds up to a genuinely massive increase in
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overall volume. So the ocean is literally just
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physically swelling up. The sea level rises even
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without a single extra drop of water being added
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from outside the system. Exactly. Careful measurements
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of ocean heat content taken from this whole global
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network of submerged instruments show the oceans
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have absorbed a staggering amount of trapped
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heat. So thermal expansion accounts for a major
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portion of the sea level rise we've measured
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over the past century. Wow. Okay, but there is
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a second driver. And this one does involve adding
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genuinely new water to the ocean. And here is
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where I had to correct a massive misconception
5:55
I had about exactly which ice we should actually
5:57
be worrying about. Right. This is a very common
5:59
point of confusion because we hear a lot of news
6:01
about shrinking Arctic sea ice. Yes. The frozen
6:04
seawater that forms and floats directly on the
6:07
ocean's surface. The ice the polar bears are
6:10
standing on in all the documentaries. Right.
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Sea ice. My instinct is to say that when that
6:15
ice melts. the sea level goes up. But the research
6:19
says that's wrong. It goes back to Archimedes'
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principle, doesn't it? It does. Think about,
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well, a glass of water filled right to the very
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brim with a big ice cube floating in it. Okay.
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When that ice cube melts, the glass won't overflow.
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Because the floating ice is already displacing
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exactly its own weight in water. Oh, so the volume
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it takes up under the surface is exactly equal
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to the water it turns into when it melts. Exactly.
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Meaning the melting of sea ice in the Arctic
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is, well, it's terrible for polar bears and for
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the ecosystem, but it isn't flooding coastal
6:49
cities. Okay, wait. So does it matter at all
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for sea level then? It matters for the climate.
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The loss of sea ice is a profound indicator of
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a warming climate, and it drastically reduces
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how much sunlight the planet reflects back into
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space, which, you know, just accelerates the
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overall warming. Ah, so it makes the thermal
7:07
expansion worse. Right. But its melting does
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not meaningfully raise global sea levels directly.
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The real culprit when it comes to adding new
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water is land ice. Land ice? Yes. We're talking
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about mountain glaciers and the two vast ice
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sheets covering Greenland and Antarctica. Because
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they are sitting on continental rock, they aren't
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in the bathtub yet. That's the key. They hold
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the overwhelming majority of the world's fresh
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water. just locked up on land. When large chunks
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of this ice break off and slide into the sea,
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a process called calving, or when it melts and
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runs off as rivers, it is dumping completely
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new water into the ocean basin. Okay, so we have
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thermal expansion making the existing water swell
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and melting land ice dumping new water into the
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system. This seems like it should just raise
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the water level everywhere equally, like a filling
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pool, but... But it doesn't. Right. Here's where
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it gets really interesting. I mean, reading this
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part of the deep... dive completely broke my
8:03
brain. It says that melting ice absolutely does
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not raise sea levels evenly. No, it does not.
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In fact, if you are standing right next to the
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Greenland ice sheet as it melts, the sea level
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on your local beach might actually fall. It might
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actually drop. How on earth is that possible?
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I mean, it's melting right there. What's fascinating
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here is that we have to talk about gravity. Gravity.
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Yeah. We usually only think of gravity in terms
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of like the earth. pulling an apple to the ground.
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But everything with mass has its own gravitational
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pull. Okay. And a vast ice sheet like Greenland's
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contains an immense mass of ice. I mean, we are
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talking about a frozen block that is miles thick,
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covering landmass the size of Mexico. I mean,
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I know mountains have mass, but I never considered
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that ice could have enough gravity to actually
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pull on an ocean. It does. The mass is so tremendous
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that it exerts a strong gravitational pull on
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everything around it, particularly the surrounding
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seawater. Wow. The ocean water is miserably pulled
9:02
toward the ice sheet. It effectively drags the
9:04
water and piles it up unusually high near the
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coast. So the sea level there is artificially
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elevated by the sheer gravitational attraction
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of the ice. That is mind -bending. Ice is literally
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pulling the ocean up against itself like a magnet
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pulling on iron filings. That's a great way to
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visualize it. Yeah. So consider what happens
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as that ice sheet melts. It is losing hundreds
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of... Billions of tons of mass. Right. And as
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the mass decreases, its gravitational pull weakens.
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And because the gravitational magnet is getting
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weaker, the seawater that was piled up against
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the coast isn't being held there anymore. Exactly.
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The water begins to relax. It sloshes back outward,
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away from the now less massive ice sheet, and
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redistributes itself toward distant parts of
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the ocean. So Greenland dumps billions of gallons
9:53
of new water into the ocean, but the ocean level
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actually drops locally because the water flows
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away. It flows away and piles up in places far
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across the globe, sometimes rising dramatically
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on the exact opposite side of the planet. That
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is wild. Scientists actually call this a sea
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level fingerprint. Each melting ice sheet, whether
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it's Greenland or specific regions of Antarctica,
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stamps its own unique calculable pattern of rising
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and falling water across the globe. Okay, so...
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If Antarctica's ice melts, it might drastically
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raise the sea level on the coast of North America,
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while Greenland's melt might drastically raise
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it somewhere in the southern hemisphere. Yes,
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precisely. The water is literally just warping
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around the globe. It is a deeply complex redistribution
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of mass. So the water itself is being pushed
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and pulled by these invisible gravitational fingerprints.
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But to make things even more chaotic for our
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hypothetical coastal city you're living in, the
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ground beneath your feet isn't sitting still
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either. No, it's not. And this introduces another
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critical concept, which is relative sea level.
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Right, relative sea level, which is just how
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high the water appears to rise from the perspective
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of someone standing on a specific dock, right?
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It's the reality local residents actually experience.
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Exactly. If the water is rising... but the land
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you are standing on is simultaneously sinking,
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your relative sea level is going to climb incredibly
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fast. Yeah, that makes sense. And that sinking
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of land is our first major localized variable.
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It's known as subsidence. So why is the land
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sinking out from under people? Like, what's causing
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that? Well, in many low -lying coastal cities
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built on river deltas, the land is composed of
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soft sediment that slowly compacts naturally
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over time. But human activity heavily. heavily
11:39
accelerates this. How so? When a city pumps vast
11:42
amounts of groundwater or, say, extracts oil
11:45
and gas from deep underground aquifers and reservoirs,
11:49
it creates these empty void spaces beneath the
11:52
surface. Ah. I was trying to picture the mechanics
11:54
of this, and I keep coming back to a jelly donut.
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A jelly donut? Yeah, like if you have a plump
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donut and you take a syringe and suck the jelly
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out of the middle, the top layer of dough has
12:03
nothing supporting it anymore, so it just caves
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in. Well, yeah, actually. The sediment layers
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above those depleted aquifers behave the exact
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same way. They gradually compact and settle downward
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into the empty space. Wow. So for cities built
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on this settling ground, the residents face a
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compounding crisis. The ocean is rising globally.
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And their city block is dropping locally. That
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is a terrible combination. But the research also
12:26
highlighted the flip side, right? Where the land
12:28
is actually rising up to meet the ocean. Yes.
12:31
Through a really fascinating geological process
12:33
called post -glacial rebound. Or sometimes it's
12:37
called glacial isostatic adjustment. Okay, that
12:39
sounds incredibly technical. But it connects
12:43
back to the Ice Age, right? It does. During the
12:46
last glacial period, enormous ice sheets pressed
12:49
down on parts of the northern hemisphere, places
12:52
like Scandinavia and parts of Canada. Okay. They
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pressed down with such tremendous weight that
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the underlying crust of the Earth was literally
13:00
compressed downward into the molten mantle below.
13:03
Oh, so the crust is slightly flexible, like sitting
13:06
on a really firm memory foam mattress. Exactly
13:08
like a memory foam mattress. Like if a heavy
13:10
person sits on it for a long time, it leaves
13:12
a deep depression. And when they finally stand
13:15
up, the foam doesn't just snap back instantly.
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It slowly puffs back up over time. That's perfectly
13:20
put. The ice melted tens of thousands of years
13:23
ago, lifting that heavy weight. But the Earth's
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mantle is incredibly viscous, like thick syrup.
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So it's still moving. Yes. The crust is still
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slowly springing back upward today. So if you
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live in one of these rebounding regions, the
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land beneath your feet is moving upward fast
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enough that your relative sea level might actually
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be falling. Even though the total amount of water
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in the global bath cub is increasing. Exactly.
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Furthermore, we have to factor in ocean currents.
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Right. Currents. The ocean is threaded with massive
13:54
currents driven by wind, temperature differences,
13:57
and the Earth's rotation. And these currents
14:00
move. enormous volumes of water around, but they
14:04
don't do it evenly. Yeah, what really stood out
14:06
in the sources was the U .S. East Coast. It mentioned
14:09
that shifting ocean circulation patterns are
14:11
causing warm water to physically mound up slightly
14:14
along that coastline. Yeah, when currents create
14:16
a bottleneck or shift direction, warm water accumulates.
14:20
Yeah. And where you have concentrated warm water,
14:22
you get more of that localized thermal expansion
14:25
we discussed earlier. The dance floor. Right,
14:27
the energetic dancers taking up more space. Okay,
14:29
let me just, let me take a breath and try to
14:31
stack all this up because it is honestly dizzying.
14:33
That's a lot. lot. We have thermal expansion
14:36
swelling the water from heat. We have melting
14:38
ice sheets creating a gravity fingerprints that
14:41
pull the ocean across the globe. We have the
14:43
land itself moving up or down like a deflating
14:45
donut or a rebounding mattress. And we have shifting
14:49
currents mounding hot water in random places.
14:52
That's the picture. Yeah. With all this localized
14:54
chaos happening at the exact same time. How do
14:58
scientists actually figure out the global average?
15:00
I mean, it seems impossible to decouple the ocean
15:03
from the land. It's an incredible scientific
15:05
challenge. Yeah. And it requires multiple sort
15:08
of complementary tools to get it right. The oldest
15:11
method relies on the tide gauge. Which is essentially
15:14
a very sophisticated ruler attached to a coastal
15:16
pier continuously measuring the water height,
15:19
right? Right. And tide gauges provide an invaluable
15:21
historical record stretching back. Well over
15:24
a century. But as you just pointed out, a tide
15:27
gauge captures the total localized mass. Because
15:30
it's attached to the land. Exactly. Because it
15:32
is physically anchored to the land. It measures
15:34
the ocean rise combined with any local land sinking
15:37
or rebounding. It gives you the relative sea
15:39
level. Which is exactly what a coastal mayor
15:42
needs to know for city planning. But it's terrible
15:45
if you want to know what the total volume of
15:46
the global ocean is actually doing. Precisely.
15:50
To find the true global trend, you need an instrument
15:53
that completely ignores the land. And that is
15:55
where satellite altimetry comes in. Space lasers.
15:58
Basically, yes. Since the early 1990s, satellites
16:02
orbiting overhead have been bouncing precisely
16:04
timed radar pulses off the ocean surface and
16:08
measuring exactly how long they take to return.
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And because the satellite is orbiting in space,
16:12
it doesn't care if the ground in Florida is sinking
16:14
or the ground in Canada is rising. It measures
16:17
the height of the sea surface relative to the
16:19
center of the Earth. It provides a clean, absolute
16:22
global reference frame. Additionally, satellites
16:24
can measure the water height far out into the
16:26
deep open ocean, where you could never practically
16:29
install a tide gauge anyway. Oh, that makes sense.
16:31
By cross -checking the historical coastal data
16:34
from tide gauges with the absolute global data
16:37
from satellites, scientists have built independently
16:39
corroborated confidence in the overall trend.
16:42
Okay, so what does this all mean for us, like
16:46
for you listening to this? We know the global
16:48
average is rising a few millimeters a year. But
16:51
I still struggle with the scale. Why does a rise
16:54
of a few millimeters pose an existential threat
16:56
to island nations or river deltas? A millimeter
17:00
is tiny. It is tiny. But you have to remember,
17:03
sea level rise does not act alone. It functions
17:06
as a rising baseline. A rising baseline, meaning
17:09
the starting floor is permanently higher. Yes.
17:12
Think about all the temporary natural events
17:14
that push water onshore. A normal high tide,
17:18
a storm surge from a passing low pressure system,
17:20
or just a period of unusually strong onshore
17:23
winds. Right, the stuff that happens all the
17:25
time. Right. Coastal infrastructure like seawalls
17:27
and storm drains, they're specifically engineered
17:30
to handle those temporary spikes based on historical
17:32
averages. Okay, I see where this is going. If
17:35
you permanently raise the floor that those spikes
17:37
launch from, even by just a few inches over a
17:39
couple of decades, the math totally changes.
17:41
It totally changes. An entirely ordinary weather
17:44
event, one that historically would have just
17:46
splashed harmlessly against a seawall, now suddenly
17:50
overtops it because it started from a higher
17:52
launching pad. Wow. Which brings us full circle
17:54
to the sunny day flooding on that clear coastal
17:58
street we talked about at the beginning. It wasn't
18:00
a freak storm. It was just a regular high tide
18:02
doing what it always does. But it arrived on
18:05
top of a permanently elevated baseline. So the
18:08
water had nowhere to go but back up through the
18:10
city's storm drains. And for low -lying island
18:13
nations, some of which max out at just a meter
18:16
or two above the ocean across their entire landmass.
18:19
This permanently rising baseline isn't just an
18:22
infrastructure headache. No. It dictates whether
18:24
the land remains habitable at all. So the measurements
18:27
are incredibly strong and the physics of the
18:29
localized threats are very clear. But as we reach
18:32
the final section of our deep dive here, the
18:34
research makes a really crucial point about the
18:36
nature of science. There is an area where scientists
18:39
remain honestly, genuinely uncertain. Yes, and
18:42
it is vital to distinguish between honest scientific
18:45
uncertainty and overarching doubt. Right. The
18:49
broad physical reality that the ocean is warming,
18:51
expanding, and absorbing melting land ice is
18:55
not in dispute. What is genuinely difficult to
18:58
pin down is exactly how fast sea levels will
19:01
rise in the coming decades. Okay, but why is
19:03
the speed so hard to predict? Don't we have supercomputers
19:06
modeling every drop of water? We do, but we run
19:09
into deeply complex ice physics. Specifically,
19:12
a phenomenon known as marine ice sheet instability.
19:15
Marine ice sheet instability. Yes. It involves
19:19
the unique geography of Antarctica. Enormous
19:22
sections of the Antarctic ice sheet actually
19:24
rest on bedrock that is below sea level. Wait,
19:27
the ice is sitting on land, but the land is shaped
19:29
like a bowl that dips underneath the ocean. It
19:31
rests on a reverse slope. The bedrock slants
19:33
downward as you move inland. Oh, I see. The point
19:36
where the grounded ice meets the ocean water
19:38
is called the grounding line. If warming ocean
19:41
currents melt that grounding line from below,
19:44
the ice detaches from the bedrock. And because
19:46
it's on a downward slope? Exactly. Because it's
19:49
on a downward slope, seawater can rush further
19:52
underneath, lifting and melting more ice in a
19:54
runaway, self -reinforcing collapse. Oh, wow.
19:57
So the whole sheet just slides downhill into
19:59
the ocean faster and faster. And modeling the
20:02
exact tipping point and speed of that structural
20:05
collapse pushes the absolute limits of current
20:07
glaciology. Plus, on top of the complex ice physics,
20:11
there is the unpredictable human element, right?
20:13
The future rate of rise depends heavily on human
20:16
choices about greenhouse gas emissions. We haven't
20:18
even decided how much carbon we're emitting tomorrow,
20:20
let alone in 20 years. Which is exactly why credible
20:23
science presents future sea level rise as a range
20:26
of possibilities tied to different scenarios,
20:28
rather than a single falsely precise number.
20:31
Right. A range of projections isn't a weakness.
20:33
It is the most honest representation of a future
20:36
that relies on unwritten... variables. It's just
20:39
an incredible look at how hyper -connected this
20:41
system really is. I mean, we started this deep
20:43
dive talking about a bathtub, and we ended up
20:46
with a globally warping patchwork of ice gravity
20:49
fingerprints pulling water across hemispheres,
20:52
subsiding ground, deflating like a donut, land
20:55
rebounding from ancient glaciers, and shifting
20:58
ocean currents. It's all connected. Every single
21:01
beach and seawall has its own completely unique
21:04
combination of these forces. A completely individualized
21:07
threat. profile for every mile of coastline?
21:10
Yep. Given that hyper -local reality, it leaves
21:13
you with something incredibly practical to consider.
21:15
I mean, think about it. How might the very concept
21:18
of a 30 -year coastal mortgage or beachfront
21:20
real estate fundamentally change in the near
21:23
future? That's the million -dollar question.
21:25
Truly. Will financial markets, banks, and insurance
21:28
companies start mathematically pricing in glacial
21:30
rebound or marine ice sheet instability before
21:33
the water even reaches the doorstep? It's a complex
21:36
planetary... physics problem that is rapidly
21:38
transforming into a very real neighborhood financial
21:41
problem. Just something to mull over the next
21:43
time you walk past a perfectly sunny, clear coastal
21:46
street and notice the seawater quietly bubbling
21:49
up from the drain at your feet.